Extend StreamChangeCache to support multiple entities per stream ID (#7303)

First some background: StreamChangeCache is used to keep track of what "entities" have 
changed since a given stream ID. So for example, we might use it to keep track of when the last
to-device message for a given user was received [1], and hence whether we need to pull any to-device messages from the database on a sync [2].

Now, it turns out that StreamChangeCache didn't support more than one thing being changed at
a given stream_id (this was part of the problem with #7206). However, it's entirely valid to send
to-device messages to more than one user at a time.

As it turns out, this did in fact work, because *some* methods of StreamChangeCache coped
ok with having multiple things changing on the same stream ID, and it seems we never actually
use the methods which don't work on the stream change caches where we allow multiple
changes at the same stream ID. But that feels horribly fragile, hence: let's update
StreamChangeCache to properly support this, and add some typing and some more tests while
we're at it.

[1]: https://github.com/matrix-org/synapse/blob/release-v1.12.3/synapse/storage/data_stores/main/deviceinbox.py#L301
[2]: https://github.com/matrix-org/synapse/blob/release-v1.12.3/synapse/storage/data_stores/main/deviceinbox.py#L47-L51

1 files changed, 60 insertions, 9 deletions

diff --git a/tests/util/test_stream_change_cache.py b/tests/util/test_stream_change_cache.py

index 72a9de5370..6857933540 100644
--- a/tests/util/test_stream_change_cache.py
+++ b/tests/util/test_stream_change_cache.py
@@ -28,18 +28,26 @@ class StreamChangeCacheTests(unittest.TestCase):
         cache.entity_has_changed("user@foo.com", 6)
         cache.entity_has_changed("bar@baz.net", 7)
 
+        # also test multiple things changing on the same stream ID
+        cache.entity_has_changed("user2@foo.com", 8)
+        cache.entity_has_changed("bar2@baz.net", 8)
+
         # If it's been changed after that stream position, return True
         self.assertTrue(cache.has_entity_changed("user@foo.com", 4))
         self.assertTrue(cache.has_entity_changed("bar@baz.net", 4))
+        self.assertTrue(cache.has_entity_changed("bar2@baz.net", 4))
+        self.assertTrue(cache.has_entity_changed("user2@foo.com", 4))
 
         # If it's been changed at that stream position, return False
         self.assertFalse(cache.has_entity_changed("user@foo.com", 6))
+        self.assertFalse(cache.has_entity_changed("user2@foo.com", 8))
 
         # If there's no changes after that stream position, return False
         self.assertFalse(cache.has_entity_changed("user@foo.com", 7))
+        self.assertFalse(cache.has_entity_changed("user2@foo.com", 9))
 
         # If the entity does not exist, return False.
-        self.assertFalse(cache.has_entity_changed("not@here.website", 7))
+        self.assertFalse(cache.has_entity_changed("not@here.website", 9))
 
         # If we request before the stream cache's earliest known position,
         # return True, whether it's a known entity or not.
@@ -47,7 +55,7 @@ class StreamChangeCacheTests(unittest.TestCase):
         self.assertTrue(cache.has_entity_changed("not@here.website", 0))
 
     @patch("synapse.util.caches.CACHE_SIZE_FACTOR", 1.0)
-    def test_has_entity_changed_pops_off_start(self):
+    def test_entity_has_changed_pops_off_start(self):
         """
         StreamChangeCache.entity_has_changed will respect the max size and
         purge the oldest items upon reaching that max size.
@@ -64,11 +72,20 @@ class StreamChangeCacheTests(unittest.TestCase):
         # The oldest item has been popped off
         self.assertTrue("user@foo.com" not in cache._entity_to_key)
 
+        self.assertEqual(
+            cache.get_all_entities_changed(2), ["bar@baz.net", "user@elsewhere.org"],
+        )
+        self.assertIsNone(cache.get_all_entities_changed(1))
+
         # If we update an existing entity, it keeps the two existing entities
         cache.entity_has_changed("bar@baz.net", 5)
         self.assertEqual(
             {"bar@baz.net", "user@elsewhere.org"}, set(cache._entity_to_key)
         )
+        self.assertEqual(
+            cache.get_all_entities_changed(2), ["user@elsewhere.org", "bar@baz.net"],
+        )
+        self.assertIsNone(cache.get_all_entities_changed(1))
 
     def test_get_all_entities_changed(self):
         """
@@ -80,18 +97,52 @@ class StreamChangeCacheTests(unittest.TestCase):
 
         cache.entity_has_changed("user@foo.com", 2)
         cache.entity_has_changed("bar@baz.net", 3)
+        cache.entity_has_changed("anotheruser@foo.com", 3)
         cache.entity_has_changed("user@elsewhere.org", 4)
 
-        self.assertEqual(
-            cache.get_all_entities_changed(1),
-            ["user@foo.com", "bar@baz.net", "user@elsewhere.org"],
-        )
-        self.assertEqual(
-            cache.get_all_entities_changed(2), ["bar@baz.net", "user@elsewhere.org"]
-        )
+        r = cache.get_all_entities_changed(1)
+
+        # either of these are valid
+        ok1 = [
+            "user@foo.com",
+            "bar@baz.net",
+            "anotheruser@foo.com",
+            "user@elsewhere.org",
+        ]
+        ok2 = [
+            "user@foo.com",
+            "anotheruser@foo.com",
+            "bar@baz.net",
+            "user@elsewhere.org",
+        ]
+        self.assertTrue(r == ok1 or r == ok2)
+
+        r = cache.get_all_entities_changed(2)
+        self.assertTrue(r == ok1[1:] or r == ok2[1:])
+
         self.assertEqual(cache.get_all_entities_changed(3), ["user@elsewhere.org"])
         self.assertEqual(cache.get_all_entities_changed(0), None)
 
+        # ... later, things gest more updates
+        cache.entity_has_changed("user@foo.com", 5)
+        cache.entity_has_changed("bar@baz.net", 5)
+        cache.entity_has_changed("anotheruser@foo.com", 6)
+
+        ok1 = [
+            "user@elsewhere.org",
+            "user@foo.com",
+            "bar@baz.net",
+            "anotheruser@foo.com",
+        ]
+        ok2 = [
+            "user@elsewhere.org",
+            "bar@baz.net",
+            "user@foo.com",
+            "anotheruser@foo.com",
+        ]
+        r = cache.get_all_entities_changed(3)
+        self.assertTrue(r == ok1 or r == ok2)
+
     def test_has_any_entity_changed(self):
         """
         StreamChangeCache.has_any_entity_changed will return True if any