Fix room upgrades creating an empty room when auth fails (#12696)

Signed-off-by: Sean Quah <seanq@element.io>

Author: squahtx

changelog.d/12696.bugfix

@@ -0,0 +1 @@
+Fix a long-standing bug where an empty room would be created when a user with an insufficient power level tried to upgrade a room.

synapse/handlers/room.py

@@ -33,6 +33,7 @@
 import attr
 from typing_extensions import TypedDict
 
+import synapse.events.snapshot
 from synapse.api.constants import (
     EventContentFields,
     EventTypes,
@@ -77,7 +78,6 @@
     create_requester,
 )
 from synapse.util import stringutils
-from synapse.util.async_helpers import Linearizer
 from synapse.util.caches.response_cache import ResponseCache
 from synapse.util.stringutils import parse_and_validate_server_name
 from synapse.visibility import filter_events_for_client
@@ -155,9 +155,6 @@ def __init__(self, hs: "HomeServer"):
 
         self._replication = hs.get_replication_data_handler()
 
-        # 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
@@ -200,6 +197,39 @@ async def upgrade_room(
                     400, "An upgrade for this room is currently in progress"
                 )
 
+        # Check whether the room exists and 404 if it doesn't.
+        # We could go straight for the auth check, but that will raise a 403 instead.
+        old_room = await self.store.get_room(old_room_id)
+        if old_room is None:
+            raise NotFoundError("Unknown room id %s" % (old_room_id,))
+
+        new_room_id = self._generate_room_id()
+
+        # Check whether the user has the power level to carry out the upgrade.
+        # `check_auth_rules_from_context` will check that they are in the room and have
+        # the required power level to send the tombstone event.
+        (
+            tombstone_event,
+            tombstone_context,
+        ) = await self.event_creation_handler.create_event(
+            requester,
+            {
+                "type": EventTypes.Tombstone,
+                "state_key": "",
+                "room_id": old_room_id,
+                "sender": user_id,
+                "content": {
+                    "body": "This room has been replaced",
+                    "replacement_room": new_room_id,
+                },
+            },
+        )
+        old_room_version = await self.store.get_room_version(old_room_id)
+        validate_event_for_room_version(old_room_version, tombstone_event)
+        await self._event_auth_handler.check_auth_rules_from_context(
+            old_room_version, tombstone_event, tombstone_context
+        )
+
         # Upgrade the room
         #
         # If this user has sent multiple upgrade requests for the same room
@@ -210,60 +240,51 @@ async def upgrade_room(
             self._upgrade_room,
             requester,
             old_room_id,
-            new_version,  # args for _upgrade_room
+            old_room,  # args for _upgrade_room
+            new_room_id,
+            new_version,
+            tombstone_event,
+            tombstone_context,
         )
 
         return ret
 
     async def _upgrade_room(
-        self, requester: Requester, old_room_id: str, new_version: RoomVersion
+        self,
+        requester: Requester,
+        old_room_id: str,
+        old_room: Dict[str, Any],
+        new_room_id: str,
+        new_version: RoomVersion,
+        tombstone_event: EventBase,
+        tombstone_context: synapse.events.snapshot.EventContext,
     ) -> str:
         """
         Args:
             requester: the user requesting the upgrade
             old_room_id: the id of the room to be replaced
-            new_versions: the version to upgrade the room to
+            old_room: a dict containing room information for the room to be replaced,
+                as returned by `RoomWorkerStore.get_room`.
+            new_room_id: the id of the replacement room
+            new_version: the version to upgrade the room to
+            tombstone_event: the tombstone event to send to the old room
+            tombstone_context: the context for the tombstone event
 
         Raises:
             ShadowBanError if the requester is shadow-banned.
         """
         user_id = requester.user.to_string()
         assert self.hs.is_mine_id(user_id), "User must be our own: %s" % (user_id,)
 
-        # start by allocating a new room id
-        r = await self.store.get_room(old_room_id)
-        if r is None:
-            raise NotFoundError("Unknown room id %s" % (old_room_id,))
-        new_room_id = await self._generate_room_id(
-            creator_id=user_id,
-            is_public=r["is_public"],
-            room_version=new_version,
-        )
-
         logger.info("Creating new room %s to replace %s", new_room_id, old_room_id)
 
