Rename pagination&purge locks and add comments explaining them (#16112)

4 files changed, 22 insertions, 13 deletions

diff --git a/synapse/handlers/message.py b/synapse/handlers/message.py
index d485f21e49..a74db1dccf 100644
--- a/synapse/handlers/message.py
+++ b/synapse/handlers/message.py
@@ -53,7 +53,7 @@ from synapse.events.snapshot import EventContext, UnpersistedEventContextBase
 from synapse.events.utils import SerializeEventConfig, maybe_upsert_event_field
 from synapse.events.validator import EventValidator
 from synapse.handlers.directory import DirectoryHandler
-from synapse.handlers.worker_lock import DELETE_ROOM_LOCK_NAME
+from synapse.handlers.worker_lock import NEW_EVENT_DURING_PURGE_LOCK_NAME
 from synapse.logging import opentracing
 from synapse.logging.context import make_deferred_yieldable, run_in_background
 from synapse.metrics.background_process_metrics import run_as_background_process
@@ -1034,7 +1034,7 @@ class EventCreationHandler:
                     )
 
         async with self._worker_lock_handler.acquire_read_write_lock(
-            DELETE_ROOM_LOCK_NAME, room_id, write=False
+            NEW_EVENT_DURING_PURGE_LOCK_NAME, room_id, write=False
         ):
             return await self._create_and_send_nonmember_event_locked(
                 requester=requester,
@@ -1978,7 +1978,7 @@ class EventCreationHandler:
 
         for room_id in room_ids:
             async with self._worker_lock_handler.acquire_read_write_lock(
-                DELETE_ROOM_LOCK_NAME, room_id, write=False
+                NEW_EVENT_DURING_PURGE_LOCK_NAME, room_id, write=False
             ):
                 dummy_event_sent = await self._send_dummy_event_for_room(room_id)
 
diff --git a/synapse/handlers/pagination.py b/synapse/handlers/pagination.py
index da34658470..1be6ebc6d9 100644
--- a/synapse/handlers/pagination.py
+++ b/synapse/handlers/pagination.py
@@ -24,6 +24,7 @@ from synapse.api.errors import SynapseError
 from synapse.api.filtering import Filter
 from synapse.events.utils import SerializeEventConfig
 from synapse.handlers.room import ShutdownRoomResponse
+from synapse.handlers.worker_lock import NEW_EVENT_DURING_PURGE_LOCK_NAME
 from synapse.logging.opentracing import trace
 from synapse.metrics.background_process_metrics import run_as_background_process
 from synapse.rest.admin._base import assert_user_is_admin
@@ -46,9 +47,10 @@ logger = logging.getLogger(__name__)
 BACKFILL_BECAUSE_TOO_MANY_GAPS_THRESHOLD = 3
 
 
-PURGE_HISTORY_LOCK_NAME = "purge_history_lock"
-
-DELETE_ROOM_LOCK_NAME = "delete_room_lock"
+# This is used to avoid purging a room several time at the same moment,
+# and also paginating during a purge. Pagination can trigger backfill,
+# which would create old events locally, and would potentially clash with the room delete.
+PURGE_PAGINATION_LOCK_NAME = "purge_pagination_lock"
 
 
 @attr.s(slots=True, auto_attribs=True)
@@ -363,7 +365,7 @@ class PaginationHandler:
         self._purges_in_progress_by_room.add(room_id)
         try:
             async with self._worker_locks.acquire_read_write_lock(
-                PURGE_HISTORY_LOCK_NAME, room_id, write=True
+                PURGE_PAGINATION_LOCK_NAME, room_id, write=True
             ):
                 await self._storage_controllers.purge_events.purge_history(
                     room_id, token, delete_local_events
@@ -421,7 +423,10 @@ class PaginationHandler:
             force: set true to skip checking for joined users.
         """
         async with self._worker_locks.acquire_multi_read_write_lock(
-            [(PURGE_HISTORY_LOCK_NAME, room_id), (DELETE_ROOM_LOCK_NAME, room_id)],
+            [
+                (PURGE_PAGINATION_LOCK_NAME, room_id),
+                (NEW_EVENT_DURING_PURGE_LOCK_NAME, room_id),
+            ],
             write=True,
         ):
             # first check that we have no users in this room
@@ -483,7 +488,7 @@ class PaginationHandler:
         room_token = from_token.room_key
 
         async with self._worker_locks.acquire_read_write_lock(
-            PURGE_HISTORY_LOCK_NAME, room_id, write=False
+            PURGE_PAGINATION_LOCK_NAME, room_id, write=False
         ):
             (membership, member_event_id) = (None, None)
             if not use_admin_priviledge:
@@ -761,7 +766,7 @@ class PaginationHandler:
         self._purges_in_progress_by_room.add(room_id)
         try:
             async with self._worker_locks.acquire_read_write_lock(
-                PURGE_HISTORY_LOCK_NAME, room_id, write=True
+                PURGE_PAGINATION_LOCK_NAME, room_id, write=True
             ):
                 self._delete_by_id[delete_id].status = DeleteStatus.STATUS_SHUTTING_DOWN
                 self._delete_by_id[
diff --git a/synapse/handlers/room_member.py b/synapse/handlers/room_member.py
index e3cdf2bc61..1d8d4a72e7 100644
--- a/synapse/handlers/room_member.py
+++ b/synapse/handlers/room_member.py
@@ -39,7 +39,7 @@ from synapse.events import EventBase
 from synapse.events.snapshot import EventContext
 from synapse.handlers.profile import MAX_AVATAR_URL_LEN, MAX_DISPLAYNAME_LEN
 from synapse.handlers.state_deltas import MatchChange, StateDeltasHandler
-from synapse.handlers.worker_lock import DELETE_ROOM_LOCK_NAME
+from synapse.handlers.worker_lock import NEW_EVENT_DURING_PURGE_LOCK_NAME
 from synapse.logging import opentracing
 from synapse.metrics import event_processing_positions
 from synapse.metrics.background_process_metrics import run_as_background_process
@@ -621,7 +621,7 @@ class RoomMemberHandler(metaclass=abc.ABCMeta):
         async with self.member_as_limiter.queue(as_id):
             async with self.member_linearizer.queue(key):
                 async with self._worker_lock_handler.acquire_read_write_lock(
-                    DELETE_ROOM_LOCK_NAME, room_id, write=False
+                    NEW_EVENT_DURING_PURGE_LOCK_NAME, room_id, write=False
                 ):
                     with opentracing.start_active_span("update_membership_locked"):
                         result = await self.update_membership_locked(
diff --git a/synapse/handlers/worker_lock.py b/synapse/handlers/worker_lock.py
index 72df773a86..58efe7116b 100644
--- a/synapse/handlers/worker_lock.py
+++ b/synapse/handlers/worker_lock.py
@@ -42,7 +42,11 @@ if TYPE_CHECKING:
     from synapse.server import HomeServer
 
 
-DELETE_ROOM_LOCK_NAME = "delete_room_lock"
+# This lock is used to avoid creating an event while we are purging the room.
+# We take a read lock when creating an event, and a write one when purging a room.
+# This is because it is fine to create several events concurrently, since referenced events
+# will not disappear under our feet as long as we don't delete the room.
+NEW_EVENT_DURING_PURGE_LOCK_NAME = "new_event_during_purge_lock"
 
 
 class WorkerLocksHandler: