Prevent multiple upgrades on the same room at once (#5051)

Closes #4583

Does slightly less than #5045, which prevented a room from being upgraded multiple times, one after another. This PR still allows that, but just prevents two from happening at the same time.

Mostly just to mitigate the fact that servers are slow and it can take a moment for the room upgrade to actually complete. We don't want people sending another request to upgrade the room when really they just thought the first didn't go through.

changelog.d/5051.bugfix

@@ -0,0 +1 @@
+Prevent >1 room upgrades happening simultaneously on the same room.

synapse/handlers/room.py

@@ -32,6 +32,7 @@ from synapse.storage.state import StateFilter
 from synapse.types import RoomAlias, RoomID, RoomStreamToken, StreamToken, UserID
 from synapse.util import stringutils
 from synapse.util.async_helpers import Linearizer
+from synapse.util.caches.response_cache import ResponseCache
 from synapse.visibility import filter_events_for_client
 
 from ._base import BaseHandler
@@ -40,6 +41,8 @@ logger = logging.getLogger(__name__)
 
 id_server_scheme = "https://"
 
+FIVE_MINUTES_IN_MS = 5 * 60 * 1000
+
 
 class RoomCreationHandler(BaseHandler):
 
@@ -75,6 +78,12 @@ class RoomCreationHandler(BaseHandler):
         # linearizer to stop two upgrades happening at once
         self._upgrade_linearizer = Linearizer("room_upgrade_linearizer")
 
+        # If a user tries to update the same room multiple times in quick
+        # succession, only process the first attempt and return its result to
+        # subsequent requests
+        self._upgrade_response_cache = ResponseCache(
+            hs, "room_upgrade", timeout_ms=FIVE_MINUTES_IN_MS
+        )
         self._server_notices_mxid = hs.config.server_notices_mxid
 
         self.third_party_event_rules = hs.get_third_party_event_rules()
@@ -95,7 +104,36 @@ class RoomCreationHandler(BaseHandler):
 
         user_id = requester.user.to_string()
 
-        with (yield self._upgrade_linearizer.queue(old_room_id)):
+        # Check if this room is already being upgraded by another person
+        for key in self._upgrade_response_cache.pending_result_cache:
+            if key[0] == old_room_id and key[1] != user_id:
+                # Two different people are trying to upgrade the same room.
+                # Send the second an error.
+                #
+                # Note that this of course only gets caught if both users are
+                # on the same homeserver.
+                raise SynapseError(
+                    400, "An upgrade for this room is currently in progress"
+                )
+
+        # Upgrade the room
+        #
+        # If this user has sent multiple upgrade requests for the same room
+        # and one of them is not complete yet, cache the response and
+        # return it to all subsequent requests
+        ret = yield self._upgrade_response_cache.wrap(
+            (old_room_id, user_id),
+            self._upgrade_room,
+            requester,
+            old_room_id,
+            new_version,  # args for _upgrade_room
+        )
+        defer.returnValue(ret)
+
+    @defer.inlineCallbacks
+    def _upgrade_room(self, requester, old_room_id, new_version):
+        user_id = requester.user.to_string()
+
         # start by allocating a new room id
         r = yield self.store.get_room(old_room_id)
         if r is None:

synapse/util/caches/response_cache.py

@@ -137,7 +137,7 @@ class ResponseCache(object):
 
             *args: positional parameters to pass to the callback, if it is used
 
-            **kwargs: named paramters to pass to the callback, if it is used
+            **kwargs: named parameters to pass to the callback, if it is used
 
         Returns:
             twisted.internet.defer.Deferred: yieldable result