-        # we create and auth the tombstone event before properly creating the new
-        # room, to check our user has perms in the old room.
-        (
-            tombstone_event,
-            tombstone_context,
-        ) = await self.event_creation_handler.create_event(
-            requester,
-            {
-                "type": EventTypes.Tombstone,
-                "state_key": "",
-                "room_id": old_room_id,
-                "sender": user_id,
-                "content": {
-                    "body": "This room has been replaced",
-                    "replacement_room": new_room_id,
-                },
-            },
-        )
-        old_room_version = await self.store.get_room_version(old_room_id)
-        validate_event_for_room_version(old_room_version, tombstone_event)
-        await self._event_auth_handler.check_auth_rules_from_context(
-            old_room_version, tombstone_event, tombstone_context
+        # create the new room. may raise a `StoreError` in the exceedingly unlikely
+        # event of a room ID collision.
+        await self.store.store_room(
+            room_id=new_room_id,
+            room_creator_user_id=user_id,
+            is_public=old_room["is_public"],
+            room_version=new_version,
         )
 
         await self.clone_existing_room(
@@ -782,7 +803,7 @@ async def create_room(
         visibility = config.get("visibility", "private")
         is_public = visibility == "public"
 
-        room_id = await self._generate_room_id(
+        room_id = await self._generate_and_create_room_id(
             creator_id=user_id,
             is_public=is_public,
             room_version=room_version,
@@ -1104,7 +1125,26 @@ async def send(etype: str, content: JsonDict, **kwargs: Any) -> int:
 
         return last_sent_stream_id
 
-    async def _generate_room_id(
+    def _generate_room_id(self) -> str:
+        """Generates a random room ID.
+
+        Room IDs look like "!opaque_id:domain" and are case-sensitive as per the spec
+        at https://spec.matrix.org/v1.2/appendices/#room-ids-and-event-ids.
+
+        Does not check for collisions with existing rooms or prevent future calls from
+        returning the same room ID. To ensure the uniqueness of a new room ID, use
+        `_generate_and_create_room_id` instead.
+
+        Synapse's room IDs are 18 [a-zA-Z] characters long, which comes out to around
+        102 bits.
+
+        Returns:
+            A random room ID of the form "!opaque_id:domain".
+        """
+        random_string = stringutils.random_string(18)
+        return RoomID(random_string, self.hs.hostname).to_string()
+
+    async def _generate_and_create_room_id(
         self,
         creator_id: str,
         is_public: bool,
@@ -1115,8 +1155,7 @@ async def _generate_room_id(
         attempts = 0
         while attempts < 5:
             try:
-                random_string = stringutils.random_string(18)
-                gen_room_id = RoomID(random_string, self.hs.hostname).to_string()
+                gen_room_id = self._generate_room_id()
                 await self.store.store_room(
                     room_id=gen_room_id,
                     room_creator_user_id=creator_id,

tests/replication/test_sharded_event_persister.py

@@ -14,7 +14,6 @@
 import logging
 from unittest.mock import patch
 
-from synapse.api.room_versions import RoomVersion
 from synapse.rest import admin
 from synapse.rest.client import login, room, sync
 from synapse.storage.util.id_generators import MultiWriterIdGenerator
@@ -64,21 +63,10 @@ def _create_room(self, room_id: str, user_id: str, tok: str):
 
         # We control the room ID generation by patching out the
         # `_generate_room_id` method
-        async def generate_room(
-            creator_id: str, is_public: bool, room_version: RoomVersion
-        ):
-            await self.store.store_room(
-                room_id=room_id,
-                room_creator_user_id=creator_id,
-                is_public=is_public,
-                room_version=room_version,
-            )
-            return room_id
-
         with patch(
             "synapse.handlers.room.RoomCreationHandler._generate_room_id"
         ) as mock:
-            mock.side_effect = generate_room
+            mock.side_effect = lambda: room_id
             self.helper.create_room_as(user_id, tok=tok)
 
     def test_basic(self):