Handle missing previous read marker event

changelog.d/15464.bugfix

@@ -0,0 +1 @@
+Don't fail moving read marker if current set to a missing event. Contributed by Nick @ Beeper (@fizzadar).

synapse/handlers/read_marker.py

@@ -15,6 +15,7 @@
 import logging
 from typing import TYPE_CHECKING
 
+from synapse.api.errors import SynapseError
 from synapse.util.async_helpers import Linearizer
 
 if TYPE_CHECKING:
@@ -46,12 +47,21 @@ async def received_client_read_marker(
             )
 
             should_update = True
+            # Get event ordering, this also ensures we know about the event
+            event_ordering = await self.store.get_event_ordering(event_id)
 
             if existing_read_marker:
-                # Only update if the new marker is ahead in the stream
-                should_update = await self.store.is_event_after(
-                    event_id, existing_read_marker["event_id"]
-                )
+                try:
+                    old_event_ordering = await self.store.get_event_ordering(
+                        existing_read_marker["event_id"]
+                    )
+                except SynapseError:
+                    # Old event no longer exists, assume new is ahead. This may
+                    # happen if the old event was removed due to retention.
+                    pass
+                else:
+                    # Only update if the new marker is ahead in the stream
+                    should_update = event_ordering > old_event_ordering
 
             if should_update:
                 content = {"event_id": event_id}

synapse/storage/databases/main/events_worker.py

@@ -1976,12 +1976,6 @@ def get_deltas_for_stream_id_txn(
 
         return rows, to_token, True
 
-    async def is_event_after(self, event_id1: str, event_id2: str) -> bool:
-        """Returns True if event_id1 is after event_id2 in the stream"""
-        to_1, so_1 = await self.get_event_ordering(event_id1)
-        to_2, so_2 = await self.get_event_ordering(event_id2)
-        return (to_1, so_1) > (to_2, so_2)
-
     @cached(max_entries=5000)
     async def get_event_ordering(self, event_id: str) -> Tuple[int, int]:
         res = await self.db_pool.simple_select_one(