From 88764dc3e9bf04f4ea293223c7abec452d879dae Mon Sep 17 00:00:00 2001
From: Richard van der Hoff <richard@matrix.org>
Date: Thu, 15 Oct 2020 18:14:33 +0100
Subject: [PATCH 1/3] Optimise createRoom with multiple invites

By not dropping the membership lock between invites, we can stop joins from
grabbing the lock when we're half-done and slowing the whole thing down.

(Is this a DoS vector? If someone invites thousands of people to a room, they
will receive the invite, but won't be able to accept it until the invites have
stopped churning? Though we should guard against abusive createRooms anyway.)
---
 synapse/handlers/room.py        | 29 ++++++++++++++++++-----------
 synapse/handlers/room_member.py |  8 ++++++--
 2 files changed, 24 insertions(+), 13 deletions(-)

diff --git a/synapse/handlers/room.py b/synapse/handlers/room.py
index d5f7c78edf52..842dc5c6b962 100644
--- a/synapse/handlers/room.py
+++ b/synapse/handlers/room.py
@@ -762,22 +762,29 @@ async def create_room(
                 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 8feba8c90a39..fef5559ddc5d 100644
--- a/synapse/handlers/room_member.py
+++ b/synapse/handlers/room_member.py
@@ -310,7 +310,7 @@ async def update_membership(
         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,
@@ -325,7 +325,7 @@ async def update_membership(
 
         return result
 
-    async def _update_membership(
+    async def update_membership_locked(
         self,
         requester: Requester,
         target: UserID,
@@ -338,6 +338,10 @@ async def _update_membership(
         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 = {}

From 172fe51d43ffbb567b19caf83ab4ed50fe1444f8 Mon Sep 17 00:00:00 2001
From: Richard van der Hoff <richard@matrix.org>
Date: Thu, 15 Oct 2020 18:18:48 +0100
Subject: [PATCH 2/3] changelog

---
 changelog.d/8559.misc | 1 +
 1 file changed, 1 insertion(+)
 create mode 100644 changelog.d/8559.misc

diff --git a/changelog.d/8559.misc b/changelog.d/8559.misc
new file mode 100644
index 000000000000..50cec6dbf694
--- /dev/null
+++ b/changelog.d/8559.misc
@@ -0,0 +1 @@
+Optimise `/createRoom` fwith multiple invited users.

From db62e76c27042c4043844ba325c26df28ecfb066 Mon Sep 17 00:00:00 2001
From: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>
Date: Thu, 15 Oct 2020 18:31:54 +0100
Subject: [PATCH 3/3] Update 8559.misc

---
 changelog.d/8559.misc | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/changelog.d/8559.misc b/changelog.d/8559.misc
index 50cec6dbf694..d7bd00964e0a 100644
--- a/changelog.d/8559.misc
+++ b/changelog.d/8559.misc
@@ -1 +1 @@
-Optimise `/createRoom` fwith multiple invited users.
+Optimise `/createRoom` with multiple invited users.