Sticky Events: don't insert them if they are spam_checker_spammy

synapse/storage/databases/main/sticky_events.py

@@ -211,10 +211,14 @@ class StickyEventsWorkerStore(StateGroupWorkerStore, CacheInvalidationWorkerStor
         Skips inserting events:
             - if they are considered spammy by the policy server;
               (unsure if correct, track: https://github.com/matrix-org/matrix-spec-proposals/pull/4354#discussion_r2727593350)
+            - if they are considered spammy by a Synapse spam checker module;
             - if they are rejected;
             - if they are outliers (they should be reconsidered for insertion when de-outliered); or
             - if they are not sticky (e.g. if the stickiness expired).
 
+        Note: Soft-failed sticky events ARE inserted, as their soft-failed status
+            could be re-evaluated later.
+
         Skipping the insertion of these types of 'invalid' events is useful for performance reasons because
         they would fill up the table yet we wouldn't show them to clients anyway.
 
@@ -230,7 +234,12 @@ class StickyEventsWorkerStore(StateGroupWorkerStore, CacheInvalidationWorkerStor
         sticky_events: list[tuple[EventBase, int]] = []
         for ev in events:
             # MSC: Note: policy servers and other similar antispam techniques still apply to these events.
-            if ev.internal_metadata.policy_server_spammy:
+            # We don't filter out soft-failed events altogether (in case they get re-evaluated later),
+            # so filter out `spam_checker_spammy` events specifically as we don't want to re-evaluate _those_ later.
+            if (
+                ev.internal_metadata.policy_server_spammy
+                or ev.internal_metadata.spam_checker_spammy
+            ):
                 continue
             # We shouldn't be passed rejected events, but if we do, we filter them out too.
             if ev.rejected_reason is not None: