summary refs log tree commit diff
path: root/synapse/handlers/federation.py
blob: 9f773aefa7fa66a29e56dc297a3edf91d34f5d53 (plain) (blame)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
1652
1653
1654
1655
1656
1657
1658
1659
1660
1661
1662
1663
1664
1665
1666
1667
1668
1669
1670
1671
1672
1673
1674
1675
1676
1677
1678
1679
1680
1681
1682
1683
1684
1685
1686
1687
1688
1689
1690
1691
1692
1693
1694
1695
1696
1697
1698
1699
1700
1701
1702
1703
1704
1705
1706
1707
1708
1709
1710
1711
1712
1713
1714
1715
1716
1717
1718
1719
1720
1721
1722
1723
1724
1725
1726
1727
1728
1729
1730
1731
1732
1733
1734
1735
1736
1737
1738
1739
1740
1741
1742
1743
1744
1745
1746
1747
1748
1749
1750
1751
1752
1753
1754
1755
1756
1757
1758
1759
1760
1761
1762
1763
1764
1765
1766
1767
1768
1769
1770
1771
1772
1773
1774
1775
1776
1777
1778
1779
1780
1781
1782
1783
1784
1785
1786
1787
1788
1789
1790
1791
1792
1793
1794
1795
1796
1797
1798
1799
1800
1801
1802
1803
1804
1805
1806
1807
1808
1809
1810
1811
1812
1813
1814
1815
1816
1817
1818
1819
1820
1821
1822
1823
1824
1825
1826
1827
1828
1829
1830
1831
1832
1833
1834
1835
1836
1837
1838
1839
1840
1841
1842
1843
1844
1845
1846
1847
1848
1849
1850
1851
1852
1853
1854
1855
1856
1857
1858
1859
1860
1861
1862
1863
1864
1865
1866
1867
1868
1869
1870
1871
1872
1873
1874
1875
1876
1877
1878
1879
1880
1881
1882
1883
1884
1885
1886
1887
1888
1889
1890
1891
1892
1893
1894
1895
1896
1897
1898
1899
1900
1901
1902
1903
1904
1905
1906
1907
1908
1909
1910
1911
1912
1913
1914
1915
1916
1917
1918
1919
1920
1921
1922
1923
1924
1925
1926
1927
1928
1929
1930
1931
1932
1933
1934
1935
1936
1937
1938
1939
1940
1941
1942
1943
1944
1945
1946
1947
1948
1949
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020
2021
2022
2023
2024
2025
2026
2027
2028
2029
2030
2031
2032
2033
2034
2035
2036
2037
2038
2039
2040
2041
2042
2043
2044
2045
2046
2047
2048
2049
2050
2051
2052
2053
2054
2055
2056
2057
2058
2059
2060
2061
2062
2063
2064
2065
2066
2067
2068
2069
2070
2071
2072
2073
2074
2075
2076
2077
2078
2079
2080
2081
2082
2083
2084
2085
2086
2087
2088
2089
2090
2091
2092
2093
2094
2095
2096
2097
2098
2099
2100
2101
2102
2103
2104
2105
2106
2107
2108
2109
2110
2111
2112
2113
2114
2115
2116
2117
2118
2119
2120
2121
2122
2123
2124
2125
2126
2127
2128
2129
2130
2131
2132
2133
2134
2135
2136
2137
2138
2139
2140
2141
2142
2143
2144
2145
2146
2147
2148
2149
2150
2151
2152
2153
2154
2155
2156
2157
2158
2159
2160
2161
2162
2163
2164
2165
2166
2167
2168
2169
2170
2171
2172
2173
2174
2175
2176
2177
2178
2179
2180
2181
2182
2183
2184
2185
2186
2187
2188
2189
2190
2191
2192
2193
2194
2195
2196
2197
2198
2199
2200
2201
2202
2203
2204
2205
2206
2207
2208
2209
2210
2211
2212
2213
2214
2215
2216
2217
2218
2219
2220
2221
2222
2223
2224
2225
2226
2227
2228
2229
2230
2231
2232
2233
2234
2235
2236
2237
2238
2239
2240
2241
2242
2243
2244
2245
2246
2247
2248
2249
2250
2251
2252
2253
2254
2255
2256
2257
2258
2259
2260
2261
2262
2263
2264
2265
2266
2267
2268
2269
2270
2271
2272
2273
2274
2275
2276
2277
2278
2279
2280
2281
2282
2283
2284
2285
2286
2287
2288
2289
2290
2291
2292
2293
2294
2295
2296
2297
2298
2299
2300
2301
2302
2303
2304
2305
2306
2307
2308
2309
2310
2311
2312
2313
2314
2315
2316
2317
2318
2319
2320
2321
2322
2323
2324
2325
2326
2327
2328
2329
2330
2331
2332
2333
2334
2335
2336
2337
2338
2339
2340
2341
2342
2343
2344
2345
2346
2347
2348
2349
2350
2351
2352
2353
2354
2355
2356
2357
2358
2359
2360
2361
2362
2363
2364
2365
2366
2367
2368
2369
2370
2371
2372
2373
2374
2375
2376
2377
2378
2379
2380
2381
2382
2383
2384
2385
2386
2387
2388
2389
2390
2391
2392
2393
2394
2395
2396
2397
2398
2399
2400
2401
2402
2403
2404
2405
2406
2407
2408
2409
2410
2411
2412
2413
2414
2415
2416
2417
2418
2419
2420
2421
2422
2423
2424
2425
2426
2427
2428
2429
2430
2431
2432
2433
2434
2435
2436
2437
2438
2439
2440
2441
2442
2443
2444
2445
2446
2447
2448
2449
2450
2451
2452
2453
2454
2455
2456
2457
2458
2459
2460
2461
2462
2463
2464
2465
2466
2467
2468
2469
2470
2471
2472
2473
2474
2475
2476
2477
2478
2479
2480
2481
2482
2483
2484
2485
2486
2487
2488
2489
2490
2491
2492
2493
2494
2495
2496
2497
2498
2499
2500
2501
2502
2503
2504
2505
2506
2507
2508
2509
2510
2511
2512
2513
2514
2515
2516
2517
2518
2519
2520
2521
2522
2523
2524
2525
2526
2527
2528
2529
2530
2531
2532
2533
2534
2535
2536
2537
2538
2539
2540
2541
2542
2543
2544
2545
2546
2547
2548
2549
2550
2551
2552
2553
2554
2555
2556
2557
2558
2559
2560
2561
2562
2563
2564
2565
2566
2567
2568
2569
2570
2571
2572
2573
2574
2575
2576
2577
2578
2579
2580
2581
2582
2583
2584
2585
2586
2587
2588
2589
2590
2591
2592
2593
2594
2595
2596
2597
2598
2599
2600
2601
2602
2603
2604
2605
2606
2607
2608
2609
2610
2611
2612
2613
2614
2615
2616
2617
2618
2619
2620
2621
2622
2623
2624
2625
2626
2627
2628
2629
2630
2631
2632
2633
2634
2635
2636
2637
2638
2639
2640
2641
2642
2643
2644
2645
2646
2647
2648
2649
2650
2651
2652
2653
2654
2655
2656
2657
2658
2659
2660
2661
2662
2663
2664
2665
2666
2667
2668
2669
2670
2671
2672
2673
2674
2675
2676
2677
2678
2679
2680
2681
2682
2683
2684
2685
2686
2687
2688
2689
2690
2691
2692
2693
2694
2695
2696
2697
2698
2699
2700
2701
2702
2703
2704
2705
2706
2707
2708
2709
2710
2711
2712
2713
2714
2715
2716
2717
2718
2719
2720
2721
2722
2723
2724
2725
2726
2727
2728
2729
2730
2731
2732
2733
2734
2735
2736
2737
2738
2739
2740
2741
2742
2743
2744
2745
2746
2747
2748
2749
2750
2751
2752
2753
2754
2755
2756
2757
2758
2759
2760
2761
2762
2763
2764
2765
2766
2767
2768
2769
2770
2771
2772
2773
2774
2775
2776
2777
2778
2779
2780
2781
2782
2783
2784
2785
2786
2787
2788
2789
2790
2791
2792
2793
2794
2795
2796
2797
2798
2799
2800
2801
2802
2803
2804
2805
2806
2807
2808
2809
2810
2811
2812
2813
2814
2815
2816
2817
2818
2819
2820
2821
2822
2823
2824
2825
2826
2827
2828
2829
2830
2831
2832
2833
2834
2835
2836
2837
2838
2839
2840
2841
2842
2843
2844
2845
2846
2847
2848
2849
2850
2851
2852
2853
2854
2855
2856
2857
2858
2859
2860
2861
2862
2863
2864
2865
2866
2867
2868
2869
2870
2871
2872
2873
2874
2875
2876
2877
2878
2879
2880
2881
2882
2883
2884
2885
2886
2887
2888
2889
2890
2891
2892
2893
2894
2895
2896
2897
2898
2899
2900
2901
2902
2903
2904
2905
2906
2907
2908
2909
2910
2911
2912
2913
2914
2915
2916
2917
2918
2919
2920
2921
2922
2923
2924
2925
2926
2927
2928
2929
2930
2931
2932
2933
2934
2935
2936
2937
2938
2939
2940
2941
2942
2943
2944
2945
2946
2947
2948
2949
2950
2951
2952
2953
2954
2955
2956
2957
2958
2959
2960
2961
2962
2963
2964
2965
2966
2967
2968
2969
2970
2971
2972
2973
2974
2975
2976
2977
2978
2979
2980
2981
2982
2983
2984
2985
2986
2987
2988
2989
2990
2991
2992
2993
2994
2995
2996
2997
2998
2999
3000
3001
3002
3003
3004
3005
3006
3007
3008
3009
3010
3011
3012
3013
3014
3015
3016
3017
3018
3019
3020
3021
3022
3023
3024
3025
3026
3027
3028
3029
3030
3031
3032
3033
3034
3035
3036
3037
3038
3039
3040
3041
3042
3043
3044
3045
3046
# -*- coding: utf-8 -*-
# Copyright 2014-2016 OpenMarket Ltd
# Copyright 2017-2018 New Vector Ltd
# Copyright 2019 The Matrix.org Foundation C.I.C.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

"""Contains handlers for federation events."""

import itertools
import logging
from collections.abc import Container
from http import HTTPStatus
from typing import Dict, Iterable, List, Optional, Sequence, Tuple, Union

import attr
from signedjson.key import decode_verify_key_bytes
from signedjson.sign import verify_signed_json
from unpaddedbase64 import decode_base64

from twisted.internet import defer

from synapse import event_auth
from synapse.api.constants import (
    EventTypes,
    Membership,
    RejectedReason,
    RoomEncryptionAlgorithms,
)
from synapse.api.errors import (
    AuthError,
    CodeMessageException,
    Codes,
    FederationDeniedError,
    FederationError,
    HttpResponseException,
    NotFoundError,
    RequestSendFailed,
    SynapseError,
)
from synapse.api.room_versions import KNOWN_ROOM_VERSIONS, RoomVersion, RoomVersions
from synapse.crypto.event_signing import compute_event_signature
from synapse.event_auth import auth_types_for_event
from synapse.events import EventBase
from synapse.events.snapshot import EventContext
from synapse.events.validator import EventValidator
from synapse.handlers._base import BaseHandler
from synapse.logging.context import (
    make_deferred_yieldable,
    nested_logging_context,
    preserve_fn,
    run_in_background,
)
from synapse.logging.utils import log_function
from synapse.metrics.background_process_metrics import run_as_background_process
from synapse.replication.http.devices import ReplicationUserDevicesResyncRestServlet
from synapse.replication.http.federation import (
    ReplicationCleanRoomRestServlet,
    ReplicationFederationSendEventsRestServlet,
    ReplicationStoreRoomOnInviteRestServlet,
)
from synapse.state import StateResolutionStore, resolve_events_with_store
from synapse.storage.databases.main.events_worker import EventRedactBehaviour
from synapse.types import (
    JsonDict,
    MutableStateMap,
    PersistedEventPosition,
    RoomStreamToken,
    StateMap,
    UserID,
    get_domain_from_id,
)
from synapse.util.async_helpers import Linearizer, concurrently_execute
from synapse.util.retryutils import NotRetryingDestination
from synapse.util.stringutils import shortstr
from synapse.visibility import filter_events_for_server

logger = logging.getLogger(__name__)


