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add lock for background task monitor to make sure there is only ever one running per database at the same time

background-job-details
Nils Dijk 2022-08-09 22:38:42 +02:00 committed by Jelte Fennema
parent 1b66403cb9
commit 3db49b43c2
2 changed files with 21 additions and 3 deletions
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21
src/backend/distributed/utils/background_jobs.c
View File

@ -28,6 +28,7 @@
#include "distributed/metadata_cache.h"
#include "distributed/metadata_utility.h"
#include "distributed/shard_cleaner.h"
#include "distributed/resource_lock.h"
bool BackgroundTaskMonitorDebugDelay = false;
@ -120,13 +121,30 @@ CitusBackgroundTaskMonitorMain(Datum arg)
/* connect to database, after that we can actually access catalogs */
BackgroundWorkerInitializeConnectionByOid(databaseOid, extensionOwner, 0);
/* TODO get lock to make sure there is only one worker running per databasse */
/*
* There should be exactly one background task monitor running, running multiple would
* cause conflicts on processing the tasks in the catalog table as well as violate
* parallelism guarantees. To make sure there is at most, exactly one backend running
* we take a session lock on the CITUS_BACKGROUND_TASK_MONITOR operation.
*/
LOCKTAG tag = { 0 };
SET_LOCKTAG_CITUS_OPERATION(tag, CITUS_BACKGROUND_TASK_MONITOR);
const bool sessionLock = true;
const bool dontWait = true;
LockAcquireResult locked =
LockAcquire(&tag, AccessExclusiveLock, sessionLock, dontWait);
if (locked == LOCKACQUIRE_NOT_AVAIL)
{
ereport(ERROR, (errmsg("background task monitor already running for database")));
exit(0);
}
/* make worker recognizable in pg_stat_activity */
pgstat_report_appname("citus background task monitor");
ereport(LOG, (errmsg("citus background task monitor")));
/* TODO this is here for debugging purposses, remove before merge. */
if (BackgroundTaskMonitorDebugDelay)
{
pg_usleep(30 * 1000 * 1000);
@ -147,7 +165,6 @@ CitusBackgroundTaskMonitorMain(Datum arg)
StartTransactionCommand();
PushActiveSnapshot(GetTransactionSnapshot());
/* TODO have an actual function to check if the worker is still running */
ResetRunningBackgroundTasks();
PopActiveSnapshot();

3
src/include/distributed/resource_lock.h
View File

@ -48,7 +48,8 @@ typedef enum AdvisoryLocktagClass
/* CitusOperations has constants for citus operations */
typedef enum CitusOperations
{
CITUS_TRANSACTION_RECOVERY = 0
CITUS_TRANSACTION_RECOVERY = 0,
CITUS_BACKGROUND_TASK_MONITOR = 1
} CitusOperations;
/* reuse advisory lock, but with different, unused field 4 (4)*/