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Commit58454d0

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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 parent142cfd3 commit58454d0

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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
@@ -79,6 +79,8 @@ typedef struct FixedParallelState
7979
PGPROC*parallel_master_pgproc;
8080
pid_tparallel_master_pid;
8181
BackendIdparallel_master_backend_id;
82+
TimestampTzxact_ts;
83+
TimestampTzstmt_ts;
8284

8385
/* Mutex protects remaining fields. */
8486
slock_tmutex;
@@ -289,6 +291,8 @@ InitializeParallelDSM(ParallelContext *pcxt)
289291
fps->parallel_master_pgproc=MyProc;
290292
fps->parallel_master_pid=MyProcPid;
291293
fps->parallel_master_backend_id=MyBackendId;
294+
fps->xact_ts=GetCurrentTransactionStartTimestamp();
295+
fps->stmt_ts=GetCurrentStatementStartTimestamp();
292296
SpinLockInit(&fps->mutex);
293297
fps->last_xlog_end=0;
294298
shm_toc_insert(pcxt->toc,PARALLEL_KEY_FIXED,fps);
@@ -1118,6 +1122,13 @@ ParallelWorkerMain(Datum main_arg)
11181122
fps->parallel_master_pid))
11191123
return;
11201124

1125+
/*
1126+
* Restore transaction and statement start-time timestamps. This must
1127+
* happen before anything that would start a transaction, else asserts in
1128+
* xact.c will fire.
1129+
*/
1130+
SetParallelStartTimestamps(fps->xact_ts,fps->stmt_ts);
1131+
11211132
/*
11221133
* Identify the entry point to be called. In theory this could result in
11231134
* 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
@@ -701,6 +701,22 @@ GetCurrentCommandId(bool used)
701701
returncurrentCommandId;
702702
}
703703

704+
/*
705+
*SetParallelStartTimestamps
706+
*
707+
* In a parallel worker, we should inherit the parent transaction's
708+
* timestamps rather than setting our own. The parallel worker
709+
* infrastructure must call this to provide those values before
710+
* calling StartTransaction() or SetCurrentStatementStartTimestamp().
711+
*/
712+
void
713+
SetParallelStartTimestamps(TimestampTzxact_ts,TimestampTzstmt_ts)
714+
{
715+
Assert(IsParallelWorker());
716+
xactStartTimestamp=xact_ts;
717+
stmtStartTimestamp=stmt_ts;
718+
}
719+
704720
/*
705721
*GetCurrentTransactionStartTimestamp
706722
*/
@@ -735,11 +751,17 @@ GetCurrentTransactionStopTimestamp(void)
735751

736752
/*
737753
*SetCurrentStatementStartTimestamp
754+
*
755+
* In a parallel worker, this should already have been provided by a call
756+
* to SetParallelStartTimestamps().
738757
*/
739758
void
740759
SetCurrentStatementStartTimestamp(void)
741760
{
742-
stmtStartTimestamp=GetCurrentTimestamp();
761+
if (!IsParallelWorker())
762+
stmtStartTimestamp=GetCurrentTimestamp();
763+
else
764+
Assert(stmtStartTimestamp!=0);
743765
}
744766

745767
/*
@@ -1893,10 +1915,16 @@ StartTransaction(void)
18931915
/*
18941916
* set transaction_timestamp() (a/k/a now()). We want this to be the same
18951917
* as the first command's statement_timestamp(), so don't do a fresh
1896-
* GetCurrentTimestamp() call (which'd be expensive anyway). Also, mark
1897-
* xactStopTimestamp as unset.
1918+
* GetCurrentTimestamp() call (which'd be expensive anyway). In a
1919+
* parallel worker, this should already have been provided by a call to
1920+
* SetParallelStartTimestamps().
1921+
*
1922+
* Also, mark xactStopTimestamp as unset.
18981923
*/
1899-
xactStartTimestamp=stmtStartTimestamp;
1924+
if (!IsParallelWorker())
1925+
xactStartTimestamp=stmtStartTimestamp;
1926+
else
1927+
Assert(xactStartTimestamp!=0);
19001928
xactStopTimestamp=0;
19011929
pgstat_report_xact_timestamp(xactStartTimestamp);
19021930

‎src/include/access/xact.h

Lines changed: 1 addition & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -337,6 +337,7 @@ extern SubTransactionId GetCurrentSubTransactionId(void);
337337
externvoidMarkCurrentTransactionIdLoggedIfAny(void);
338338
externboolSubTransactionIsActive(SubTransactionIdsubxid);
339339
externCommandIdGetCurrentCommandId(boolused);
340+
externvoidSetParallelStartTimestamps(TimestampTzxact_ts,TimestampTzstmt_ts);
340341
externTimestampTzGetCurrentTransactionStartTimestamp(void);
341342
externTimestampTzGetCurrentStatementStartTimestamp(void);
342343
externTimestampTzGetCurrentTransactionStopTimestamp(void);

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