Add a mechanism to await un-partial-stating

When we join a room via the faster-joins mechanism, we end up with "partial
state" at some points on the event DAG. Many parts of the codebase need to
wait for the full state to load. So, we implement a mechanism to keep track of
which events have partial state, and wait for them to be fully-populated.

changelog.d/12399.misc

@@ -0,0 +1 @@
+Preparation for faster-room-join work: Implement a tracking mechanism to allow functions to wait for full room state to arrive.

synapse/handlers/federation_event.py

@@ -515,6 +515,7 @@ async def update_state_for_partial_state_event(
                 )
                 return
             await self._store.update_state_for_partial_state_event(event, context)
+            self._state_store.notify_event_un_partial_stated(event.event_id)
 
     async def backfill(
         self, dest: str, room_id: str, limit: int, extremities: Collection[str]

synapse/storage/databases/main/events_worker.py

@@ -1956,7 +1956,15 @@ def get_event_id_for_timestamp_txn(txn: LoggingTransaction) -> Optional[str]:
     async def get_partial_state_events(
         self, event_ids: Collection[str]
     ) -> Dict[str, bool]:
-        """Checks which of the given events have partial state"""
+        """Checks which of the given events have partial state
+
+        Args:
+            event_ids: the events we want to check for partial state.
+
+        Returns:
+            a dict mapping from event id to partial-stateness. We return True for
+            any of the events which are unknown (or are outliers).
+        """
         result = await self.db_pool.simple_select_many_batch(
             table="partial_state_events",
             column="event_id",

synapse/storage/state.py

@@ -31,6 +31,7 @@
 
 from synapse.api.constants import EventTypes
 from synapse.events import EventBase
+from synapse.storage.util.partial_state_events_tracker import PartialStateEventsTracker
 from synapse.types import MutableStateMap, StateKey, StateMap
 
 if TYPE_CHECKING:
@@ -542,6 +543,10 @@ class StateGroupStorage:
 
     def __init__(self, hs: "HomeServer", stores: "Databases"):
         self.stores = stores
+        self._partial_state_events_tracker = PartialStateEventsTracker(stores.main)
+
+    def notify_event_un_partial_stated(self, event_id: str) -> None:
+        self._partial_state_events_tracker.notify_un_partial_stated(event_id)
 
     async def get_state_group_delta(
         self, state_group: int

synapse/storage/util/partial_state_events_tracker.py

@@ -0,0 +1,114 @@
+# Copyright 2022 The Matrix.org Foundation C.I.C.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import logging
+from collections import defaultdict
+from typing import Collection, Dict, Set
+
+from twisted.internet import defer
+from twisted.internet.defer import Deferred
+
+from synapse.logging.context import PreserveLoggingContext, make_deferred_yieldable
+from synapse.storage.databases.main.events_worker import EventsWorkerStore
+from synapse.util import unwrapFirstError
+
+logger = logging.getLogger(__name__)
+
+
+class PartialStateEventsTracker:
+    """Keeps track of which events have partial state, after a partial-state join"""
+
+    def __init__(self, store: EventsWorkerStore):
+        self._store = store
+        self._observers: Dict[str, Set[Deferred]] = defaultdict(set)
+
+    def notify_un_partial_stated(self, event_id: str) -> None:
+        """Notify that we now have full state for a given event
+
+        Called by the state-resynchronization loop whenever we resynchronize the state
+        for a particular event. Unblocks any callers to await_full_state() for that
+        event.
+
+        Args:
+            event_id: the event that now has full state.
+        """
+        observers = self._observers.pop(event_id, None)
+        if not observers:
+            return
+        logger.info(
+            "Notifying %i things waiting for un-partial-stating of event %s",
+            len(observers),
+            event_id,
+        )
+        with PreserveLoggingContext():
+            for o in observers:
+                o.callback(None)
+
+    async def await_full_state(self, event_ids: Collection[str]) -> None:
+        """Wait for all the given events to have full state.
+
+        Args:
+            event_ids: the list of event ids that we want full state for
+        """
+        # first try the happy path: if there are no partial-state events, we can return
+        # quickly
+        partial_state_event_ids = [
+            ev
+            for ev, p in (await self._store.get_partial_state_events(event_ids)).items()
+            if p
+        ]
+
+        if not partial_state_event_ids:
+            return
+
+        logger.info(
+            "Awaiting un-partial-stating of events %s",
+            partial_state_event_ids,
+            stack_info=True,
+        )
+
+        # create an observer for each lazy-joined event
+        observers = {event_id: Deferred() for event_id in partial_state_event_ids}
+        for event_id, observer in observers.items():
+            self._observers[event_id].add(observer)
+
+        try:
+            # some of them may have been un-lazy-joined between us checking the db and
+            # registering the observer, in which case we'd wait forever for the
+            # notification. Call back the observers now.
+            for event_id, partial in (
+                await self._store.get_partial_state_events(observers.keys())
+            ).items():
+                if not partial:
+                    observers[event_id].callback(None)
+
+            await make_deferred_yieldable(
+                defer.gatherResults(
+                    observers.values(),
+                    consumeErrors=True,
+                )
+            ).addErrback(unwrapFirstError)
+            logger.info("Events %s all un-partial-stated", observers.keys())
+        finally:
+            # remove any observers we created. This should happen when the notification
+            # is received, but that might not happen for two reasons:
+            #   (a) we're bailing out early on an exception (including us being
+            #       cancelled during the await)
+            #   (b) the event got de-lazy-joined before we set up the observer.
+            for event_id, observer in observers.items():
+                observer_set = self._observers.get(event_id)
+                if observer_set:
+                    observer_set.discard(observer)
+                    if not observer_set:
+                        del self._observers[event_id]

