3 files changed, 25 insertions, 13 deletions
diff --git a/changelog.d/8559.misc b/changelog.d/8559.misc
new file mode 100644
index 0000000000..d7bd00964e
--- /dev/null
+++ b/changelog.d/8559.misc
@@ -0,0 +1 @@
+Optimise `/createRoom` with multiple invited users.
diff --git a/synapse/handlers/room.py b/synapse/handlers/room.py
index c5b1f1f1e1..e73031475f 100644
--- a/synapse/handlers/room.py
+++ b/synapse/handlers/room.py
@@ -771,22 +771,29 @@ class RoomCreationHandler(BaseHandler):
                 ratelimit=False,
             )
 
-        for invitee in invite_list:
+        # we avoid dropping the lock between invites, as otherwise joins can
+        # start coming in and making the createRoom slow.
+        #
+        # we also don't need to check the requester's shadow-ban here, as we
+        # have already done so above (and potentially emptied invite_list).
+        with (await self.room_member_handler.member_linearizer.queue((room_id,))):
             content = {}
             is_direct = config.get("is_direct", None)
             if is_direct:
                 content["is_direct"] = is_direct
 
-            # Note that update_membership with an action of "invite" can raise a
-            # ShadowBanError, but this was handled above by emptying invite_list.
-            _, last_stream_id = await self.room_member_handler.update_membership(
-                requester,
-                UserID.from_string(invitee),
-                room_id,
-                "invite",
-                ratelimit=False,
-                content=content,
-            )
+            for invitee in invite_list:
+                (
+                    _,
+                    last_stream_id,
+                ) = await self.room_member_handler.update_membership_locked(
+                    requester,
+                    UserID.from_string(invitee),
+                    room_id,
+                    "invite",
+                    ratelimit=False,
+                    content=content,
+                )
 
         for invite_3pid in invite_3pid_list:
             id_server = invite_3pid["id_server"]
diff --git a/synapse/handlers/room_member.py b/synapse/handlers/room_member.py
index ec784030e9..7cd858b7db 100644
--- a/synapse/handlers/room_member.py
+++ b/synapse/handlers/room_member.py
@@ -307,7 +307,7 @@ class RoomMemberHandler(metaclass=abc.ABCMeta):
         key = (room_id,)
 
         with (await self.member_linearizer.queue(key)):
-            result = await self._update_membership(
+            result = await self.update_membership_locked(
                 requester,
                 target,
                 room_id,
@@ -322,7 +322,7 @@ class RoomMemberHandler(metaclass=abc.ABCMeta):
 
         return result
 
-    async def _update_membership(
+    async def update_membership_locked(
         self,
         requester: Requester,
         target: UserID,
@@ -335,6 +335,10 @@ class RoomMemberHandler(metaclass=abc.ABCMeta):
         content: Optional[dict] = None,
         require_consent: bool = True,
     ) -> Tuple[str, int]:
+        """Helper for update_membership.
+
+        Assumes that the membership linearizer is already held for the room.
+        """
         content_specified = bool(content)
         if content is None:
             content = {}