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Fix race in MultiWriterIdGenerator
(#11045)
The race allowed the current position to advance too far when stream IDs are still being persisted. This happened when it received a new stream ID from a remote write between a new stream ID being allocated and it being added to the set of unpersisted stream IDs. Fixes #9424.
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changelog.d/11045.bugfix
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changelog.d/11045.bugfix
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Fix a long-standing bug when using multiple event persister workers where events were not correctly sent down `/sync` due to a race.
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@ -36,7 +36,7 @@ from typing import (
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)
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import attr
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from sortedcontainers import SortedSet
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from sortedcontainers import SortedList, SortedSet
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from synapse.metrics.background_process_metrics import run_as_background_process
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from synapse.storage.database import (
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@ -265,6 +265,15 @@ class MultiWriterIdGenerator:
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# should be less than the minimum of this set (if not empty).
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self._unfinished_ids: SortedSet[int] = SortedSet()
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# We also need to track when we've requested some new stream IDs but
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# they haven't yet been added to the `_unfinished_ids` set. Every time
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# we request a new stream ID we add the current max stream ID to the
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# list, and remove it once we've added the newly allocated IDs to the
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# `_unfinished_ids` set. This means that we *may* be allocated stream
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# IDs above those in the list, and so we can't advance the local current
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# position beyond the minimum stream ID in this list.
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self._in_flight_fetches: SortedList[int] = SortedList()
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# Set of local IDs that we've processed that are larger than the current
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# position, due to there being smaller unpersisted IDs.
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self._finished_ids: Set[int] = set()
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@ -290,6 +299,9 @@ class MultiWriterIdGenerator:
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)
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self._known_persisted_positions: List[int] = []
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# The maximum stream ID that we have seen been allocated across any writer.
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self._max_seen_allocated_stream_id = 1
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self._sequence_gen = PostgresSequenceGenerator(sequence_name)
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# We check that the table and sequence haven't diverged.
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@ -305,6 +317,10 @@ class MultiWriterIdGenerator:
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# This goes and fills out the above state from the database.
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self._load_current_ids(db_conn, tables)
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self._max_seen_allocated_stream_id = max(
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self._current_positions.values(), default=1
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)
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def _load_current_ids(
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self,
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db_conn: LoggingDatabaseConnection,
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@ -411,10 +427,32 @@ class MultiWriterIdGenerator:
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cur.close()
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def _load_next_id_txn(self, txn: Cursor) -> int:
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return self._sequence_gen.get_next_id_txn(txn)
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stream_ids = self._load_next_mult_id_txn(txn, 1)
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return stream_ids[0]
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def _load_next_mult_id_txn(self, txn: Cursor, n: int) -> List[int]:
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return self._sequence_gen.get_next_mult_txn(txn, n)
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# We need to track that we've requested some more stream IDs, and what
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# the current max allocated stream ID is. This is to prevent a race
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# where we've been allocated stream IDs but they have not yet been added
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# to the `_unfinished_ids` set, allowing the current position to advance
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# past them.
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with self._lock:
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current_max = self._max_seen_allocated_stream_id
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self._in_flight_fetches.add(current_max)
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try:
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stream_ids = self._sequence_gen.get_next_mult_txn(txn, n)
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with self._lock:
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self._unfinished_ids.update(stream_ids)
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self._max_seen_allocated_stream_id = max(
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self._max_seen_allocated_stream_id, self._unfinished_ids[-1]
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)
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finally:
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with self._lock:
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self._in_flight_fetches.remove(current_max)
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return stream_ids
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def get_next(self) -> AsyncContextManager[int]:
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"""
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@ -463,9 +501,6 @@ class MultiWriterIdGenerator:
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next_id = self._load_next_id_txn(txn)
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with self._lock:
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self._unfinished_ids.add(next_id)
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txn.call_after(self._mark_id_as_finished, next_id)
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txn.call_on_exception(self._mark_id_as_finished, next_id)
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@ -497,15 +532,27 @@ class MultiWriterIdGenerator:
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new_cur: Optional[int] = None
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if self._unfinished_ids:
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if self._unfinished_ids or self._in_flight_fetches:
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# If there are unfinished IDs then the new position will be the
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# largest finished ID less than the minimum unfinished ID.
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# largest finished ID strictly less than the minimum unfinished
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# ID.
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# The minimum unfinished ID needs to take account of both
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# `_unfinished_ids` and `_in_flight_fetches`.
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if self._unfinished_ids and self._in_flight_fetches:
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# `_in_flight_fetches` stores the maximum safe stream ID, so
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# we add one to make it equivalent to the minimum unsafe ID.
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min_unfinished = min(
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self._unfinished_ids[0], self._in_flight_fetches[0] + 1
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)
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elif self._in_flight_fetches:
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min_unfinished = self._in_flight_fetches[0] + 1
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else:
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min_unfinished = self._unfinished_ids[0]
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finished = set()
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min_unfinshed = self._unfinished_ids[0]
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for s in self._finished_ids:
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if s < min_unfinshed:
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if s < min_unfinished:
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if new_cur is None or new_cur < s:
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new_cur = s
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else:
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@ -575,6 +622,10 @@ class MultiWriterIdGenerator:
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new_id, self._current_positions.get(instance_name, 0)
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)
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self._max_seen_allocated_stream_id = max(
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self._max_seen_allocated_stream_id, new_id
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)
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self._add_persisted_position(new_id)
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def get_persisted_upto_position(self) -> int:
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@ -605,7 +656,11 @@ class MultiWriterIdGenerator:
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# to report a recent position when asked, rather than a potentially old
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# one (if this instance hasn't written anything for a while).
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our_current_position = self._current_positions.get(self._instance_name)
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if our_current_position and not self._unfinished_ids:
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if (
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our_current_position
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and not self._unfinished_ids
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and not self._in_flight_fetches
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):
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self._current_positions[self._instance_name] = max(
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our_current_position, new_id
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)
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@ -697,9 +752,6 @@ class _MultiWriterCtxManager:
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db_autocommit=True,
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)
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with self.id_gen._lock:
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self.id_gen._unfinished_ids.update(self.stream_ids)
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if self.multiple_ids is None:
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return self.stream_ids[0] * self.id_gen._return_factor
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else:
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