@attr.s(slots=True)
class _NewEventInfo:
    """Holds information about a received event, ready for passing to _handle_new_events

    Attributes:
        event: the received event

        state: the state at that event

        auth_events: the auth_event map for that event
    """

    event = attr.ib(type=EventBase)
    state = attr.ib(type=Optional[Sequence[EventBase]], default=None)
    auth_events = attr.ib(type=Optional[MutableStateMap[EventBase]], default=None)


class FederationHandler(BaseHandler):
    """Handles events that originated from federation.
        Responsible for:
        a) handling received Pdus before handing them on as Events to the rest
        of the homeserver (including auth and state conflict resoultion)
        b) converting events that were produced by local clients that may need
        to be sent to remote homeservers.
        c) doing the necessary dances to invite remote users and join remote
        rooms.
    """

    def __init__(self, hs):
        super().__init__(hs)

        self.hs = hs

        self.store = hs.get_datastore()
        self.storage = hs.get_storage()
        self.state_store = self.storage.state
        self.federation_client = hs.get_federation_client()
        self.state_handler = hs.get_state_handler()
        self.server_name = hs.hostname
        self.keyring = hs.get_keyring()
        self.action_generator = hs.get_action_generator()
        self.is_mine_id = hs.is_mine_id
        self.spam_checker = hs.get_spam_checker()
        self.event_creation_handler = hs.get_event_creation_handler()
        self._message_handler = hs.get_message_handler()
        self._server_notices_mxid = hs.config.server_notices_mxid
        self.config = hs.config
        self.http_client = hs.get_simple_http_client()
        self._instance_name = hs.get_instance_name()
        self._replication = hs.get_replication_data_handler()

        self._send_events = ReplicationFederationSendEventsRestServlet.make_client(hs)
        self._clean_room_for_join_client = ReplicationCleanRoomRestServlet.make_client(
            hs
        )

        if hs.config.worker_app:
            self._user_device_resync = ReplicationUserDevicesResyncRestServlet.make_client(
                hs
            )
            self._maybe_store_room_on_invite = ReplicationStoreRoomOnInviteRestServlet.make_client(
                hs
            )
        else:
            self._device_list_updater = hs.get_device_handler().device_list_updater
            self._maybe_store_room_on_invite = self.store.maybe_store_room_on_invite

        # When joining a room we need to queue any events for that room up
        self.room_queues = {}
        self._room_pdu_linearizer = Linearizer("fed_room_pdu")

        self.third_party_event_rules = hs.get_third_party_event_rules()

        self._ephemeral_messages_enabled = hs.config.enable_ephemeral_messages

    async def on_receive_pdu(self, origin, pdu, sent_to_us_directly=False) -> None:
        """ Process a PDU received via a federation /send/ transaction, or
        via backfill of missing prev_events

        Args:
            origin (str): server which initiated the /send/ transaction. Will
                be used to fetch missing events or state.
            pdu (FrozenEvent): received PDU
            sent_to_us_directly (bool): True if this event was pushed to us; False if
                we pulled it as the result of a missing prev_event.
        """

        room_id = pdu.room_id
        event_id = pdu.event_id

        logger.info("handling received PDU: %s", pdu)

        # We reprocess pdus when we have seen them only as outliers
        existing = await self.store.get_event(
            event_id, allow_none=True, allow_rejected=True
        )

        # FIXME: Currently we fetch an event again when we already have it
        # if it has been marked as an outlier.

        already_seen = existing and (
            not existing.internal_metadata.is_outlier()
            or pdu.internal_metadata.is_outlier()
        )
        if already_seen:
            logger.debug("[%s %s]: Already seen pdu", room_id, event_id)
            return

        # do some initial sanity-checking of the event. In particular, make
        # sure it doesn't have hundreds of prev_events or auth_events, which
        # could cause a huge state resolution or cascade of event fetches.
        try:
            self._sanity_check_event(pdu)
        except SynapseError as err:
            logger.warning(
                "[%s %s] Received event failed sanity checks", room_id, event_id
            )
            raise FederationError("ERROR", err.code, err.msg, affected=pdu.event_id)

        # If we are currently in the process of joining this room, then we
        # queue up events for later processing.
        if room_id in self.room_queues:
            logger.info(
                "[%s %s] Queuing PDU from %s for now: join in progress",
                room_id,
                event_id,
                origin,
            )
            self.room_queues[room_id].append((pdu, origin))
            return

        # If we're not in the room just ditch the event entirely. This is
        # probably an old server that has come back and thinks we're still in
        # the room (or we've been rejoined to the room by a state reset).
        #
        # Note that if we were never in the room then we would have already
        # dropped the event, since we wouldn't know the room version.
        is_in_room = await self.auth.check_host_in_room(room_id, self.server_name)
        if not is_in_room:
            logger.info(
                "[%s %s] Ignoring PDU from %s as we're not in the room",
                room_id,
                event_id,
                origin,
            )
            return None

        state = None

        # Get missing pdus if necessary.
        if not pdu.internal_metadata.is_outlier():
            # We only backfill backwards to the min depth.
            min_depth = await self.get_min_depth_for_context(pdu.room_id)

            logger.debug("[%s %s] min_depth: %d", room_id, event_id, min_depth)

            prevs = set(pdu.prev_event_ids())
            seen = await self.store.have_events_in_timeline(prevs)

            if min_depth is not None and pdu.depth < min_depth:
                # This is so that we don't notify the user about this
                # message, to work around the fact that some events will
                # reference really really old events we really don't want to
                # send to the clients.
                pdu.internal_metadata.outlier = True
            elif min_depth is not None and pdu.depth > min_depth:
                missing_prevs = prevs - seen
                if sent_to_us_directly and missing_prevs:
                    # If we're missing stuff, ensure we only fetch stuff one
                    # at a time.
                    logger.info(
                        "[%s %s] Acquiring room lock to fetch %d missing prev_events: %s",
                        room_id,
                        event_id,
                        len(missing_prevs),
                        shortstr(missing_prevs),
                    )
                    with (await self._room_pdu_linearizer.queue(pdu.room_id)):
                        logger.info(
                            "[%s %s] Acquired room lock to fetch %d missing prev_events",
                            room_id,
                            event_id,
                            len(missing_prevs),
                        )

                        try:
                            await self._get_missing_events_for_pdu(
                                origin, pdu, prevs, min_depth
                            )
                        except Exception as e:
                            raise Exception(
                                "Error fetching missing prev_events for %s: %s"
                                % (event_id, e)
                            )

                        # Update the set of things we've seen after trying to
                        # fetch the missing stuff
                        seen = await self.store.have_events_in_timeline(prevs)

                        if not prevs - seen:
                            logger.info(
                                "[%s %s] Found all missing prev_events",
                                room_id,
                                event_id,
                            )

            if prevs - seen:
                # We've still not been able to get all of the prev_events for this event.
                #
                # In this case, we need to fall back to asking another server in the
                # federation for the state at this event. That's ok provided we then
                # resolve the state against other bits of the DAG before using it (which
                # will ensure that you can't just take over a room by sending an event,
                # withholding its prev_events, and declaring yourself to be an admin in
                # the subsequent state request).
                #
                # Now, if we're pulling this event as a missing prev_event, then clearly
                # this event is not going to become the only forward-extremity and we are
                # guaranteed to resolve its state against our existing forward
                # extremities, so that should be fine.
                #
                # On the other hand, if this event was pushed to us, it is possible for
                # it to become the only forward-extremity in the room, and we would then
                # trust its state to be the state for the whole room. This is very bad.
                # Further, if the event was pushed to us, there is no excuse for us not to
                # have all the prev_events. We therefore reject any such events.
                #
                # XXX this really feels like it could/should be merged with the above,
                # but there is an interaction with min_depth that I'm not really
                # following.

                if sent_to_us_directly:
                    logger.warning(
                        "[%s %s] Rejecting: failed to fetch %d prev events: %s",
                        room_id,
                        event_id,
                        len(prevs - seen),
                        shortstr(prevs - seen),
                    )
                    raise FederationError(
                        "ERROR",
                        403,
                        (
                            "Your server isn't divulging details about prev_events "
                            "referenced in this event."
                        ),
                        affected=pdu.event_id,
                    )

                logger.info(
                    "Event %s is missing prev_events: calculating state for a "
                    "backwards extremity",
                    event_id,
                )

                # Calculate the state after each of the previous events, and
                # resolve them to find the correct state at the current event.
                event_map = {event_id: pdu}
                try:
                    # Get the state of the events we know about
                    ours = await self.state_store.get_state_groups_ids(room_id, seen)

                    # state_maps is a list of mappings from (type, state_key) to event_id
                    state_maps = list(ours.values())  # type: List[StateMap[str]]

                    # we don't need this any more, let's delete it.
                    del ours

                    # Ask the remote server for the states we don't
                    # know about
                    for p in prevs - seen:
                        logger.info(
                            "Requesting state at missing prev_event %s", event_id,
                        )

                        with nested_logging_context(p):
                            # note that if any of the missing prevs share missing state or
                            # auth events, the requests to fetch those events are deduped
                            # by the get_pdu_cache in federation_client.
                            (remote_state, _,) = await self._get_state_for_room(
                                origin, room_id, p, include_event_in_state=True
                            )

                            remote_state_map = {
                                (x.type, x.state_key): x.event_id for x in remote_state
                            }
                            state_maps.append(remote_state_map)

                            for x in remote_state:
                                event_map[x.event_id] = x

                    room_version = await self.store.get_room_version_id(room_id)
                    state_map = await resolve_events_with_store(
                        self.clock,
                        room_id,
                        room_version,
                        state_maps,
                        event_map,
                        state_res_store=StateResolutionStore(self.store),
                    )

                    # We need to give _process_received_pdu the actual state events
                    # rather than event ids, so generate that now.

                    # First though we need to fetch all the events that are in
                    # state_map, so we can build up the state below.
                    evs = await self.store.get_events(
                        list(state_map.values()),
                        get_prev_content=False,
                        redact_behaviour=EventRedactBehaviour.AS_IS,
                    )
                    event_map.update(evs)

                    state = [event_map[e] for e in state_map.values()]
                except Exception:
                    logger.warning(
                        "[%s %s] Error attempting to resolve state at missing "
                        "prev_events",
                        room_id,
                        event_id,
                        exc_info=True,
                    )
                    raise FederationError(
                        "ERROR",
                        403,
                        "We can't get valid state history.",
                        affected=event_id,
                    )

        await self._process_received_pdu(origin, pdu, state=state)

    async def _get_missing_events_for_pdu(self, origin, pdu, prevs, min_depth):
        """
        Args:
            origin (str): Origin of the pdu. Will be called to get the missing events
            pdu: received pdu
            prevs (set(str)): List of event ids which we are missing
            min_depth (int): Minimum depth of events to return.
        """

        room_id = pdu.room_id
        event_id = pdu.event_id

        seen = await self.store.have_events_in_timeline(prevs)

        if not prevs - seen:
            return

        latest_list = await self.store.get_latest_event_ids_in_room(room_id)

        # We add the prev events that we have seen to the latest
        # list to ensure the remote server doesn't give them to us
        latest = set(latest_list)
        latest |= seen

        logger.info(
            "[%s %s]: Requesting missing events between %s and %s",
            room_id,
            event_id,
            shortstr(latest),
            event_id,
        )

        # XXX: we set timeout to 10s to help workaround
        # https://github.com/matrix-org/synapse/issues/1733.
        # The reason is to avoid holding the linearizer lock
        # whilst processing inbound /send transactions, causing
        # FDs to stack up and block other inbound transactions
        # which empirically can currently take up to 30 minutes.
        #
        # N.B. this explicitly disables retry attempts.
        #
        # N.B. this also increases our chances of falling back to
        # fetching fresh state for the room if the missing event
        # can't be found, which slightly reduces our security.
        # it may also increase our DAG extremity count for the room,
        # causing additional state resolution?  See #1760.
        # However, fetching state doesn't hold the linearizer lock
        # apparently.
        #
        # see https://github.com/matrix-org/synapse/pull/1744
        #
        # ----
        #
        # Update richvdh 2018/09/18: There are a number of problems with timing this
        # request out agressively on the client side:
        #
        # - it plays badly with the server-side rate-limiter, which starts tarpitting you
        #   if you send too many requests at once, so you end up with the server carefully
        #   working through the backlog of your requests, which you have already timed
        #   out.
        #
        # - for this request in particular, we now (as of
        #   https://github.com/matrix-org/synapse/pull/3456) reject any PDUs where the
        #   server can't produce a plausible-looking set of prev_events - so we becone
        #   much more likely to reject the event.
        #
        # - contrary to what it says above, we do *not* fall back to fetching fresh state
        #   for the room if get_missing_events times out. Rather, we give up processing
        #   the PDU whose prevs we are missing, which then makes it much more likely that
        #   we'll end up back here for the *next* PDU in the list, which exacerbates the
        #   problem.
        #
        # - the agressive 10s timeout was introduced to deal with incoming federation
        #   requests taking 8 hours to process. It's not entirely clear why that was going
        #   on; certainly there were other issues causing traffic storms which are now
        #   resolved, and I think in any case we may be more sensible about our locking
        #   now. We're *certainly* more sensible about our logging.
        #
        # All that said: Let's try increasing the timout to 60s and see what happens.

        try:
            missing_events = await self.federation_client.get_missing_events(
                origin,
                room_id,
                earliest_events_ids=list(latest),
                latest_events=[pdu],
                limit=10,
                min_depth=min_depth,
                timeout=60000,
            )
        except (RequestSendFailed, HttpResponseException, NotRetryingDestination) as e:
            # We failed to get the missing events, but since we need to handle
            # the case of `get_missing_events` not returning the necessary
            # events anyway, it is safe to simply log the error and continue.
            logger.warning(
                "[%s %s]: Failed to get prev_events: %s", room_id, event_id, e
            )
            return

        logger.info(
            "[%s %s]: Got %d prev_events: %s",
            room_id,
            event_id,
            len(missing_events),
            shortstr(missing_events),
        )

        # We want to sort these by depth so we process them and
        # tell clients about them in order.
        missing_events.sort(key=lambda x: x.depth)

        for ev in missing_events:
            logger.info(
                "[%s %s] Handling received prev_event %s",
                room_id,
                event_id,
                ev.event_id,
            )
            with nested_logging_context(ev.event_id):
                try:
                    await self.on_receive_pdu(origin, ev, sent_to_us_directly=False)
                except FederationError as e:
                    if e.code == 403:
                        logger.warning(
                            "[%s %s] Received prev_event %s failed history check.",
                            room_id,
                            event_id,
                            ev.event_id,
                        )
                    else:
                        raise

    async def _get_state_for_room(
        self,
        destination: str,
        room_id: str,
        event_id: str,
        include_event_in_state: bool = False,
    ) -> Tuple[List[EventBase], List[EventBase]]:
        """Requests all of the room state at a given event from a remote homeserver.

        Args:
            destination: The remote homeserver to query for the state.
            room_id: The id of the room we're interested in.
            event_id: The id of the event we want the state at.
            include_event_in_state: if true, the event itself will be included in the
                returned state event list.

        Returns:
            A list of events in the state, possibly including the event itself, and
            a list of events in the auth chain for the given event.
        """
        (
            state_event_ids,
            auth_event_ids,
        ) = await self.federation_client.get_room_state_ids(
            destination, room_id, event_id=event_id
        )

        desired_events = set(state_event_ids + auth_event_ids)

        if include_event_in_state:
            desired_events.add(event_id)

        event_map = await self._get_events_from_store_or_dest(
            destination, room_id, desired_events
        )

        failed_to_fetch = desired_events - event_map.keys()
        if failed_to_fetch:
            logger.warning(
                "Failed to fetch missing state/auth events for %s %s",
                event_id,
                failed_to_fetch,
            )

        remote_state = [
            event_map[e_id] for e_id in state_event_ids if e_id in event_map
        ]

        if include_event_in_state:
            remote_event = event_map.get(event_id)
            if not remote_event:
                raise Exception("Unable to get missing prev_event %s" % (event_id,))
            if remote_event.is_state() and remote_event.rejected_reason is None:
                remote_state.append(remote_event)

        auth_chain = [event_map[e_id] for e_id in auth_event_ids if e_id in event_map]
        auth_chain.sort(key=lambda e: e.depth)

        return remote_state, auth_chain

    async def _get_events_from_store_or_dest(
        self, destination: str, room_id: str, event_ids: Iterable[str]
    ) -> Dict[str, EventBase]:
        """Fetch events from a remote destination, checking if we already have them.

        Persists any events we don't already have as outliers.

        If we fail to fetch any of the events, a warning will be logged, and the event
        will be omitted from the result. Likewise, any events which turn out not to
        be in the given room.

        This function *does not* automatically get missing auth events of the
        newly fetched events. Callers must include the full auth chain of
        of the missing events in the `event_ids` argument, to ensure that any
        missing auth events are correctly fetched.

        Returns:
            map from event_id to event
        """
        fetched_events = await self.store.get_events(event_ids, allow_rejected=True)

        missing_events = set(event_ids) - fetched_events.keys()

        if missing_events:
            logger.debug(
                "Fetching unknown state/auth events %s for room %s",
                missing_events,
                room_id,
            )

            await self._get_events_and_persist(
                destination=destination, room_id=room_id, events=missing_events
            )

            # we need to make sure we re-load from the database to get the rejected
            # state correct.
            fetched_events.update(
                (await self.store.get_events(missing_events, allow_rejected=True))
            )

        # check for events which were in the wrong room.
        #
        # this can happen if a remote server claims that the state or
        # auth_events at an event in room A are actually events in room B

        bad_events = [
            (event_id, event.room_id)
            for event_id, event in fetched_events.items()
            if event.room_id != room_id
        ]

        for bad_event_id, bad_room_id in bad_events:
            # This is a bogus situation, but since we may only discover it a long time
            # after it happened, we try our best to carry on, by just omitting the
            # bad events from the returned auth/state set.
            logger.warning(
                "Remote server %s claims event %s in room %s is an auth/state "
                "event in room %s",
                destination,
                bad_event_id,
                bad_room_id,
                room_id,
            )

            del fetched_events[bad_event_id]

        return fetched_events

    async def _process_received_pdu(
        self, origin: str, event: EventBase, state: Optional[Iterable[EventBase]],
    ):
        """ Called when we have a new pdu. We need to do auth checks and put it
        through the StateHandler.

        Args:
            origin: server sending the event

            event: event to be persisted

            state: Normally None, but if we are handling a gap in the graph
                (ie, we are missing one or more prev_events), the resolved state at the
                event
        """
        room_id = event.room_id
        event_id = event.event_id

        logger.debug("[%s %s] Processing event: %s", room_id, event_id, event)

        try:
            await self._handle_new_event(origin, event, state=state)
        except AuthError as e:
            raise FederationError("ERROR", e.code, e.msg, affected=event.event_id)

        # For encrypted messages we check that we know about the sending device,
        # if we don't then we mark the device cache for that user as stale.
        if event.type == EventTypes.Encrypted:
            device_id = event.content.get("device_id")
            sender_key = event.content.get("sender_key")

            cached_devices = await self.store.get_cached_devices_for_user(event.sender)

            resync = False  # Whether we should resync device lists.

            device = None
            if device_id is not None:
                device = cached_devices.get(device_id)
                if device is None:
                    logger.info(
                        "Received event from remote device not in our cache: %s %s",
                        event.sender,
                        device_id,
                    )
                    resync = True

            # We also check if the `sender_key` matches what we expect.
            if sender_key is not None:
                # Figure out what sender key we're expecting. If we know the
                # device and recognize the algorithm then we can work out the
                # exact key to expect. Otherwise check it matches any key we
                # have for that device.

                current_keys = []  # type: Container[str]

                if device:
                    keys = device.get("keys", {}).get("keys", {})

                    if (
                        event.content.get("algorithm")
                        == RoomEncryptionAlgorithms.MEGOLM_V1_AES_SHA2
                    ):
                        # For this algorithm we expect a curve25519 key.
                        key_name = "curve25519:%s" % (device_id,)
                        current_keys = [keys.get(key_name)]
                    else:
                        # We don't know understand the algorithm, so we just
                        # check it matches a key for the device.
                        current_keys = keys.values()
                elif device_id:
                    # We don't have any keys for the device ID.
                    pass
                else:
                    # The event didn't include a device ID, so we just look for
                    # keys across all devices.
                    current_keys = [
                        key
                        for device in cached_devices.values()
                        for key in device.get("keys", {}).get("keys", {}).values()
                    ]

                # We now check that the sender key matches (one of) the expected
                # keys.
                if sender_key not in current_keys:
                    logger.info(
                        "Received event from remote device with unexpected sender key: %s %s: %s",
                        event.sender,
                        device_id or "<no device_id>",
                        sender_key,
                    )
                    resync = True

            if resync:
                run_as_background_process(
                    "resync_device_due_to_pdu", self._resync_device, event.sender
                )

    async def _resync_device(self, sender: str) -> None:
        """We have detected that the device list for the given user may be out
        of sync, so we try and resync them.
        """

        try:
            await self.store.mark_remote_user_device_cache_as_stale(sender)

            # Immediately attempt a resync in the background
            if self.config.worker_app:
                await self._user_device_resync(user_id=sender)
            else:
                await self._device_list_updater.user_device_resync(sender)
        except Exception:
            logger.exception("Failed to resync device for %s", sender)

    @log_function
    async def backfill(self, dest, room_id, limit, extremities):
        """ Trigger a backfill request to `dest` for the given `room_id`

        This will attempt to get more events from the remote. If the other side
        has no new events to offer, this will return an empty list.

        As the events are received, we check their signatures, and also do some
        sanity-checking on them. If any of the backfilled events are invalid,
        this method throws a SynapseError.

        TODO: make this more useful to distinguish failures of the remote
        server from invalid events (there is probably no point in trying to
        re-fetch invalid events from every other HS in the room.)
        """
        if dest == self.server_name:
            raise SynapseError(400, "Can't backfill from self.")

        events = await self.federation_client.backfill(
            dest, room_id, limit=limit, extremities=extremities
        )

        # ideally we'd sanity check the events here for excess prev_events etc,
        # but it's hard to reject events at this point without completely
        # breaking backfill in the same way that it is currently broken by
        # events whose signature we cannot verify (#3121).
        #
        # So for now we accept the events anyway. #3124 tracks this.
        #
        # for ev in events:
        #     self._sanity_check_event(ev)

        # Don't bother processing events we already have.
        seen_events = await self.store.have_events_in_timeline(
            {e.event_id for e in events}
        )

        events = [e for e in events if e.event_id not in seen_events]

        if not events:
            return []

        event_map = {e.event_id: e for e in events}

        event_ids = {e.event_id for e in events}

        # build a list of events whose prev_events weren't in the batch.
        # (XXX: this will include events whose prev_events we already have; that doesn't
        # sound right?)
        edges = [ev.event_id for ev in events if set(ev.prev_event_ids()) - event_ids]

        logger.info("backfill: Got %d events with %d edges", len(events), len(edges))

        # For each edge get the current state.

        auth_events = {}
        state_events = {}
        events_to_state = {}
        for e_id in edges:
            state, auth = await self._get_state_for_room(
                destination=dest,
                room_id=room_id,
                event_id=e_id,
                include_event_in_state=False,
            )
            auth_events.update({a.event_id: a for a in auth})
            auth_events.update({s.event_id: s for s in state})
            state_events.update({s.event_id: s for s in state})
            events_to_state[e_id] = state

        required_auth = {
            a_id
            for event in events
            + list(state_events.values())
            + list(auth_events.values())
            for a_id in event.auth_event_ids()
        }
        auth_events.update(
            {e_id: event_map[e_id] for e_id in required_auth if e_id in event_map}
        )

        ev_infos = []

        # Step 1: persist the events in the chunk we fetched state for (i.e.
        # the backwards extremities), with custom auth events and state
        for e_id in events_to_state:
            # For paranoia we ensure that these events are marked as
            # non-outliers
            ev = event_map[e_id]
            assert not ev.internal_metadata.is_outlier()

            ev_infos.append(
                _NewEventInfo(
                    event=ev,
                    state=events_to_state[e_id],
                    auth_events={
                        (
                            auth_events[a_id].type,
                            auth_events[a_id].state_key,
                        ): auth_events[a_id]
                        for a_id in ev.auth_event_ids()
                        if a_id in auth_events
                    },
                )
            )

        if ev_infos:
            await self._handle_new_events(dest, room_id, ev_infos, backfilled=True)

        # Step 2: Persist the rest of the events in the chunk one by one
        events.sort(key=lambda e: e.depth)

        for event in events:
            if event in events_to_state:
                continue

            # For paranoia we ensure that these events are marked as
            # non-outliers
            assert not event.internal_metadata.is_outlier()

            # We store these one at a time since each event depends on the
            # previous to work out the state.
            # TODO: We can probably do something more clever here.
            await self._handle_new_event(dest, event, backfilled=True)

        return events

    async def maybe_backfill(
        self, room_id: str, current_depth: int, limit: int
    ) -> bool:
        """Checks the database to see if we should backfill before paginating,
        and if so do.

