Prevent multiple upgrades on the same room at once

changelog.d/5051.bugfix

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

synapse/handlers/room.py

@@ -70,6 +70,7 @@ def __init__(self, hs):
         self.spam_checker = hs.get_spam_checker()
         self.event_creation_handler = hs.get_event_creation_handler()
         self.room_member_handler = hs.get_room_member_handler()
+        self.currently_upgrading_rooms = {}
 
         # linearizer to stop two upgrades happening at once
         self._upgrade_linearizer = Linearizer("room_upgrade_linearizer")
@@ -90,6 +91,12 @@ def upgrade_room(self, requester, old_room_id, new_version):
 
         user_id = requester.user.to_string()
 
+        if old_room_id in self.currently_upgrading_rooms:
+            raise SynapseError(
+                400, "An upgrade for this room is currently in progress."
+            )
+        self.currently_upgrading_rooms[old_room_id] = True
+
         with (yield self._upgrade_linearizer.queue(old_room_id)):
             # start by allocating a new room id
             r = yield self.store.get_room(old_room_id)
@@ -149,6 +156,8 @@ def upgrade_room(self, requester, old_room_id, new_version):
                 requester, old_room_id, new_room_id, old_room_state,
             )
 
+            del self.currently_upgrading_rooms[old_room_id]
+
             defer.returnValue(new_room_id)
 
     @defer.inlineCallbacks