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Track ongoing event fetches correctly in the presence of failure (#11240)

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When an event fetcher aborts due to an exception, `_event_fetch_ongoing`
must be decremented, otherwise the event fetcher would never be
replaced. If enough event fetchers were to fail, no more events would be
fetched and requests would get stuck waiting for events.
This commit is contained in:
Sean Quah 2021-11-04 10:33:53 +00:00 committed by GitHub
parent a271e233e9
commit 8eec25a1d9
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2 changed files with 33 additions and 20 deletions
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1
changelog.d/11240.bugfix Normal file
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@ -0,0 +1 @@
Fix a long-standing bug where all requests that read events from the database could get stuck as a result of losing the database connection.

52
synapse/storage/databases/main/events_worker.py
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@ -28,6 +28,7 @@ from typing import (
import attr
from constantly import NamedConstant, Names
from prometheus_client import Gauge
from typing_extensions import Literal
from twisted.internet import defer
@ -81,6 +82,12 @@ EVENT_QUEUE_ITERATIONS = 3 # No. times we block waiting for requests for events
EVENT_QUEUE_TIMEOUT_S = 0.1 # Timeout when waiting for requests for events
event_fetch_ongoing_gauge = Gauge(
"synapse_event_fetch_ongoing",
"The number of event fetchers that are running",
)
@attr.s(slots=True, auto_attribs=True)
class _EventCacheEntry:
event: EventBase
@ -222,6 +229,7 @@ class EventsWorkerStore(SQLBaseStore):
self._event_fetch_lock = threading.Condition()
self._event_fetch_list = []
self._event_fetch_ongoing = 0
event_fetch_ongoing_gauge.set(self._event_fetch_ongoing)
# We define this sequence here so that it can be referenced from both
# the DataStore and PersistEventStore.
@ -732,28 +740,31 @@ class EventsWorkerStore(SQLBaseStore):
"""Takes a database connection and waits for requests for events from
the _event_fetch_list queue.
"""
i = 0
while True:
with self._event_fetch_lock:
event_list = self._event_fetch_list
self._event_fetch_list = []
try:
i = 0
while True:
with self._event_fetch_lock:
event_list = self._event_fetch_list
self._event_fetch_list = []
if not event_list:
single_threaded = self.database_engine.single_threaded
if (
not self.USE_DEDICATED_DB_THREADS_FOR_EVENT_FETCHING
or single_threaded
or i > EVENT_QUEUE_ITERATIONS
):
self._event_fetch_ongoing -= 1
return
else:
self._event_fetch_lock.wait(EVENT_QUEUE_TIMEOUT_S)
i += 1
continue
i = 0
if not event_list:
single_threaded = self.database_engine.single_threaded
if (
not self.USE_DEDICATED_DB_THREADS_FOR_EVENT_FETCHING
or single_threaded
or i > EVENT_QUEUE_ITERATIONS
):
break
else:
self._event_fetch_lock.wait(EVENT_QUEUE_TIMEOUT_S)
i += 1
continue
i = 0
self._fetch_event_list(conn, event_list)
self._fetch_event_list(conn, event_list)
finally:
self._event_fetch_ongoing -= 1
event_fetch_ongoing_gauge.set(self._event_fetch_ongoing)
def _fetch_event_list(
self, conn: Connection, event_list: List[Tuple[List[str], defer.Deferred]]
@ -977,6 +988,7 @@ class EventsWorkerStore(SQLBaseStore):
if self._event_fetch_ongoing < EVENT_QUEUE_THREADS:
self._event_fetch_ongoing += 1
event_fetch_ongoing_gauge.set(self._event_fetch_ongoing)
should_start = True
else:
should_start = False