        Args:
            room_id
            current_depth: The depth from which we're paginating from. This is
                used to decide if we should backfill and what extremities to
                use.
            limit: The number of events that the pagination request will
                return. This is used as part of the heuristic to decide if we
                should back paginate.
        """
        extremities = await self.store.get_oldest_events_with_depth_in_room(room_id)

        if not extremities:
            logger.debug("Not backfilling as no extremeties found.")
            return False

        # We only want to paginate if we can actually see the events we'll get,
        # as otherwise we'll just spend a lot of resources to get redacted
        # events.
        #
        # We do this by filtering all the backwards extremities and seeing if
        # any remain. Given we don't have the extremity events themselves, we
        # need to actually check the events that reference them.
        #
        # *Note*: the spec wants us to keep backfilling until we reach the start
        # of the room in case we are allowed to see some of the history. However
        # in practice that causes more issues than its worth, as a) its
        # relatively rare for there to be any visible history and b) even when
        # there is its often sufficiently long ago that clients would stop
        # attempting to paginate before backfill reached the visible history.
        #
        # TODO: If we do do a backfill then we should filter the backwards
        #   extremities to only include those that point to visible portions of
        #   history.
        #
        # TODO: Correctly handle the case where we are allowed to see the
        #   forward event but not the backward extremity, e.g. in the case of
        #   initial join of the server where we are allowed to see the join
        #   event but not anything before it. This would require looking at the
        #   state *before* the event, ignoring the special casing certain event
        #   types have.

        forward_events = await self.store.get_successor_events(list(extremities))

        extremities_events = await self.store.get_events(
            forward_events,
            redact_behaviour=EventRedactBehaviour.AS_IS,
            get_prev_content=False,
        )

        # We set `check_history_visibility_only` as we might otherwise get false
        # positives from users having been erased.
        filtered_extremities = await filter_events_for_server(
            self.storage,
            self.server_name,
            list(extremities_events.values()),
            redact=False,
            check_history_visibility_only=True,
        )

        if not filtered_extremities:
            return False

        # Check if we reached a point where we should start backfilling.
        sorted_extremeties_tuple = sorted(extremities.items(), key=lambda e: -int(e[1]))
        max_depth = sorted_extremeties_tuple[0][1]

        # If we're approaching an extremity we trigger a backfill, otherwise we
        # no-op.
        #
        # We chose twice the limit here as then clients paginating backwards
        # will send pagination requests that trigger backfill at least twice
        # using the most recent extremity before it gets removed (see below). We
        # chose more than one times the limit in case of failure, but choosing a
        # much larger factor will result in triggering a backfill request much
        # earlier than necessary.
        if current_depth - 2 * limit > max_depth:
            logger.debug(
                "Not backfilling as we don't need to. %d < %d - 2 * %d",
                max_depth,
                current_depth,
                limit,
            )
            return False

        logger.debug(
            "room_id: %s, backfill: current_depth: %s, max_depth: %s, extrems: %s",
            room_id,
            current_depth,
            max_depth,
            sorted_extremeties_tuple,
        )

        # We ignore extremities that have a greater depth than our current depth
        # as:
        #    1. we don't really care about getting events that have happened
        #       before our current position; and
        #    2. we have likely previously tried and failed to backfill from that
        #       extremity, so to avoid getting "stuck" requesting the same
        #       backfill repeatedly we drop those extremities.
        filtered_sorted_extremeties_tuple = [
            t for t in sorted_extremeties_tuple if int(t[1]) <= current_depth
        ]

        # However, we need to check that the filtered extremities are non-empty.
        # If they are empty then either we can a) bail or b) still attempt to
        # backill. We opt to try backfilling anyway just in case we do get
        # relevant events.
        if filtered_sorted_extremeties_tuple:
            sorted_extremeties_tuple = filtered_sorted_extremeties_tuple

        # We don't want to specify too many extremities as it causes the backfill
        # request URI to be too long.
        extremities = dict(sorted_extremeties_tuple[:5])

        # Now we need to decide which hosts to hit first.

        # First we try hosts that are already in the room
        # TODO: HEURISTIC ALERT.

        curr_state = await self.state_handler.get_current_state(room_id)

        def get_domains_from_state(state):
            """Get joined domains from state

            Args:
                state (dict[tuple, FrozenEvent]): State map from type/state
                    key to event.

            Returns:
                list[tuple[str, int]]: Returns a list of servers with the
                lowest depth of their joins. Sorted by lowest depth first.
            """
            joined_users = [
                (state_key, int(event.depth))
                for (e_type, state_key), event in state.items()
                if e_type == EventTypes.Member and event.membership == Membership.JOIN
            ]

            joined_domains = {}  # type: Dict[str, int]
            for u, d in joined_users:
                try:
                    dom = get_domain_from_id(u)
                    old_d = joined_domains.get(dom)
                    if old_d:
                        joined_domains[dom] = min(d, old_d)
                    else:
                        joined_domains[dom] = d
                except Exception:
                    pass

            return sorted(joined_domains.items(), key=lambda d: d[1])

        curr_domains = get_domains_from_state(curr_state)

        likely_domains = [
            domain for domain, depth in curr_domains if domain != self.server_name
        ]

        async def try_backfill(domains):
            # TODO: Should we try multiple of these at a time?
            for dom in domains:
                try:
                    await self.backfill(
                        dom, room_id, limit=100, extremities=extremities
                    )
                    # If this succeeded then we probably already have the
                    # appropriate stuff.
                    # TODO: We can probably do something more intelligent here.
                    return True
                except SynapseError as e:
                    logger.info("Failed to backfill from %s because %s", dom, e)
                    continue
                except HttpResponseException as e:
                    if 400 <= e.code < 500:
                        raise e.to_synapse_error()

                    logger.info("Failed to backfill from %s because %s", dom, e)
                    continue
                except CodeMessageException as e:
                    if 400 <= e.code < 500:
                        raise

                    logger.info("Failed to backfill from %s because %s", dom, e)
                    continue
                except NotRetryingDestination as e:
                    logger.info(str(e))
                    continue
                except RequestSendFailed as e:
                    logger.info("Falied to get backfill from %s because %s", dom, e)
                    continue
                except FederationDeniedError as e:
                    logger.info(e)
                    continue
                except Exception as e:
                    logger.exception("Failed to backfill from %s because %s", dom, e)
                    continue

            return False

        success = await try_backfill(likely_domains)
        if success:
            return True

        # Huh, well *those* domains didn't work out. Lets try some domains
        # from the time.

        tried_domains = set(likely_domains)
        tried_domains.add(self.server_name)

        event_ids = list(extremities.keys())

        logger.debug("calling resolve_state_groups in _maybe_backfill")
        resolve = preserve_fn(self.state_handler.resolve_state_groups_for_events)
        states = await make_deferred_yieldable(
            defer.gatherResults(
                [resolve(room_id, [e]) for e in event_ids], consumeErrors=True
            )
        )

        # dict[str, dict[tuple, str]], a map from event_id to state map of
        # event_ids.
        states = dict(zip(event_ids, [s.state for s in states]))

        state_map = await self.store.get_events(
            [e_id for ids in states.values() for e_id in ids.values()],
            get_prev_content=False,
        )
        states = {
            key: {
                k: state_map[e_id]
                for k, e_id in state_dict.items()
                if e_id in state_map
            }
            for key, state_dict in states.items()
        }

        for e_id, _ in sorted_extremeties_tuple:
            likely_domains = get_domains_from_state(states[e_id])

            success = await try_backfill(
                [dom for dom, _ in likely_domains if dom not in tried_domains]
            )
            if success:
                return True

            tried_domains.update(dom for dom, _ in likely_domains)

        return False

    async def _get_events_and_persist(
        self, destination: str, room_id: str, events: Iterable[str]
    ):
        """Fetch the given events from a server, and persist them as outliers.

        This function *does not* recursively get missing auth events of the
        newly fetched events. Callers must include in the `events` argument
        any missing events from the auth chain.

        Logs a warning if we can't find the given event.
        """

        room_version = await self.store.get_room_version(room_id)

        event_map = {}  # type: Dict[str, EventBase]

        async def get_event(event_id: str):
            with nested_logging_context(event_id):
                try:
                    event = await self.federation_client.get_pdu(
                        [destination], event_id, room_version, outlier=True,
                    )
                    if event is None:
                        logger.warning(
                            "Server %s didn't return event %s", destination, event_id,
                        )
                        return

                    event_map[event.event_id] = event

                except Exception as e:
                    logger.warning(
                        "Error fetching missing state/auth event %s: %s %s",
                        event_id,
                        type(e),
                        e,
                    )

        await concurrently_execute(get_event, events, 5)

        # Make a map of auth events for each event. We do this after fetching
        # all the events as some of the events' auth events will be in the list
        # of requested events.

        auth_events = [
            aid
            for event in event_map.values()
            for aid in event.auth_event_ids()
            if aid not in event_map
        ]
        persisted_events = await self.store.get_events(
            auth_events, allow_rejected=True,
        )

        event_infos = []
        for event in event_map.values():
            auth = {}
            for auth_event_id in event.auth_event_ids():
                ae = persisted_events.get(auth_event_id) or event_map.get(auth_event_id)
                if ae:
                    auth[(ae.type, ae.state_key)] = ae
                else:
                    logger.info("Missing auth event %s", auth_event_id)

            event_infos.append(_NewEventInfo(event, None, auth))

        await self._handle_new_events(
            destination, room_id, event_infos,
        )

    def _sanity_check_event(self, ev):
        """
        Do some early sanity checks of a received event

        In particular, checks it doesn't have an excessive number of
        prev_events or auth_events, which could cause a huge state resolution
        or cascade of event fetches.

        Args:
            ev (synapse.events.EventBase): event to be checked

        Returns: None

        Raises:
            SynapseError if the event does not pass muster
        """
        if len(ev.prev_event_ids()) > 20:
            logger.warning(
                "Rejecting event %s which has %i prev_events",
                ev.event_id,
                len(ev.prev_event_ids()),
            )
            raise SynapseError(HTTPStatus.BAD_REQUEST, "Too many prev_events")

        if len(ev.auth_event_ids()) > 10:
            logger.warning(
                "Rejecting event %s which has %i auth_events",
                ev.event_id,
                len(ev.auth_event_ids()),
            )
            raise SynapseError(HTTPStatus.BAD_REQUEST, "Too many auth_events")

    async def send_invite(self, target_host, event):
        """ Sends the invite to the remote server for signing.

        Invites must be signed by the invitee's server before distribution.
        """
        pdu = await self.federation_client.send_invite(
            destination=target_host,
            room_id=event.room_id,
            event_id=event.event_id,
            pdu=event,
        )

        return pdu

    async def on_event_auth(self, event_id: str) -> List[EventBase]:
        event = await self.store.get_event(event_id)
        auth = await self.store.get_auth_chain(
            list(event.auth_event_ids()), include_given=True
        )
        return list(auth)

    async def do_invite_join(
        self, target_hosts: Iterable[str], room_id: str, joinee: str, content: JsonDict
    ) -> Tuple[str, int]:
        """ Attempts to join the `joinee` to the room `room_id` via the
        servers contained in `target_hosts`.

        This first triggers a /make_join/ request that returns a partial
        event that we can fill out and sign. This is then sent to the
        remote server via /send_join/ which responds with the state at that
        event and the auth_chains.

        We suspend processing of any received events from this room until we
        have finished processing the join.

        Args:
            target_hosts: List of servers to attempt to join the room with.

            room_id: The ID of the room to join.

            joinee: The User ID of the joining user.

            content: The event content to use for the join event.
        """
        # TODO: We should be able to call this on workers, but the upgrading of
        # room stuff after join currently doesn't work on workers.
        assert self.config.worker.worker_app is None

        logger.debug("Joining %s to %s", joinee, room_id)

        origin, event, room_version_obj = await self._make_and_verify_event(
            target_hosts,
            room_id,
            joinee,
            "join",
            content,
            params={"ver": KNOWN_ROOM_VERSIONS},
        )

        # This shouldn't happen, because the RoomMemberHandler has a
        # linearizer lock which only allows one operation per user per room
        # at a time - so this is just paranoia.
        assert room_id not in self.room_queues

        self.room_queues[room_id] = []

        await self._clean_room_for_join(room_id)

        handled_events = set()

        try:
            # Try the host we successfully got a response to /make_join/
            # request first.
            host_list = list(target_hosts)
            try:
                host_list.remove(origin)
                host_list.insert(0, origin)
            except ValueError:
                pass

            ret = await self.federation_client.send_join(
                host_list, event, room_version_obj
            )

            origin = ret["origin"]
            state = ret["state"]
            auth_chain = ret["auth_chain"]
            auth_chain.sort(key=lambda e: e.depth)

            handled_events.update([s.event_id for s in state])
            handled_events.update([a.event_id for a in auth_chain])
            handled_events.add(event.event_id)

            logger.debug("do_invite_join auth_chain: %s", auth_chain)
            logger.debug("do_invite_join state: %s", state)

            logger.debug("do_invite_join event: %s", event)

            # if this is the first time we've joined this room, it's time to add
            # a row to `rooms` with the correct room version. If there's already a
            # row there, we should override it, since it may have been populated
            # based on an invite request which lied about the room version.
            #
            # federation_client.send_join has already checked that the room
            # version in the received create event is the same as room_version_obj,
            # so we can rely on it now.
            #
            await self.store.upsert_room_on_join(
                room_id=room_id, room_version=room_version_obj,
            )

            max_stream_id = await self._persist_auth_tree(
                origin, room_id, auth_chain, state, event, room_version_obj
            )

            # We wait here until this instance has seen the events come down
            # replication (if we're using replication) as the below uses caches.
            await self._replication.wait_for_stream_position(
                self.config.worker.events_shard_config.get_instance(room_id),
                "events",
                max_stream_id,
            )

            # Check whether this room is the result of an upgrade of a room we already know
            # about. If so, migrate over user information
            predecessor = await self.store.get_room_predecessor(room_id)
            if not predecessor or not isinstance(predecessor.get("room_id"), str):
                return event.event_id, max_stream_id
            old_room_id = predecessor["room_id"]
            logger.debug(
                "Found predecessor for %s during remote join: %s", room_id, old_room_id
            )

            # We retrieve the room member handler here as to not cause a cyclic dependency
            member_handler = self.hs.get_room_member_handler()
            await member_handler.transfer_room_state_on_room_upgrade(
                old_room_id, room_id
            )

            logger.debug("Finished joining %s to %s", joinee, room_id)
            return event.event_id, max_stream_id
        finally:
            room_queue = self.room_queues[room_id]
            del self.room_queues[room_id]

            # we don't need to wait for the queued events to be processed -
            # it's just a best-effort thing at this point. We do want to do
            # them roughly in order, though, otherwise we'll end up making
            # lots of requests for missing prev_events which we do actually
            # have. Hence we fire off the background task, but don't wait for it.

            run_in_background(self._handle_queued_pdus, room_queue)

    async def _handle_queued_pdus(self, room_queue):
        """Process PDUs which got queued up while we were busy send_joining.

        Args:
            room_queue (list[FrozenEvent, str]): list of PDUs to be processed
                and the servers that sent them
        """
        for p, origin in room_queue:
            try:
                logger.info(
                    "Processing queued PDU %s which was received "
                    "while we were joining %s",
                    p.event_id,
                    p.room_id,
                )
                with nested_logging_context(p.event_id):
                    await self.on_receive_pdu(origin, p, sent_to_us_directly=True)
            except Exception as e:
                logger.warning(
                    "Error handling queued PDU %s from %s: %s", p.event_id, origin, e
                )

    async def on_make_join_request(
        self, origin: str, room_id: str, user_id: str
    ) -> EventBase:
        """ We've received a /make_join/ request, so we create a partial
        join event for the room and return that. We do *not* persist or
        process it until the other server has signed it and sent it back.

        Args:
            origin: The (verified) server name of the requesting server.
            room_id: Room to create join event in
            user_id: The user to create the join for
        """
        if get_domain_from_id(user_id) != origin:
            logger.info(
                "Got /make_join request for user %r from different origin %s, ignoring",
                user_id,
                origin,
            )
            raise SynapseError(403, "User not from origin", Codes.FORBIDDEN)

        # checking the room version will check that we've actually heard of the room
        # (and return a 404 otherwise)
        room_version = await self.store.get_room_version_id(room_id)

        # now check that we are *still* in the room
        is_in_room = await self.auth.check_host_in_room(room_id, self.server_name)
        if not is_in_room:
            logger.info(
                "Got /make_join request for room %s we are no longer in", room_id,
            )
            raise NotFoundError("Not an active room on this server")

        event_content = {"membership": Membership.JOIN}

        builder = self.event_builder_factory.new(
            room_version,
            {
                "type": EventTypes.Member,
                "content": event_content,
                "room_id": room_id,
                "sender": user_id,
                "state_key": user_id,
            },
        )

        try:
            event, context = await self.event_creation_handler.create_new_client_event(
                builder=builder
            )
        except AuthError as e:
            logger.warning("Failed to create join to %s because %s", room_id, e)
            raise e

        event_allowed = await self.third_party_event_rules.check_event_allowed(
            event, context
        )
        if not event_allowed:
            logger.info("Creation of join %s forbidden by third-party rules", event)
            raise SynapseError(
                403, "This event is not allowed in this context", Codes.FORBIDDEN
            )

        # The remote hasn't signed it yet, obviously. We'll do the full checks
        # when we get the event back in `on_send_join_request`
        await self.auth.check_from_context(
            room_version, event, context, do_sig_check=False
        )

        return event

    async def on_send_join_request(self, origin, pdu):
        """ We have received a join event for a room. Fully process it and
        respond with the current state and auth chains.
        """
        event = pdu

        logger.debug(
            "on_send_join_request from %s: Got event: %s, signatures: %s",
            origin,
            event.event_id,
            event.signatures,
        )

        if get_domain_from_id(event.sender) != origin:
            logger.info(
                "Got /send_join request for user %r from different origin %s",
                event.sender,
                origin,
            )
            raise SynapseError(403, "User not from origin", Codes.FORBIDDEN)

        event.internal_metadata.outlier = False
        # Send this event on behalf of the origin server.
        #
        # The reasons we have the destination server rather than the origin
        # server send it are slightly mysterious: the origin server should have
        # all the neccessary state once it gets the response to the send_join,
        # so it could send the event itself if it wanted to. It may be that
        # doing it this way reduces failure modes, or avoids certain attacks
        # where a new server selectively tells a subset of the federation that
        # it has joined.
        #
        # The fact is that, as of the current writing, Synapse doesn't send out
        # the join event over federation after joining, and changing it now
        # would introduce the danger of backwards-compatibility problems.
        event.internal_metadata.send_on_behalf_of = origin

        context = await self._handle_new_event(origin, event)

        event_allowed = await self.third_party_event_rules.check_event_allowed(
            event, context
        )
        if not event_allowed:
            logger.info("Sending of join %s forbidden by third-party rules", event)
            raise SynapseError(
                403, "This event is not allowed in this context", Codes.FORBIDDEN
            )

        logger.debug(
            "on_send_join_request: After _handle_new_event: %s, sigs: %s",
            event.event_id,
            event.signatures,
        )

        prev_state_ids = await context.get_prev_state_ids()

        state_ids = list(prev_state_ids.values())
        auth_chain = await self.store.get_auth_chain(state_ids)

        state = await self.store.get_events(list(prev_state_ids.values()))

        return {"state": list(state.values()), "auth_chain": auth_chain}

    async def on_invite_request(
        self, origin: str, event: EventBase, room_version: RoomVersion
    ):
        """ We've got an invite event. Process and persist it. Sign it.

        Respond with the now signed event.
        """
        if event.state_key is None:
            raise SynapseError(400, "The invite event did not have a state key")

        is_blocked = await self.store.is_room_blocked(event.room_id)
        if is_blocked:
            raise SynapseError(403, "This room has been blocked on this server")

        if self.hs.config.block_non_admin_invites:
            raise SynapseError(403, "This server does not accept room invites")

        if not self.spam_checker.user_may_invite(
            event.sender, event.state_key, event.room_id
        ):
            raise SynapseError(
                403, "This user is not permitted to send invites to this server/user"
            )

        membership = event.content.get("membership")
        if event.type != EventTypes.Member or membership != Membership.INVITE:
            raise SynapseError(400, "The event was not an m.room.member invite event")

        sender_domain = get_domain_from_id(event.sender)
        if sender_domain != origin:
            raise SynapseError(
                400, "The invite event was not from the server sending it"
            )

        if not self.is_mine_id(event.state_key):
            raise SynapseError(400, "The invite event must be for this server")

        # block any attempts to invite the server notices mxid
        if event.state_key == self._server_notices_mxid:
            raise SynapseError(HTTPStatus.FORBIDDEN, "Cannot invite this user")

        # keep a record of the room version, if we don't yet know it.
        # (this may get overwritten if we later get a different room version in a
        # join dance).
        await self._maybe_store_room_on_invite(
            room_id=event.room_id, room_version=room_version
        )

        event.internal_metadata.outlier = True
        event.internal_metadata.out_of_band_membership = True

        event.signatures.update(
            compute_event_signature(
                room_version,
                event.get_pdu_json(),
                self.hs.hostname,
                self.hs.signing_key,
            )
        )

        context = await self.state_handler.compute_event_context(event)
        await self.persist_events_and_notify(event.room_id, [(event, context)])

        return event

    async def do_remotely_reject_invite(
        self, target_hosts: Iterable[str], room_id: str, user_id: str, content: JsonDict
    ) -> Tuple[EventBase, int]:
        origin, event, room_version = await self._make_and_verify_event(
            target_hosts, room_id, user_id, "leave", content=content
        )
        # Mark as outlier as we don't have any state for this event; we're not
        # even in the room.
        event.internal_metadata.outlier = True
        event.internal_metadata.out_of_band_membership = True

        # Try the host that we succesfully called /make_leave/ on first for
        # the /send_leave/ request.
        host_list = list(target_hosts)
        try:
            host_list.remove(origin)
            host_list.insert(0, origin)
        except ValueError:
            pass

        await self.federation_client.send_leave(host_list, event)

        context = await self.state_handler.compute_event_context(event)
        stream_id = await self.persist_events_and_notify(
            event.room_id, [(event, context)]
        )

        return event, stream_id

    async def _make_and_verify_event(
        self,
        target_hosts: Iterable[str],
        room_id: str,
        user_id: str,
        membership: str,
        content: JsonDict = {},
        params: Optional[Dict[str, Union[str, Iterable[str]]]] = None,
    ) -> Tuple[str, EventBase, RoomVersion]:
        (
            origin,
            event,
            room_version,
        ) = await self.federation_client.make_membership_event(
            target_hosts, room_id, user_id, membership, content, params=params
        )

        logger.debug("Got response to make_%s: %s", membership, event)

        # We should assert some things.
        # FIXME: Do this in a nicer way
        assert event.type == EventTypes.Member
        assert event.user_id == user_id
        assert event.state_key == user_id
        assert event.room_id == room_id
        return origin, event, room_version

    async def on_make_leave_request(
        self, origin: str, room_id: str, user_id: str
    ) -> EventBase:
        """ We've received a /make_leave/ request, so we create a partial
        leave event for the room and return that. We do *not* persist or
        process it until the other server has signed it and sent it back.

        Args:
            origin: The (verified) server name of the requesting server.
            room_id: Room to create leave event in
            user_id: The user to create the leave for
        """
        if get_domain_from_id(user_id) != origin:
            logger.info(
                "Got /make_leave request for user %r from different origin %s, ignoring",
                user_id,
                origin,
            )
            raise SynapseError(403, "User not from origin", Codes.FORBIDDEN)

        room_version = await self.store.get_room_version_id(room_id)
        builder = self.event_builder_factory.new(
            room_version,
            {
                "type": EventTypes.Member,
                "content": {"membership": Membership.LEAVE},
                "room_id": room_id,
                "sender": user_id,
                "state_key": user_id,
            },
        )

        event, context = await self.event_creation_handler.create_new_client_event(
            builder=builder
        )

        event_allowed = await self.third_party_event_rules.check_event_allowed(
            event, context
        )
        if not event_allowed:
            logger.warning("Creation of leave %s forbidden by third-party rules", event)
            raise SynapseError(
                403, "This event is not allowed in this context", Codes.FORBIDDEN
            )

        try:
            # The remote hasn't signed it yet, obviously. We'll do the full checks
            # when we get the event back in `on_send_leave_request`
            await self.auth.check_from_context(
                room_version, event, context, do_sig_check=False
            )
        except AuthError as e:
            logger.warning("Failed to create new leave %r because %s", event, e)
            raise e

        return event

    async def on_send_leave_request(self, origin, pdu):
        """ We have received a leave event for a room. Fully process it."""
        event = pdu

        logger.debug(
            "on_send_leave_request: Got event: %s, signatures: %s",
            event.event_id,
            event.signatures,
        )

        if get_domain_from_id(event.sender) != origin:
            logger.info(
                "Got /send_leave request for user %r from different origin %s",
                event.sender,
                origin,
            )
            raise SynapseError(403, "User not from origin", Codes.FORBIDDEN)

        event.internal_metadata.outlier = False

        context = await self._handle_new_event(origin, event)

        event_allowed = await self.third_party_event_rules.check_event_allowed(
            event, context
        )
        if not event_allowed:
            logger.info("Sending of leave %s forbidden by third-party rules", event)
            raise SynapseError(
                403, "This event is not allowed in this context", Codes.FORBIDDEN
            )

        logger.debug(
            "on_send_leave_request: After _handle_new_event: %s, sigs: %s",
            event.event_id,
            event.signatures,
        )

        return None

    async def get_state_for_pdu(self, room_id: str, event_id: str) -> List[EventBase]:
        """Returns the state at the event. i.e. not including said event.
        """

        event = await self.store.get_event(event_id, check_room_id=room_id)

        state_groups = await self.state_store.get_state_groups(room_id, [event_id])

        if state_groups:
            _, state = list(state_groups.items()).pop()
            results = {(e.type, e.state_key): e for e in state}

            if event.is_state():
                # Get previous state
                if "replaces_state" in event.unsigned:
                    prev_id = event.unsigned["replaces_state"]
                    if prev_id != event.event_id:
                        prev_event = await self.store.get_event(prev_id)
                        results[(event.type, event.state_key)] = prev_event
                else:
                    del results[(event.type, event.state_key)]

            res = list(results.values())
            return res
        else:
            return []

    async def get_state_ids_for_pdu(self, room_id: str, event_id: str) -> List[str]:
        """Returns the state at the event. i.e. not including said event.
        """
        event = await self.store.get_event(event_id, check_room_id=room_id)

        state_groups = await self.state_store.get_state_groups_ids(room_id, [event_id])

        if state_groups:
            _, state = list(state_groups.items()).pop()
            results = state

            if event.is_state():
                # Get previous state
                if "replaces_state" in event.unsigned:
                    prev_id = event.unsigned["replaces_state"]
                    if prev_id != event.event_id:
                        results[(event.type, event.state_key)] = prev_id
                else:
                    results.pop((event.type, event.state_key), None)

            return list(results.values())
        else:
            return []

    @log_function
    async def on_backfill_request(
        self, origin: str, room_id: str, pdu_list: List[str], limit: int
    ) -> List[EventBase]:
        in_room = await self.auth.check_host_in_room(room_id, origin)
        if not in_room:
            raise AuthError(403, "Host not in room.")

        # Synapse asks for 100 events per backfill request. Do not allow more.
        limit = min(limit, 100)

        events = await self.store.get_backfill_events(room_id, pdu_list, limit)

        events = await filter_events_for_server(self.storage, origin, events)

        return events

    @log_function
    async def get_persisted_pdu(
        self, origin: str, event_id: str
    ) -> Optional[EventBase]:
        """Get an event from the database for the given server.

        Args:
            origin: hostname of server which is requesting the event; we
               will check that the server is allowed to see it.
            event_id: id of the event being requested

        Returns:
            None if we know nothing about the event; otherwise the (possibly-redacted) event.

        Raises:
            AuthError if the server is not currently in the room
        """
        event = await self.store.get_event(
            event_id, allow_none=True, allow_rejected=True
        )

        if event:
            in_room = await self.auth.check_host_in_room(event.room_id, origin)
            if not in_room:
                raise AuthError(403, "Host not in room.")

            events = await filter_events_for_server(self.storage, origin, [event])
            event = events[0]
            return event
        else:
            return None

    async def get_min_depth_for_context(self, context):
        return await self.store.get_min_depth(context)

    async def _handle_new_event(
        self, origin, event, state=None, auth_events=None, backfilled=False
    ):
        context = await self._prep_event(
            origin, event, state=state, auth_events=auth_events, backfilled=backfilled
        )

        try:
            if (
                not event.internal_metadata.is_outlier()
                and not backfilled
                and not context.rejected
            ):
                await self.action_generator.handle_push_actions_for_event(
                    event, context
                )

            await self.persist_events_and_notify(
                event.room_id, [(event, context)], backfilled=backfilled
            )
        except Exception:
            run_in_background(
                self.store.remove_push_actions_from_staging, event.event_id
            )
            raise

        return context

    async def _handle_new_events(
        self,
        origin: str,
        room_id: str,
        event_infos: Iterable[_NewEventInfo],
        backfilled: bool = False,
    ) -> None:
        """Creates the appropriate contexts and persists events. The events
        should not depend on one another, e.g. this should be used to persist
        a bunch of outliers, but not a chunk of individual events that depend
        on each other for state calculations.

        Notifies about the events where appropriate.
        """

        async def prep(ev_info: _NewEventInfo):
            event = ev_info.event
            with nested_logging_context(suffix=event.event_id):
                res = await self._prep_event(
                    origin,
                    event,
                    state=ev_info.state,
                    auth_events=ev_info.auth_events,
                    backfilled=backfilled,
                )
            return res

        contexts = await make_deferred_yieldable(
            defer.gatherResults(
                [run_in_background(prep, ev_info) for ev_info in event_infos],
                consumeErrors=True,
            )
        )

        await self.persist_events_and_notify(
            room_id,
            [
                (ev_info.event, context)
                for ev_info, context in zip(event_infos, contexts)
            ],
            backfilled=backfilled,
        )

    async def _persist_auth_tree(
        self,
        origin: str,
        room_id: str,
        auth_events: List[EventBase],
        state: List[EventBase],
        event: EventBase,
        room_version: RoomVersion,
    ) -> int:
        """Checks the auth chain is valid (and passes auth checks) for the
        state and event. Then persists the auth chain and state atomically.
        Persists the event separately. Notifies about the persisted events
        where appropriate.

        Will attempt to fetch missing auth events.

        Args:
            origin: Where the events came from
            room_id,
            auth_events
            state
            event
            room_version: The room version we expect this room to have, and
                will raise if it doesn't match the version in the create event.
        """
        events_to_context = {}
        for e in itertools.chain(auth_events, state):
            e.internal_metadata.outlier = True
            ctx = await self.state_handler.compute_event_context(e)
            events_to_context[e.event_id] = ctx

        event_map = {
            e.event_id: e for e in itertools.chain(auth_events, state, [event])
        }

        create_event = None
        for e in auth_events:
            if (e.type, e.state_key) == (EventTypes.Create, ""):
                create_event = e
                break

        if create_event is None:
            # If the state doesn't have a create event then the room is
            # invalid, and it would fail auth checks anyway.
            raise SynapseError(400, "No create event in state")

        room_version_id = create_event.content.get(
            "room_version", RoomVersions.V1.identifier
        )

        if room_version.identifier != room_version_id:
            raise SynapseError(400, "Room version mismatch")

        missing_auth_events = set()
        for e in itertools.chain(auth_events, state, [event]):
            for e_id in e.auth_event_ids():
                if e_id not in event_map:
                    missing_auth_events.add(e_id)

        for e_id in missing_auth_events:
            m_ev = await self.federation_client.get_pdu(
                [origin], e_id, room_version=room_version, outlier=True, timeout=10000,
            )
            if m_ev and m_ev.event_id == e_id:
                event_map[e_id] = m_ev
            else:
                logger.info("Failed to find auth event %r", e_id)

        for e in itertools.chain(auth_events, state, [event]):
            auth_for_e = {
                (event_map[e_id].type, event_map[e_id].state_key): event_map[e_id]
                for e_id in e.auth_event_ids()
                if e_id in event_map
            }
            if create_event:
                auth_for_e[(EventTypes.Create, "")] = create_event

            try:
                event_auth.check(room_version, e, auth_events=auth_for_e)
            except SynapseError as err:
                # we may get SynapseErrors here as well as AuthErrors. For
                # instance, there are a couple of (ancient) events in some
                # rooms whose senders do not have the correct sigil; these
                # cause SynapseErrors in auth.check. We don't want to give up
                # the attempt to federate altogether in such cases.

                logger.warning("Rejecting %s because %s", e.event_id, err.msg)

                if e == event:
                    raise
                events_to_context[e.event_id].rejected = RejectedReason.AUTH_ERROR

        await self.persist_events_and_notify(
            room_id,
            [
                (e, events_to_context[e.event_id])
                for e in itertools.chain(auth_events, state)
            ],
        )

        new_event_context = await self.state_handler.compute_event_context(
            event, old_state=state
        )

        return await self.persist_events_and_notify(
            room_id, [(event, new_event_context)]
        )

    async def _prep_event(
        self,
        origin: str,
        event: EventBase,
        state: Optional[Iterable[EventBase]],
        auth_events: Optional[MutableStateMap[EventBase]],
        backfilled: bool,
    ) -> EventContext:
        context = await self.state_handler.compute_event_context(event, old_state=state)

        if not auth_events:
            prev_state_ids = await context.get_prev_state_ids()
            auth_events_ids = self.auth.compute_auth_events(
                event, prev_state_ids, for_verification=True
            )
            auth_events_x = await self.store.get_events(auth_events_ids)
            auth_events = {(e.type, e.state_key): e for e in auth_events_x.values()}

        # This is a hack to fix some old rooms where the initial join event
        # didn't reference the create event in its auth events.
        if event.type == EventTypes.Member and not event.auth_event_ids():
            if len(event.prev_event_ids()) == 1 and event.depth < 5:
                c = await self.store.get_event(
                    event.prev_event_ids()[0], allow_none=True
                )
                if c and c.type == EventTypes.Create:
                    auth_events[(c.type, c.state_key)] = c

        context = await self.do_auth(origin, event, context, auth_events=auth_events)

        if not context.rejected:
            await self._check_for_soft_fail(event, state, backfilled)

        if event.type == EventTypes.GuestAccess and not context.rejected:
            await self.maybe_kick_guest_users(event)

        return context

    async def _check_for_soft_fail(
        self, event: EventBase, state: Optional[Iterable[EventBase]], backfilled: bool
    ) -> None:
        """Checks if we should soft fail the event; if so, marks the event as
        such.

        Args:
            event
            state: The state at the event if we don't have all the event's prev events
            backfilled: Whether the event is from backfill
        """
        # For new (non-backfilled and non-outlier) events we check if the event
        # passes auth based on the current state. If it doesn't then we
        # "soft-fail" the event.
        if backfilled or event.internal_metadata.is_outlier():
            return

        extrem_ids_list = await self.store.get_latest_event_ids_in_room(event.room_id)
        extrem_ids = set(extrem_ids_list)
        prev_event_ids = set(event.prev_event_ids())

        if extrem_ids == prev_event_ids:
            # If they're the same then the current state is the same as the
            # state at the event, so no point rechecking auth for soft fail.
            return

        room_version = await self.store.get_room_version_id(event.room_id)
        room_version_obj = KNOWN_ROOM_VERSIONS[room_version]

        # Calculate the "current state".
        if state is not None:
            # If we're explicitly given the state then we won't have all the
            # prev events, and so we have a gap in the graph. In this case
            # we want to be a little careful as we might have been down for
            # a while and have an incorrect view of the current state,
            # however we still want to do checks as gaps are easy to
            # maliciously manufacture.
            #
            # So we use a "current state" that is actually a state
            # resolution across the current forward extremities and the
            # given state at the event. This should correctly handle cases
            # like bans, especially with state res v2.

            state_sets = await self.state_store.get_state_groups(
                event.room_id, extrem_ids
            )
            state_sets = list(state_sets.values())
            state_sets.append(state)
            current_states = await self.state_handler.resolve_events(
                room_version, state_sets, event
            )
            current_state_ids = {
                k: e.event_id for k, e in current_states.items()
            }  # type: StateMap[str]
        else:
            current_state_ids = await self.state_handler.get_current_state_ids(
                event.room_id, latest_event_ids=extrem_ids
            )

        logger.debug(
            "Doing soft-fail check for %s: state %s", event.event_id, current_state_ids,
        )

        # Now check if event pass auth against said current state
        auth_types = auth_types_for_event(event)
        current_state_ids_list = [
            e for k, e in current_state_ids.items() if k in auth_types
        ]

        auth_events_map = await self.store.get_events(current_state_ids_list)
        current_auth_events = {
            (e.type, e.state_key): e for e in auth_events_map.values()
        }

        try:
            event_auth.check(room_version_obj, event, auth_events=current_auth_events)
        except AuthError as e:
            logger.warning("Soft-failing %r because %s", event, e)
            event.internal_metadata.soft_failed = True

    async def on_query_auth(
        self, origin, event_id, room_id, remote_auth_chain, rejects, missing
    ):
        in_room = await self.auth.check_host_in_room(room_id, origin)
        if not in_room:
            raise AuthError(403, "Host not in room.")

        event = await self.store.get_event(event_id, check_room_id=room_id)

        # Just go through and process each event in `remote_auth_chain`. We
        # don't want to fall into the trap of `missing` being wrong.
        for e in remote_auth_chain:
            try:
                await self._handle_new_event(origin, e)
            except AuthError:
                pass

        # Now get the current auth_chain for the event.
        local_auth_chain = await self.store.get_auth_chain(
            list(event.auth_event_ids()), include_given=True
        )

        # TODO: Check if we would now reject event_id. If so we need to tell
        # everyone.

        ret = await self.construct_auth_difference(local_auth_chain, remote_auth_chain)

        logger.debug("on_query_auth returning: %s", ret)

        return ret

    async def on_get_missing_events(
        self, origin, room_id, earliest_events, latest_events, limit
    ):
        in_room = await self.auth.check_host_in_room(room_id, origin)
        if not in_room:
            raise AuthError(403, "Host not in room.")

        # Only allow up to 20 events to be retrieved per request.
        limit = min(limit, 20)

        missing_events = await self.store.get_missing_events(
            room_id=room_id,
            earliest_events=earliest_events,
            latest_events=latest_events,
            limit=limit,
        )

        missing_events = await filter_events_for_server(
            self.storage, origin, missing_events
        )

        return missing_events

    async def do_auth(
        self,
        origin: str,
        event: EventBase,
        context: EventContext,
        auth_events: MutableStateMap[EventBase],
    ) -> EventContext:
        """

        Args:
            origin:
            event:
            context:
            auth_events:
                Map from (event_type, state_key) to event

                Normally, our calculated auth_events based on the state of the room
                at the event's position in the DAG, though occasionally (eg if the
                event is an outlier), may be the auth events claimed by the remote
                server.

                Also NB that this function adds entries to it.
        Returns:
            updated context object
        """
        room_version = await self.store.get_room_version_id(event.room_id)
        room_version_obj = KNOWN_ROOM_VERSIONS[room_version]

        try:
            context = await self._update_auth_events_and_context_for_auth(
                origin, event, context, auth_events
            )
        except Exception:
            # We don't really mind if the above fails, so lets not fail
            # processing if it does. However, it really shouldn't fail so
            # let's still log as an exception since we'll still want to fix
            # any bugs.
            logger.exception(
                "Failed to double check auth events for %s with remote. "
                "Ignoring failure and continuing processing of event.",
                event.event_id,
            )

        try:
            event_auth.check(room_version_obj, event, auth_events=auth_events)
        except AuthError as e:
            logger.warning("Failed auth resolution for %r because %s", event, e)
            context.rejected = RejectedReason.AUTH_ERROR

        return context

    async def _update_auth_events_and_context_for_auth(
        self,
        origin: str,
        event: EventBase,
        context: EventContext,
        auth_events: MutableStateMap[EventBase],
    ) -> EventContext:
        """Helper for do_auth. See there for docs.

        Checks whether a given event has the expected auth events. If it
        doesn't then we talk to the remote server to compare state to see if
        we can come to a consensus (e.g. if one server missed some valid
        state).

        This attempts to resolve any potential divergence of state between
        servers, but is not essential and so failures should not block further
        processing of the event.

        Args:
            origin:
            event:
            context:

            auth_events:
                Map from (event_type, state_key) to event

                Normally, our calculated auth_events based on the state of the room
                at the event's position in the DAG, though occasionally (eg if the
                event is an outlier), may be the auth events claimed by the remote
                server.

                Also NB that this function adds entries to it.

        Returns:
            updated context
        """
        event_auth_events = set(event.auth_event_ids())

        # missing_auth is the set of the event's auth_events which we don't yet have
        # in auth_events.
        missing_auth = event_auth_events.difference(
            e.event_id for e in auth_events.values()
        )

        # if we have missing events, we need to fetch those events from somewhere.
        #
        # we start by checking if they are in the store, and then try calling /event_auth/.
        if missing_auth:
            have_events = await self.store.have_seen_events(missing_auth)
            logger.debug("Events %s are in the store", have_events)
            missing_auth.difference_update(have_events)

        if missing_auth:
            # If we don't have all the auth events, we need to get them.
            logger.info("auth_events contains unknown events: %s", missing_auth)
            try:
                try:
                    remote_auth_chain = await self.federation_client.get_event_auth(
                        origin, event.room_id, event.event_id
                    )
                except RequestSendFailed as e1:
                    # The other side isn't around or doesn't implement the
                    # endpoint, so lets just bail out.
                    logger.info("Failed to get event auth from remote: %s", e1)
                    return context

                seen_remotes = await self.store.have_seen_events(
                    [e.event_id for e in remote_auth_chain]
                )

                for e in remote_auth_chain:
                    if e.event_id in seen_remotes:
                        continue

                    if e.event_id == event.event_id:
                        continue

                    try:
                        auth_ids = e.auth_event_ids()
                        auth = {
                            (e.type, e.state_key): e
                            for e in remote_auth_chain
                            if e.event_id in auth_ids or e.type == EventTypes.Create
                        }
                        e.internal_metadata.outlier = True

                        logger.debug(
                            "do_auth %s missing_auth: %s", event.event_id, e.event_id
                        )
                        await self._handle_new_event(origin, e, auth_events=auth)

                        if e.event_id in event_auth_events:
                            auth_events[(e.type, e.state_key)] = e
                    except AuthError:
                        pass

            except Exception:
                logger.exception("Failed to get auth chain")

        if event.internal_metadata.is_outlier():
            # XXX: given that, for an outlier, we'll be working with the
            # event's *claimed* auth events rather than those we calculated:
            # (a) is there any point in this test, since different_auth below will
            # obviously be empty
            # (b) alternatively, why don't we do it earlier?
            logger.info("Skipping auth_event fetch for outlier")
            return context

        different_auth = event_auth_events.difference(
            e.event_id for e in auth_events.values()
        )

        if not different_auth:
            return context

        logger.info(
            "auth_events refers to events which are not in our calculated auth "
            "chain: %s",
            different_auth,
        )

        # XXX: currently this checks for redactions but I'm not convinced that is
        # necessary?
        different_events = await self.store.get_events_as_list(different_auth)

        for d in different_events:
            if d.room_id != event.room_id:
                logger.warning(
                    "Event %s refers to auth_event %s which is in a different room",
                    event.event_id,
                    d.event_id,
                )

                # don't attempt to resolve the claimed auth events against our own
                # in this case: just use our own auth events.
                #
                # XXX: should we reject the event in this case? It feels like we should,
                # but then shouldn't we also do so if we've failed to fetch any of the
                # auth events?
                return context

        # now we state-resolve between our own idea of the auth events, and the remote's
        # idea of them.

        local_state = auth_events.values()
        remote_auth_events = dict(auth_events)
        remote_auth_events.update({(d.type, d.state_key): d for d in different_events})
        remote_state = remote_auth_events.values()

        room_version = await self.store.get_room_version_id(event.room_id)
        new_state = await self.state_handler.resolve_events(
            room_version, (local_state, remote_state), event
        )

        logger.info(
            "After state res: updating auth_events with new state %s",
            {
                (d.type, d.state_key): d.event_id
                for d in new_state.values()
                if auth_events.get((d.type, d.state_key)) != d
            },
        )

        auth_events.update(new_state)

        context = await self._update_context_for_auth_events(
            event, context, auth_events
        )

        return context

    async def _update_context_for_auth_events(
        self, event: EventBase, context: EventContext, auth_events: StateMap[EventBase]
    ) -> EventContext:
        """Update the state_ids in an event context after auth event resolution,
        storing the changes as a new state group.

        Args:
            event: The event we're handling the context for

            context: initial event context

            auth_events: Events to update in the event context.

        Returns:
            new event context
        """
        # exclude the state key of the new event from the current_state in the context.
        if event.is_state():
            event_key = (event.type, event.state_key)  # type: Optional[Tuple[str, str]]
        else:
            event_key = None
        state_updates = {
            k: a.event_id for k, a in auth_events.items() if k != event_key
        }

        current_state_ids = await context.get_current_state_ids()
        current_state_ids = dict(current_state_ids)  # type: ignore

        current_state_ids.update(state_updates)

        prev_state_ids = await context.get_prev_state_ids()
        prev_state_ids = dict(prev_state_ids)

        prev_state_ids.update({k: a.event_id for k, a in auth_events.items()})

        # create a new state group as a delta from the existing one.
        prev_group = context.state_group
        state_group = await self.state_store.store_state_group(
            event.event_id,
            event.room_id,
            prev_group=prev_group,
            delta_ids=state_updates,
            current_state_ids=current_state_ids,
        )

        return EventContext.with_state(
            state_group=state_group,
            state_group_before_event=context.state_group_before_event,
            current_state_ids=current_state_ids,
            prev_state_ids=prev_state_ids,
            prev_group=prev_group,
            delta_ids=state_updates,
        )

    async def construct_auth_difference(
        self, local_auth: Iterable[EventBase], remote_auth: Iterable[EventBase]
    ) -> Dict:
        """ Given a local and remote auth chain, find the differences. This
        assumes that we have already processed all events in remote_auth

        Params:
            local_auth (list)
            remote_auth (list)

        Returns:
            dict
        """

        logger.debug("construct_auth_difference Start!")

        # TODO: Make sure we are OK with local_auth or remote_auth having more
        # auth events in them than strictly necessary.

        def sort_fun(ev):
            return ev.depth, ev.event_id

        logger.debug("construct_auth_difference after sort_fun!")

        # We find the differences by starting at the "bottom" of each list
        # and iterating up on both lists. The lists are ordered by depth and
        # then event_id, we iterate up both lists until we find the event ids
        # don't match. Then we look at depth/event_id to see which side is
        # missing that event, and iterate only up that list. Repeat.

        remote_list = list(remote_auth)
        remote_list.sort(key=sort_fun)

        local_list = list(local_auth)
        local_list.sort(key=sort_fun)

        local_iter = iter(local_list)
        remote_iter = iter(remote_list)

        logger.debug("construct_auth_difference before get_next!")

        def get_next(it, opt=None):
            try:
                return next(it)
            except Exception:
                return opt

        current_local = get_next(local_iter)
        current_remote = get_next(remote_iter)

        logger.debug("construct_auth_difference before while")

        missing_remotes = []
        missing_locals = []
        while current_local or current_remote:
            if current_remote is None:
                missing_locals.append(current_local)
                current_local = get_next(local_iter)
                continue

            if current_local is None:
                missing_remotes.append(current_remote)
                current_remote = get_next(remote_iter)
                continue

            if current_local.event_id == current_remote.event_id:
                current_local = get_next(local_iter)
                current_remote = get_next(remote_iter)
                continue

            if current_local.depth < current_remote.depth:
                missing_locals.append(current_local)
                current_local = get_next(local_iter)
                continue

            if current_local.depth > current_remote.depth:
                missing_remotes.append(current_remote)
                current_remote = get_next(remote_iter)
                continue

            # They have the same depth, so we fall back to the event_id order
            if current_local.event_id < current_remote.event_id:
                missing_locals.append(current_local)
                current_local = get_next(local_iter)

            if current_local.event_id > current_remote.event_id:
                missing_remotes.append(current_remote)
                current_remote = get_next(remote_iter)
                continue

        logger.debug("construct_auth_difference after while")

        # missing locals should be sent to the server
        # We should find why we are missing remotes, as they will have been
        # rejected.

