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Commit6bf278d

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Propagate xactStartTimestamp and stmtStartTimestamp to parallel workers.
Previously, a worker process would establish values for these based onits own start time. In v10 and up, this can trivially be shown to causemisbehavior of transaction_timestamp(), timestamp_in(), and relatedfunctions which are (perhaps unwisely?) marked parallel-safe. It seemslikely that other behaviors might diverge from what happens in the parentas well.It's not as trivial to demonstrate problems in 9.6 or 9.5, but I'm sureit's still possible, so back-patch to all branches containing parallelworker infrastructure.In HEAD only, mark now() and statement_timestamp() as parallel-safe(other affected functions already were). While in theory we couldstill squeeze that change into v11, it doesn't seem important enoughto force a last-minute catversion bump.Konstantin Knizhnik, whacked around a bit by meDiscussion:https://postgr.es/m/6406dbd2-5d37-4cb6-6eb2-9c44172c7e7c@postgrespro.ru
1 parentc905b67 commit6bf278d

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3 files changed

+44
-4
lines changed

3 files changed

+44
-4
lines changed

‎src/backend/access/transam/parallel.c

Lines changed: 11 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -86,6 +86,8 @@ typedef struct FixedParallelState
8686
PGPROC*parallel_master_pgproc;
8787
pid_tparallel_master_pid;
8888
BackendIdparallel_master_backend_id;
89+
TimestampTzxact_ts;
90+
TimestampTzstmt_ts;
8991

9092
/* Mutex protects remaining fields. */
9193
slock_tmutex;
@@ -321,6 +323,8 @@ InitializeParallelDSM(ParallelContext *pcxt)
321323
fps->parallel_master_pgproc=MyProc;
322324
fps->parallel_master_pid=MyProcPid;
323325
fps->parallel_master_backend_id=MyBackendId;
326+
fps->xact_ts=GetCurrentTransactionStartTimestamp();
327+
fps->stmt_ts=GetCurrentStatementStartTimestamp();
324328
SpinLockInit(&fps->mutex);
325329
fps->last_xlog_end=0;
326330
shm_toc_insert(pcxt->toc,PARALLEL_KEY_FIXED,fps);
@@ -1303,6 +1307,13 @@ ParallelWorkerMain(Datum main_arg)
13031307
fps->parallel_master_pid))
13041308
return;
13051309

1310+
/*
1311+
* Restore transaction and statement start-time timestamps. This must
1312+
* happen before anything that would start a transaction, else asserts in
1313+
* xact.c will fire.
1314+
*/
1315+
SetParallelStartTimestamps(fps->xact_ts,fps->stmt_ts);
1316+
13061317
/*
13071318
* Identify the entry point to be called. In theory this could result in
13081319
* loading an additional library, though most likely the entry point is in

‎src/backend/access/transam/xact.c

Lines changed: 32 additions & 4 deletions
Original file line numberDiff line numberDiff line change
@@ -693,6 +693,22 @@ GetCurrentCommandId(bool used)
693693
returncurrentCommandId;
694694
}
695695

696+
/*
697+
*SetParallelStartTimestamps
698+
*
699+
* In a parallel worker, we should inherit the parent transaction's
700+
* timestamps rather than setting our own. The parallel worker
701+
* infrastructure must call this to provide those values before
702+
* calling StartTransaction() or SetCurrentStatementStartTimestamp().
703+
*/
704+
void
705+
SetParallelStartTimestamps(TimestampTzxact_ts,TimestampTzstmt_ts)
706+
{
707+
Assert(IsParallelWorker());
708+
xactStartTimestamp=xact_ts;
709+
stmtStartTimestamp=stmt_ts;
710+
}
711+
696712
/*
697713
*GetCurrentTransactionStartTimestamp
698714
*/
@@ -727,11 +743,17 @@ GetCurrentTransactionStopTimestamp(void)
727743

728744
/*
729745
*SetCurrentStatementStartTimestamp
746+
*
747+
* In a parallel worker, this should already have been provided by a call
748+
* to SetParallelStartTimestamps().
730749
*/
731750
void
732751
SetCurrentStatementStartTimestamp(void)
733752
{
734-
stmtStartTimestamp=GetCurrentTimestamp();
753+
if (!IsParallelWorker())
754+
stmtStartTimestamp=GetCurrentTimestamp();
755+
else
756+
Assert(stmtStartTimestamp!=0);
735757
}
736758

737759
/*
@@ -1886,10 +1908,16 @@ StartTransaction(void)
18861908
/*
18871909
* set transaction_timestamp() (a/k/a now()). We want this to be the same
18881910
* as the first command's statement_timestamp(), so don't do a fresh
1889-
* GetCurrentTimestamp() call (which'd be expensive anyway). Also, mark
1890-
* xactStopTimestamp as unset.
1911+
* GetCurrentTimestamp() call (which'd be expensive anyway). In a
1912+
* parallel worker, this should already have been provided by a call to
1913+
* SetParallelStartTimestamps().
1914+
*
1915+
* Also, mark xactStopTimestamp as unset.
18911916
*/
1892-
xactStartTimestamp=stmtStartTimestamp;
1917+
if (!IsParallelWorker())
1918+
xactStartTimestamp=stmtStartTimestamp;
1919+
else
1920+
Assert(xactStartTimestamp!=0);
18931921
xactStopTimestamp=0;
18941922
pgstat_report_xact_timestamp(xactStartTimestamp);
18951923

‎src/include/access/xact.h

Lines changed: 1 addition & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -359,6 +359,7 @@ extern SubTransactionId GetCurrentSubTransactionId(void);
359359
externvoidMarkCurrentTransactionIdLoggedIfAny(void);
360360
externboolSubTransactionIsActive(SubTransactionIdsubxid);
361361
externCommandIdGetCurrentCommandId(boolused);
362+
externvoidSetParallelStartTimestamps(TimestampTzxact_ts,TimestampTzstmt_ts);
362363
externTimestampTzGetCurrentTransactionStartTimestamp(void);
363364
externTimestampTzGetCurrentStatementStartTimestamp(void);
364365
externTimestampTzGetCurrentTransactionStopTimestamp(void);

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