tests/storage/util/__init__.py

@@ -0,0 +1,13 @@
+# Copyright 2022 The Matrix.org Foundation C.I.C.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.

tests/storage/util/test_partial_state_events_tracker.py

@@ -0,0 +1,94 @@
+# Copyright 2022 The Matrix.org Foundation C.I.C.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from typing import Dict
+from unittest import mock
+
+from twisted.internet.defer import CancelledError, ensureDeferred
+
+from synapse.storage.util.partial_state_events_tracker import PartialStateEventsTracker
+
+from tests.unittest import TestCase
+
+
+class PartialStateEventsTrackerTestCase(TestCase):
+    def setUp(self) -> None:
+        # the results to be returned by the mocked get_partial_state_events
+        self._events_dict: Dict[str, bool] = {}
+
+        async def get_partial_state_events(events):
+            return {e: self._events_dict[e] for e in events}
+
+        self.mock_store = mock.Mock(spec_set=["get_partial_state_events"])
+        self.mock_store.get_partial_state_events.side_effect = get_partial_state_events
+
+        self.tracker = PartialStateEventsTracker(self.mock_store)
+
+    def test_does_not_block_for_full_state_events(self):
+        self._events_dict = {"event1": False, "event2": False}
+
+        self.successResultOf(
+            ensureDeferred(self.tracker.await_full_state(["event1", "event2"]))
+        )
+
+        self.mock_store.get_partial_state_events.assert_called_once_with(
+            ["event1", "event2"]
+        )
+
+    def test_blocks_for_partial_state_events(self):
+        self._events_dict = {"event1": True, "event2": False}
+
+        d = ensureDeferred(self.tracker.await_full_state(["event1", "event2"]))
+
+        # there should be no result yet
+        self.assertNoResult(d)
+
+        # notifying that the event has been de-partial-stated should unblock
+        self.tracker.notify_un_partial_stated("event1")
+        self.successResultOf(d)
+
+    def test_un_partial_state_race(self):
+        # if the event is un-partial-stated between the initial check and the
+        # registration of the listener, it should not block.
+        self._events_dict = {"event1": True, "event2": False}
+
+        async def get_partial_state_events(events):
+            res = {e: self._events_dict[e] for e in events}
+            # change the result for next time
+            self._events_dict = {"event1": False, "event2": False}
+            return res
+
+        self.mock_store.get_partial_state_events.side_effect = get_partial_state_events
+
+        self.successResultOf(
+            ensureDeferred(self.tracker.await_full_state(["event1", "event2"]))
+        )
+
+    def test_cancellation(self):
+        self._events_dict = {"event1": True, "event2": False}
+
+        d1 = ensureDeferred(self.tracker.await_full_state(["event1", "event2"]))
+        self.assertNoResult(d1)
+
+        d2 = ensureDeferred(self.tracker.await_full_state(["event1"]))
+        self.assertNoResult(d2)
+
+        d1.cancel()
+        self.assertFailure(d1, CancelledError)
+
+        # d2 should still be waiting!
+        self.assertNoResult(d2)
+
+        self.tracker.notify_un_partial_stated("event1")
+        self.successResultOf(d2)