Fix MSC3030 /timestamp_to_event returning outliers that it has no idea whether are near a gap or not (#14215)

Fix MSC3030 `/timestamp_to_event` endpoint returning `outliers` that it has no idea whether are near a gap or not (and therefore unable to determine whether it's actually the closest event). The reason Synapse doesn't know whether an `outlier` is next to a gap is because our gap checks rely on entries in the `event_edges`, `event_forward_extremeties`, and `event_backward_extremities` tables which is [not the case for `outliers`](2c63cdcc3f/docs/development/room-dag-concepts.md (outliers)).

Also fixes MSC3030 Complement `can_paginate_after_getting_remote_event_from_timestamp_to_event_endpoint` test flake.  Although this acted flakey in Complement, if `sync_partial_state` raced and beat us before `/timestamp_to_event`, then even if we retried the failing `/context` request it wouldn't work until we made this Synapse change. With this PR, Synapse will never return an `outlier` event so that test will always go and ask over federation.

Fix  https://github.com/matrix-org/synapse/issues/13944


### Why did this fail before? Why was it flakey?

Sleuthing the server logs on the [CI failure](5121449357 (step):5:5805), it looks like `hs2:/timestamp_to_event` found `$NP6-oU7mIFVyhtKfGvfrEQX949hQX-T-gvuauG6eurU` as an `outlier` event locally. Then when we went and asked for it via `/context`, since it's an `outlier`, it was filtered out of the results -> `You don't have permission to access that event.`

This is reproducible when `sync_partial_state` races and persists `$NP6-oU7mIFVyhtKfGvfrEQX949hQX-T-gvuauG6eurU` as an `outlier` before we evaluate `get_event_for_timestamp(...)`. To consistently reproduce locally, just add a delay at the [start of `get_event_for_timestamp(...)`](cb20b885cb/synapse/handlers/room.py (L1470-L1496)) so it always runs after `sync_partial_state` completes.

```py
from twisted.internet import task as twisted_task
d = twisted_task.deferLater(self.hs.get_reactor(), 3.5)
await d
```

In a run where it passes, on `hs2`, `get_event_for_timestamp(...)` finds a different event locally which is next to a gap and we request from a closer one from `hs1` which gets backfilled. And since the backfilled event is not an `outlier`, it's returned as expected during `/context`.

With this PR, Synapse will never return an `outlier` event so that test will always go and ask over federation.

synapse/storage/databases/main/events_worker.py

@@ -1971,12 +1971,17 @@ class EventsWorkerStore(SQLBaseStore):
 
         Args:
             room_id: room where the event lives
-            event_id: event to check
+            event: event to check (can't be an `outlier`)
 
         Returns:
             Boolean indicating whether it's an extremity
         """
 
+        assert not event.internal_metadata.is_outlier(), (
+            "is_event_next_to_backward_gap(...) can't be used with `outlier` events. "
+            "This function relies on `event_backward_extremities` which won't be filled in for `outliers`."
+        )
+
         def is_event_next_to_backward_gap_txn(txn: LoggingTransaction) -> bool:
             # If the event in question has any of its prev_events listed as a
             # backward extremity, it's next to a gap.
@@ -2026,12 +2031,17 @@ class EventsWorkerStore(SQLBaseStore):
 
         Args:
             room_id: room where the event lives
-            event_id: event to check
+            event: event to check (can't be an `outlier`)
 
         Returns:
             Boolean indicating whether it's an extremity
         """
 
+        assert not event.internal_metadata.is_outlier(), (
+            "is_event_next_to_forward_gap(...) can't be used with `outlier` events. "
+            "This function relies on `event_edges` and `event_forward_extremities` which won't be filled in for `outliers`."
+        )
+
         def is_event_next_to_gap_txn(txn: LoggingTransaction) -> bool:
             # If the event in question is a forward extremity, we will just
             # consider any potential forward gap as not a gap since it's one of
@@ -2112,13 +2122,33 @@ class EventsWorkerStore(SQLBaseStore):
             The closest event_id otherwise None if we can't find any event in
             the given direction.
         """
+        if direction == "b":
+            # Find closest event *before* a given timestamp. We use descending
+            # (which gives values largest to smallest) because we want the
+            # largest possible timestamp *before* the given timestamp.
+            comparison_operator = "<="
+            order = "DESC"
+        else:
+            # Find closest event *after* a given timestamp. We use ascending
+            # (which gives values smallest to largest) because we want the
+            # closest possible timestamp *after* the given timestamp.
+            comparison_operator = ">="
+            order = "ASC"
 
-        sql_template = """
+        sql_template = f"""
             SELECT event_id FROM events
             LEFT JOIN rejections USING (event_id)
             WHERE
-                origin_server_ts %s ?
-                AND room_id = ?
+                room_id = ?
+                AND origin_server_ts {comparison_operator} ?
+                /**
+                 * Make sure the event isn't an `outlier` because we have no way
+                 * to later check whether it's next to a gap. `outliers` do not
+                 * have entries in the `event_edges`, `event_forward_extremeties`,
+                 * and `event_backward_extremities` tables to check against
+                 * (used by `is_event_next_to_backward_gap` and `is_event_next_to_forward_gap`).
+                 */
+                AND NOT outlier
                 /* Make sure event is not rejected */
                 AND rejections.event_id IS NULL
             /**
@@ -2128,27 +2158,14 @@ class EventsWorkerStore(SQLBaseStore):
              * Finally, we can tie-break based on when it was received on the server
              * (`stream_ordering`).
              */
-            ORDER BY origin_server_ts %s, depth %s, stream_ordering %s
+            ORDER BY origin_server_ts {order}, depth {order}, stream_ordering {order}
             LIMIT 1;
         """
 
         def get_event_id_for_timestamp_txn(txn: LoggingTransaction) -> Optional[str]:
-            if direction == "b":
-                # Find closest event *before* a given timestamp. We use descending
-                # (which gives values largest to smallest) because we want the
-                # largest possible timestamp *before* the given timestamp.
-                comparison_operator = "<="
-                order = "DESC"
-            else:
-                # Find closest event *after* a given timestamp. We use ascending
-                # (which gives values smallest to largest) because we want the
-                # closest possible timestamp *after* the given timestamp.
-                comparison_operator = ">="
-                order = "ASC"
-
             txn.execute(
-                sql_template % (comparison_operator, order, order, order),
-                (timestamp, room_id),
+                sql_template,
+                (room_id, timestamp),
             )
             row = txn.fetchone()
             if row: