From 3c8a116e1a36a911aa92965836444f4473044f66 Mon Sep 17 00:00:00 2001
From: Kegan Dougal <7190048+kegsay@users.noreply.github.com>
Date: Wed, 18 Sep 2024 17:25:50 +0100
Subject: [PATCH] Sliding Sync: bugfix: ensure we can sync with SSS even with
 missing rooms (#17727)

Fixes https://github.com/element-hq/element-x-ios/issues/3300

Some rooms are missing from `sliding_sync_joined_rooms`. When this
happens, the first call will succeed, but any subsequent calls for this
room ID will cause the cache to return `None` for the room ID, rather
than not having the key at all. This then causes the `<=` check to
throw.

Root cause: https://github.com/element-hq/synapse/issues/17726

### Pull Request Checklist

<!-- Please read
https://element-hq.github.io/synapse/latest/development/contributing_guide.html
before submitting your pull request -->

* [x] Pull request is based on the develop branch
* [ ] Pull request includes a [changelog
file](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#changelog).
The entry should:
- Be a short description of your change which makes sense to users.
"Fixed a bug that prevented receiving messages from other servers."
instead of "Moved X method from `EventStore` to `EventWorkerStore`.".
  - Use markdown where necessary, mostly for `code blocks`.
  - End with either a period (.) or an exclamation mark (!).
  - Start with a capital letter.
- Feel free to credit yourself, by adding a sentence "Contributed by
@github_username." or "Contributed by [Your Name]." to the end of the
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* [ ] [Code
style](https://element-hq.github.io/synapse/latest/code_style.html) is
correct
(run the
[linters](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#run-the-linters))
---
 changelog.d/17727.bugfix                 | 1 +
 synapse/storage/databases/main/stream.py | 6 +++++-
 2 files changed, 6 insertions(+), 1 deletion(-)
 create mode 100644 changelog.d/17727.bugfix

diff --git a/changelog.d/17727.bugfix b/changelog.d/17727.bugfix
new file mode 100644
index 0000000000..64c6e90d87
--- /dev/null
+++ b/changelog.d/17727.bugfix
@@ -0,0 +1 @@
+Fix a bug in SSS which could prevent /sync from working for certain user accounts.
diff --git a/synapse/storage/databases/main/stream.py b/synapse/storage/databases/main/stream.py
index 94a7efee73..03b4aa3381 100644
--- a/synapse/storage/databases/main/stream.py
+++ b/synapse/storage/databases/main/stream.py
@@ -1469,6 +1469,10 @@ class StreamWorkerStore(EventsWorkerStore, SQLBaseStore):
         recheck_rooms: Set[str] = set()
         min_token = end_token.stream
         for room_id, stream in uncapped_results.items():
+            if stream is None:
+                # Despite the function not directly setting None, the cache can!
+                # See: https://github.com/element-hq/synapse/issues/17726
+                continue
             if stream <= min_token:
                 results[room_id] = stream
             else:
@@ -1495,7 +1499,7 @@ class StreamWorkerStore(EventsWorkerStore, SQLBaseStore):
     @cachedList(cached_method_name="_get_max_event_pos", list_name="room_ids")
     async def _bulk_get_max_event_pos(
         self, room_ids: StrCollection
-    ) -> Mapping[str, int]:
+    ) -> Mapping[str, Optional[int]]:
         """Fetch the max position of a persisted event in the room."""
 
         # We need to be careful not to return positions ahead of the current