Upon deactivation, forget all of the user's rooms (#17400)

This can help ensure that the rooms are eventually purged if the other
local users also forget them. Synapse already clears some of the room
information as part of the `_background_remove_left_rooms` background
task, but this doesn't catch `events`, `event_json`, etc.

changelog.d/17400.feature

@@ -0,0 +1 @@
+Forget all of a user's rooms upon deactivation, enabling future purges.

synapse/handlers/deactivate_account.py

@@ -283,6 +283,10 @@ async def _part_user(self, user_id: str) -> None:
                     ratelimit=False,
                     require_consent=False,
                 )
+
+                # Mark the room forgotten too, because they won't be able to do this
+                # for us. This may lead to the room being purged eventually.
+                await self._room_member_handler.forget(user, room_id)
             except Exception:
                 logger.exception(
                     "Failed to part user %r from room %r: ignoring and continuing",

tests/handlers/test_deactivate_account.py

@@ -461,3 +461,25 @@ def test_membership_is_redacted_upon_deactivation(self) -> None:
         # Validate that there is no displayname in any of the events
         for event in events:
             self.assertTrue("displayname" not in event.content)
+
+    def test_rooms_forgotten_upon_deactivation(self) -> None:
+        """
+        Tests that the user 'forgets' the rooms they left upon deactivation.
+        """
+        # Create a room
+        room_id = self.helper.create_room_as(
+            self.user,
+            is_public=True,
+            tok=self.token,
+        )
+
+        # Deactivate the account
+        self._deactivate_my_account()
+
+        # Get all of the user's forgotten rooms
+        forgotten_rooms = self.get_success(
+            self._store.get_forgotten_rooms_for_user(self.user)
+        )
+
+        # Validate that the created room is forgotten
+        self.assertTrue(room_id in forgotten_rooms)