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Fix race in MultiWriterIdGenerator (#11045)

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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.
This commit is contained in:
Erik Johnston 2021-10-12 14:27:09 +01:00 committed by GitHub
parent 5c35074d85
commit 333d6f4e84
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2 changed files with 68 additions and 15 deletions
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1
changelog.d/11045.bugfix Normal file
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@ -0,0 +1 @@
Fix a long-standing bug when using multiple event persister workers where events were not correctly sent down `/sync` due to a race.

82
synapse/storage/util/id_generators.py
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@ -36,7 +36,7 @@ from typing import (
)
import attr
from sortedcontainers import SortedSet
from sortedcontainers import SortedList, SortedSet
from synapse.metrics.background_process_metrics import run_as_background_process
from synapse.storage.database import (
@ -265,6 +265,15 @@ class MultiWriterIdGenerator:
# should be less than the minimum of this set (if not empty).
self._unfinished_ids: SortedSet[int] = SortedSet()
# We also need to track when we've requested some new stream IDs but
# they haven't yet been added to the `_unfinished_ids` set. Every time
# we request a new stream ID we add the current max stream ID to the
# list, and remove it once we've added the newly allocated IDs to the
# `_unfinished_ids` set. This means that we *may* be allocated stream
# IDs above those in the list, and so we can't advance the local current
# position beyond the minimum stream ID in this list.
self._in_flight_fetches: SortedList[int] = SortedList()
# Set of local IDs that we've processed that are larger than the current
# position, due to there being smaller unpersisted IDs.
self._finished_ids: Set[int] = set()
@ -290,6 +299,9 @@ class MultiWriterIdGenerator:
)
self._known_persisted_positions: List[int] = []
# The maximum stream ID that we have seen been allocated across any writer.
self._max_seen_allocated_stream_id = 1
self._sequence_gen = PostgresSequenceGenerator(sequence_name)
# We check that the table and sequence haven't diverged.
@ -305,6 +317,10 @@ class MultiWriterIdGenerator:
# This goes and fills out the above state from the database.
self._load_current_ids(db_conn, tables)
self._max_seen_allocated_stream_id = max(
self._current_positions.values(), default=1
)
def _load_current_ids(
self,
db_conn: LoggingDatabaseConnection,
@ -411,10 +427,32 @@ class MultiWriterIdGenerator:
cur.close()
def _load_next_id_txn(self, txn: Cursor) -> int:
return self._sequence_gen.get_next_id_txn(txn)
stream_ids = self._load_next_mult_id_txn(txn, 1)
return stream_ids[0]
def _load_next_mult_id_txn(self, txn: Cursor, n: int) -> List[int]:
return self._sequence_gen.get_next_mult_txn(txn, n)
# We need to track that we've requested some more stream IDs, and what
# the current max allocated stream ID is. This is to prevent a race
# where we've been allocated stream IDs but they have not yet been added
# to the `_unfinished_ids` set, allowing the current position to advance
# past them.
with self._lock:
current_max = self._max_seen_allocated_stream_id
self._in_flight_fetches.add(current_max)
try:
stream_ids = self._sequence_gen.get_next_mult_txn(txn, n)
with self._lock:
self._unfinished_ids.update(stream_ids)
self._max_seen_allocated_stream_id = max(
self._max_seen_allocated_stream_id, self._unfinished_ids[-1]
)
finally:
with self._lock:
self._in_flight_fetches.remove(current_max)
return stream_ids
def get_next(self) -> AsyncContextManager[int]:
"""
@ -463,9 +501,6 @@ class MultiWriterIdGenerator:
next_id = self._load_next_id_txn(txn)
with self._lock:
self._unfinished_ids.add(next_id)
txn.call_after(self._mark_id_as_finished, next_id)
txn.call_on_exception(self._mark_id_as_finished, next_id)
@ -497,15 +532,27 @@ class MultiWriterIdGenerator:
new_cur: Optional[int] = None
if self._unfinished_ids:
if self._unfinished_ids or self._in_flight_fetches:
# If there are unfinished IDs then the new position will be the
# largest finished ID less than the minimum unfinished ID.
# largest finished ID strictly less than the minimum unfinished
# ID.
# The minimum unfinished ID needs to take account of both
# `_unfinished_ids` and `_in_flight_fetches`.
if self._unfinished_ids and self._in_flight_fetches:
# `_in_flight_fetches` stores the maximum safe stream ID, so
# we add one to make it equivalent to the minimum unsafe ID.
min_unfinished = min(
self._unfinished_ids[0], self._in_flight_fetches[0] + 1
)
elif self._in_flight_fetches:
min_unfinished = self._in_flight_fetches[0] + 1
else:
min_unfinished = self._unfinished_ids[0]
finished = set()
min_unfinshed = self._unfinished_ids[0]
for s in self._finished_ids:
if s < min_unfinshed:
if s < min_unfinished:
if new_cur is None or new_cur < s:
new_cur = s
else:
@ -575,6 +622,10 @@ class MultiWriterIdGenerator:
new_id, self._current_positions.get(instance_name, 0)
)
self._max_seen_allocated_stream_id = max(
self._max_seen_allocated_stream_id, new_id
)
self._add_persisted_position(new_id)
def get_persisted_upto_position(self) -> int:
@ -605,7 +656,11 @@ class MultiWriterIdGenerator:
# to report a recent position when asked, rather than a potentially old
# one (if this instance hasn't written anything for a while).
our_current_position = self._current_positions.get(self._instance_name)
if our_current_position and not self._unfinished_ids:
if (
our_current_position
and not self._unfinished_ids
and not self._in_flight_fetches
):
self._current_positions[self._instance_name] = max(
our_current_position, new_id
)
@ -697,9 +752,6 @@ class _MultiWriterCtxManager:
db_autocommit=True,
)
with self.id_gen._lock:
self.id_gen._unfinished_ids.update(self.stream_ids)
if self.multiple_ids is None:
return self.stream_ids[0] * self.id_gen._return_factor
else: