Fix deduplicating of membership events to not create unused state groups. (#17164)

We try and deduplicate in two places: 1) really early on, and 2) just
before we persist the event. The first case was broken due to it
occuring before the profile information was added, and so it thought the
event contents were different.

The second case did catch it and handle it correctly, however doing so
creates a redundant state group leading to bloat.

Fixes #3791

4 files changed, 54 insertions, 35 deletions

diff --git a/changelog.d/17164.bugfix b/changelog.d/17164.bugfix
new file mode 100644
index 0000000000..597e2f14b0
--- /dev/null
+++ b/changelog.d/17164.bugfix
@@ -0,0 +1 @@
+Fix deduplicating of membership events to not create unused state groups.
diff --git a/synapse/handlers/message.py b/synapse/handlers/message.py
index ccaa5508ff..de5bd44a5f 100644
--- a/synapse/handlers/message.py
+++ b/synapse/handlers/message.py
@@ -496,13 +496,6 @@ class EventCreationHandler:
 
         self.room_prejoin_state_types = self.hs.config.api.room_prejoin_state
 
-        self.membership_types_to_include_profile_data_in = {
-            Membership.JOIN,
-            Membership.KNOCK,
-        }
-        if self.hs.config.server.include_profile_data_on_invite:
-            self.membership_types_to_include_profile_data_in.add(Membership.INVITE)
-
         self.send_event = ReplicationSendEventRestServlet.make_client(hs)
         self.send_events = ReplicationSendEventsRestServlet.make_client(hs)
 
@@ -594,8 +587,6 @@ class EventCreationHandler:
         Creates an FrozenEvent object, filling out auth_events, prev_events,
         etc.
 
-        Adds display names to Join membership events.
-
         Args:
             requester
             event_dict: An entire event
@@ -672,29 +663,6 @@ class EventCreationHandler:
 
         self.validator.validate_builder(builder)
 
-        if builder.type == EventTypes.Member:
-            membership = builder.content.get("membership", None)
-            target = UserID.from_string(builder.state_key)
-
-            if membership in self.membership_types_to_include_profile_data_in:
-                # If event doesn't include a display name, add one.
-                profile = self.profile_handler
-                content = builder.content
-
-                try:
-                    if "displayname" not in content:
-                        displayname = await profile.get_displayname(target)
-                        if displayname is not None:
-                            content["displayname"] = displayname
-                    if "avatar_url" not in content:
-                        avatar_url = await profile.get_avatar_url(target)
-                        if avatar_url is not None:
-                            content["avatar_url"] = avatar_url
-                except Exception as e:
-                    logger.info(
-                        "Failed to get profile information for %r: %s", target, e
-                    )
-
         is_exempt = await self._is_exempt_from_privacy_policy(builder, requester)
         if require_consent and not is_exempt:
             await self.assert_accepted_privacy_policy(requester)
diff --git a/synapse/handlers/room_member.py b/synapse/handlers/room_member.py
index 655c78e150..51b9772329 100644
--- a/synapse/handlers/room_member.py
+++ b/synapse/handlers/room_member.py
@@ -106,6 +106,13 @@ class RoomMemberHandler(metaclass=abc.ABCMeta):
         self.event_auth_handler = hs.get_event_auth_handler()
         self._worker_lock_handler = hs.get_worker_locks_handler()
 
+        self._membership_types_to_include_profile_data_in = {
+            Membership.JOIN,
+            Membership.KNOCK,
+        }
+        if self.hs.config.server.include_profile_data_on_invite:
+            self._membership_types_to_include_profile_data_in.add(Membership.INVITE)
+
         self.member_linearizer: Linearizer = Linearizer(name="member")
         self.member_as_limiter = Linearizer(max_count=10, name="member_as_limiter")
 
@@ -785,9 +792,8 @@ class RoomMemberHandler(metaclass=abc.ABCMeta):
         if (
             not self.allow_per_room_profiles and not is_requester_server_notices_user
         ) or requester.shadow_banned:
-            # Strip profile data, knowing that new profile data will be added to the
-            # event's content in event_creation_handler.create_event() using the target's
-            # global profile.
+            # Strip profile data, knowing that new profile data will be added to
+            # the event's content below using the target's global profile.
             content.pop("displayname", None)
             content.pop("avatar_url", None)
 
@@ -823,6 +829,29 @@ class RoomMemberHandler(metaclass=abc.ABCMeta):
         if action in ["kick", "unban"]:
             effective_membership_state = "leave"
 
+        if effective_membership_state not in Membership.LIST:
+            raise SynapseError(400, "Invalid membership key")
+
+        # Add profile data for joins etc, if no per-room profile.
+        if (
+            effective_membership_state
+            in self._membership_types_to_include_profile_data_in
+        ):
+            # If event doesn't include a display name, add one.
+            profile = self.profile_handler
+
+            try:
+                if "displayname" not in content:
+                    displayname = await profile.get_displayname(target)
+                    if displayname is not None:
+                        content["displayname"] = displayname
+                if "avatar_url" not in content:
+                    avatar_url = await profile.get_avatar_url(target)
+                    if avatar_url is not None:
+                        content["avatar_url"] = avatar_url
+            except Exception as e:
+                logger.info("Failed to get profile information for %r: %s", target, e)
+
         # if this is a join with a 3pid signature, we may need to turn a 3pid
         # invite into a normal invite before we can handle the join.
         if third_party_signed is not None:
diff --git a/tests/handlers/test_room_member.py b/tests/handlers/test_room_member.py
index df43ce581c..213a66ed1a 100644
--- a/tests/handlers/test_room_member.py
+++ b/tests/handlers/test_room_member.py
@@ -407,3 +407,24 @@ class RoomMemberMasterHandlerTestCase(HomeserverTestCase):
         self.assertFalse(
             self.get_success(self.store.did_forget(self.alice, self.room_id))
         )
+
+    def test_deduplicate_joins(self) -> None:
+        """
+        Test that calling /join multiple times does not store a new state group.
+        """
+
+        self.helper.join(self.room_id, user=self.bob, tok=self.bob_token)
+
+        sql = "SELECT COUNT(*) FROM state_groups WHERE room_id = ?"
+        rows = self.get_success(
+            self.store.db_pool.execute("test_deduplicate_joins", sql, self.room_id)
+        )
+        initial_count = rows[0][0]
+
+        self.helper.join(self.room_id, user=self.bob, tok=self.bob_token)
+        rows = self.get_success(
+            self.store.db_pool.execute("test_deduplicate_joins", sql, self.room_id)
+        )
+        new_count = rows[0][0]
+
+        self.assertEqual(initial_count, new_count)