Fix AssertionErrors after purging events (#11642)

* Fix AssertionErrors after purging events

If you purged a bunch of events from your database, and then restarted synapse
without receiving more events, then you would get a bunch of AssertionErrors on
restart.

This fixes the situation by rewinding the stream processors.

* `check-newsfragment`: ignore deleted newsfiles

1 files changed, 11 insertions, 0 deletions

diff --git a/synapse/handlers/stats.py b/synapse/handlers/stats.py

index bd3e6f2ec7..29e41a4c79 100644
--- a/synapse/handlers/stats.py
+++ b/synapse/handlers/stats.py
@@ -80,6 +80,17 @@ class StatsHandler:
         # If self.pos is None then means we haven't fetched it from DB
         if self.pos is None:
             self.pos = await self.store.get_stats_positions()
+            room_max_stream_ordering = self.store.get_room_max_stream_ordering()
+            if self.pos > room_max_stream_ordering:
+                # apparently, we've processed more events than exist in the database!
+                # this can happen if events are removed with history purge or similar.
+                logger.warning(
+                    "Event stream ordering appears to have gone backwards (%i -> %i): "
+                    "rewinding stats processor",
+                    self.pos,
+                    room_max_stream_ordering,
+                )
+                self.pos = room_max_stream_ordering
 
         # Loop round handling deltas until we're up to date