        # Remove events from missing_remotes if they are referencing a missing
        # remote. We only care about the "root" rejected ones.
        missing_remote_ids = [e.event_id for e in missing_remotes]
        base_remote_rejected = list(missing_remotes)
        for e in missing_remotes:
            for e_id in e.auth_event_ids():
                if e_id in missing_remote_ids:
                    try:
                        base_remote_rejected.remove(e)
                    except ValueError:
                        pass

        reason_map = {}

        for e in base_remote_rejected:
            reason = await self.store.get_rejection_reason(e.event_id)
            if reason is None:
                # TODO: e is not in the current state, so we should
                # construct some proof of that.
                continue

            reason_map[e.event_id] = reason

        logger.debug("construct_auth_difference returning")

        return {
            "auth_chain": local_auth,
            "rejects": {
                e.event_id: {"reason": reason_map[e.event_id], "proof": None}
                for e in base_remote_rejected
            },
            "missing": [e.event_id for e in missing_locals],
        }

    @log_function
    async def exchange_third_party_invite(
        self, sender_user_id, target_user_id, room_id, signed
    ):
        third_party_invite = {"signed": signed}

        event_dict = {
            "type": EventTypes.Member,
            "content": {
                "membership": Membership.INVITE,
                "third_party_invite": third_party_invite,
            },
            "room_id": room_id,
            "sender": sender_user_id,
            "state_key": target_user_id,
        }

        if await self.auth.check_host_in_room(room_id, self.hs.hostname):
            room_version = await self.store.get_room_version_id(room_id)
            builder = self.event_builder_factory.new(room_version, event_dict)

            EventValidator().validate_builder(builder)
            event, context = await self.event_creation_handler.create_new_client_event(
                builder=builder
            )

            event_allowed = await self.third_party_event_rules.check_event_allowed(
                event, context
            )
            if not event_allowed:
                logger.info(
                    "Creation of threepid invite %s forbidden by third-party rules",
                    event,
                )
                raise SynapseError(
                    403, "This event is not allowed in this context", Codes.FORBIDDEN
                )

            event, context = await self.add_display_name_to_third_party_invite(
                room_version, event_dict, event, context
            )

            EventValidator().validate_new(event, self.config)

            # We need to tell the transaction queue to send this out, even
            # though the sender isn't a local user.
            event.internal_metadata.send_on_behalf_of = self.hs.hostname

            try:
                await self.auth.check_from_context(room_version, event, context)
            except AuthError as e:
                logger.warning("Denying new third party invite %r because %s", event, e)
                raise e

            await self._check_signature(event, context)

            # We retrieve the room member handler here as to not cause a cyclic dependency
            member_handler = self.hs.get_room_member_handler()
            await member_handler.send_membership_event(None, event, context)
        else:
            destinations = {x.split(":", 1)[-1] for x in (sender_user_id, room_id)}
            await self.federation_client.forward_third_party_invite(
                destinations, room_id, event_dict
            )

    async def on_exchange_third_party_invite_request(
        self, room_id: str, event_dict: JsonDict
    ) -> None:
        """Handle an exchange_third_party_invite request from a remote server

        The remote server will call this when it wants to turn a 3pid invite
        into a normal m.room.member invite.

        Args:
            room_id: The ID of the room.

            event_dict (dict[str, Any]): Dictionary containing the event body.

        """
        room_version = await self.store.get_room_version_id(room_id)

        # NB: event_dict has a particular specced format we might need to fudge
        # if we change event formats too much.
        builder = self.event_builder_factory.new(room_version, event_dict)

        event, context = await self.event_creation_handler.create_new_client_event(
            builder=builder
        )

        event_allowed = await self.third_party_event_rules.check_event_allowed(
            event, context
        )
        if not event_allowed:
            logger.warning(
                "Exchange of threepid invite %s forbidden by third-party rules", event
            )
            raise SynapseError(
                403, "This event is not allowed in this context", Codes.FORBIDDEN
            )

        event, context = await self.add_display_name_to_third_party_invite(
            room_version, event_dict, event, context
        )

        try:
            await self.auth.check_from_context(room_version, event, context)
        except AuthError as e:
            logger.warning("Denying third party invite %r because %s", event, e)
            raise e
        await self._check_signature(event, context)

        # We need to tell the transaction queue to send this out, even
        # though the sender isn't a local user.
        event.internal_metadata.send_on_behalf_of = get_domain_from_id(event.sender)

        # We retrieve the room member handler here as to not cause a cyclic dependency
        member_handler = self.hs.get_room_member_handler()
        await member_handler.send_membership_event(None, event, context)

    async def add_display_name_to_third_party_invite(
        self, room_version, event_dict, event, context
    ):
        key = (
            EventTypes.ThirdPartyInvite,
            event.content["third_party_invite"]["signed"]["token"],
        )
        original_invite = None
        prev_state_ids = await context.get_prev_state_ids()
        original_invite_id = prev_state_ids.get(key)
        if original_invite_id:
            original_invite = await self.store.get_event(
                original_invite_id, allow_none=True
            )
        if original_invite:
            # If the m.room.third_party_invite event's content is empty, it means the
            # invite has been revoked. In this case, we don't have to raise an error here
            # because the auth check will fail on the invite (because it's not able to
            # fetch public keys from the m.room.third_party_invite event's content, which
            # is empty).
            display_name = original_invite.content.get("display_name")
            event_dict["content"]["third_party_invite"]["display_name"] = display_name
        else:
            logger.info(
                "Could not find invite event for third_party_invite: %r", event_dict
            )
            # We don't discard here as this is not the appropriate place to do
            # auth checks. If we need the invite and don't have it then the
            # auth check code will explode appropriately.

        builder = self.event_builder_factory.new(room_version, event_dict)
        EventValidator().validate_builder(builder)
        event, context = await self.event_creation_handler.create_new_client_event(
            builder=builder
        )
        EventValidator().validate_new(event, self.config)
        return (event, context)

    async def _check_signature(self, event, context):
        """
        Checks that the signature in the event is consistent with its invite.

        Args:
            event (Event): The m.room.member event to check
            context (EventContext):

        Raises:
            AuthError: if signature didn't match any keys, or key has been
                revoked,
            SynapseError: if a transient error meant a key couldn't be checked
                for revocation.
        """
        signed = event.content["third_party_invite"]["signed"]
        token = signed["token"]

        prev_state_ids = await context.get_prev_state_ids()
        invite_event_id = prev_state_ids.get((EventTypes.ThirdPartyInvite, token))

        invite_event = None
        if invite_event_id:
            invite_event = await self.store.get_event(invite_event_id, allow_none=True)

        if not invite_event:
            raise AuthError(403, "Could not find invite")

        logger.debug("Checking auth on event %r", event.content)

        last_exception = None  # type: Optional[Exception]

        # for each public key in the 3pid invite event
        for public_key_object in self.hs.get_auth().get_public_keys(invite_event):
            try:
                # for each sig on the third_party_invite block of the actual invite
                for server, signature_block in signed["signatures"].items():
                    for key_name, encoded_signature in signature_block.items():
                        if not key_name.startswith("ed25519:"):
                            continue

                        logger.debug(
                            "Attempting to verify sig with key %s from %r "
                            "against pubkey %r",
                            key_name,
                            server,
                            public_key_object,
                        )

                        try:
                            public_key = public_key_object["public_key"]
                            verify_key = decode_verify_key_bytes(
                                key_name, decode_base64(public_key)
                            )
                            verify_signed_json(signed, server, verify_key)
                            logger.debug(
                                "Successfully verified sig with key %s from %r "
                                "against pubkey %r",
                                key_name,
                                server,
                                public_key_object,
                            )
                        except Exception:
                            logger.info(
                                "Failed to verify sig with key %s from %r "
                                "against pubkey %r",
                                key_name,
                                server,
                                public_key_object,
                            )
                            raise
                        try:
                            if "key_validity_url" in public_key_object:
                                await self._check_key_revocation(
                                    public_key, public_key_object["key_validity_url"]
                                )
                        except Exception:
                            logger.info(
                                "Failed to query key_validity_url %s",
                                public_key_object["key_validity_url"],
                            )
                            raise
                        return
            except Exception as e:
                last_exception = e

        if last_exception is None:
            # we can only get here if get_public_keys() returned an empty list
            # TODO: make this better
            raise RuntimeError("no public key in invite event")

        raise last_exception

    async def _check_key_revocation(self, public_key, url):
        """
        Checks whether public_key has been revoked.

        Args:
            public_key (str): base-64 encoded public key.
            url (str): Key revocation URL.

        Raises:
            AuthError: if they key has been revoked.
            SynapseError: if a transient error meant a key couldn't be checked
                for revocation.
        """
        try:
            response = await self.http_client.get_json(url, {"public_key": public_key})
        except Exception:
            raise SynapseError(502, "Third party certificate could not be checked")
        if "valid" not in response or not response["valid"]:
            raise AuthError(403, "Third party certificate was invalid")

    async def persist_events_and_notify(
        self,
        room_id: str,
        event_and_contexts: Sequence[Tuple[EventBase, EventContext]],
        backfilled: bool = False,
    ) -> int:
        """Persists events and tells the notifier/pushers about them, if
        necessary.

        Args:
            room_id: The room ID of events being persisted.
            event_and_contexts: Sequence of events with their associated
                context that should be persisted. All events must belong to
                the same room.
            backfilled: Whether these events are a result of
                backfilling or not
        """
        instance = self.config.worker.events_shard_config.get_instance(room_id)
        if instance != self._instance_name:
            result = await self._send_events(
                instance_name=instance,
                store=self.store,
                room_id=room_id,
                event_and_contexts=event_and_contexts,
                backfilled=backfilled,
            )
            return result["max_stream_id"]
        else:
            max_stream_token = await self.storage.persistence.persist_events(
                event_and_contexts, backfilled=backfilled
            )

            if self._ephemeral_messages_enabled:
                for (event, context) in event_and_contexts:
                    # If there's an expiry timestamp on the event, schedule its expiry.
                    self._message_handler.maybe_schedule_expiry(event)

            if not backfilled:  # Never notify for backfilled events
                for event, _ in event_and_contexts:
                    await self._notify_persisted_event(event, max_stream_token)

            return max_stream_token.stream

    async def _notify_persisted_event(
        self, event: EventBase, max_stream_token: RoomStreamToken
    ) -> None:
        """Checks to see if notifier/pushers should be notified about the
        event or not.

        Args:
            event:
            max_stream_id: The max_stream_id returned by persist_events
        """

        extra_users = []
        if event.type == EventTypes.Member:
            target_user_id = event.state_key

            # We notify for memberships if its an invite for one of our
            # users
            if event.internal_metadata.is_outlier():
                if event.membership != Membership.INVITE:
                    if not self.is_mine_id(target_user_id):
                        return

            target_user = UserID.from_string(target_user_id)
            extra_users.append(target_user)
        elif event.internal_metadata.is_outlier():
            return

        event_pos = PersistedEventPosition(
            self._instance_name, event.internal_metadata.stream_ordering
        )
        self.notifier.on_new_room_event(
            event, event_pos, max_stream_token, extra_users=extra_users
        )

    async def _clean_room_for_join(self, room_id: str) -> None:
        """Called to clean up any data in DB for a given room, ready for the
        server to join the room.

        Args:
            room_id
        """
        if self.config.worker_app:
            await self._clean_room_for_join_client(room_id)
        else:
            await self.store.clean_room_for_join(room_id)

    async def get_room_complexity(
        self, remote_room_hosts: List[str], room_id: str
    ) -> Optional[dict]:
        """
        Fetch the complexity of a remote room over federation.

        Args:
            remote_room_hosts (list[str]): The remote servers to ask.
            room_id (str): The room ID to ask about.

        Returns:
            Dict contains the complexity
            metric versions, while None means we could not fetch the complexity.
        """

        for host in remote_room_hosts:
            res = await self.federation_client.get_room_complexity(host, room_id)

            # We got a result, return it.
            if res:
                return res

        # We fell off the bottom, couldn't get the complexity from anyone. Oh
        # well.
        return None