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Commitbdc2e7a

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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 parent60cc241 commitbdc2e7a

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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
@@ -78,6 +78,8 @@ typedef struct FixedParallelState
7878
PGPROC*parallel_master_pgproc;
7979
pid_tparallel_master_pid;
8080
BackendIdparallel_master_backend_id;
81+
TimestampTzxact_ts;
82+
TimestampTzstmt_ts;
8183

8284
/* Entrypoint for parallel workers (deprecated)! */
8385
parallel_worker_main_typeentrypoint;
@@ -324,6 +326,8 @@ InitializeParallelDSM(ParallelContext *pcxt)
324326
fps->parallel_master_pgproc=MyProc;
325327
fps->parallel_master_pid=MyProcPid;
326328
fps->parallel_master_backend_id=MyBackendId;
329+
fps->xact_ts=GetCurrentTransactionStartTimestamp();
330+
fps->stmt_ts=GetCurrentStatementStartTimestamp();
327331
fps->entrypoint=pcxt->entrypoint;
328332
SpinLockInit(&fps->mutex);
329333
fps->last_xlog_end=0;
@@ -1148,6 +1152,13 @@ ParallelWorkerMain(Datum main_arg)
11481152
fps->parallel_master_pid))
11491153
return;
11501154

1155+
/*
1156+
* Restore transaction and statement start-time timestamps. This must
1157+
* happen before anything that would start a transaction, else asserts in
1158+
* xact.c will fire.
1159+
*/
1160+
SetParallelStartTimestamps(fps->xact_ts,fps->stmt_ts);
1161+
11511162
/*
11521163
* Identify the entry point to be called. In theory this could result in
11531164
* 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
@@ -698,6 +698,22 @@ GetCurrentCommandId(bool used)
698698
returncurrentCommandId;
699699
}
700700

701+
/*
702+
*SetParallelStartTimestamps
703+
*
704+
* In a parallel worker, we should inherit the parent transaction's
705+
* timestamps rather than setting our own. The parallel worker
706+
* infrastructure must call this to provide those values before
707+
* calling StartTransaction() or SetCurrentStatementStartTimestamp().
708+
*/
709+
void
710+
SetParallelStartTimestamps(TimestampTzxact_ts,TimestampTzstmt_ts)
711+
{
712+
Assert(IsParallelWorker());
713+
xactStartTimestamp=xact_ts;
714+
stmtStartTimestamp=stmt_ts;
715+
}
716+
701717
/*
702718
*GetCurrentTransactionStartTimestamp
703719
*/
@@ -732,11 +748,17 @@ GetCurrentTransactionStopTimestamp(void)
732748

733749
/*
734750
*SetCurrentStatementStartTimestamp
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*
752+
* In a parallel worker, this should already have been provided by a call
753+
* to SetParallelStartTimestamps().
735754
*/
736755
void
737756
SetCurrentStatementStartTimestamp(void)
738757
{
739-
stmtStartTimestamp=GetCurrentTimestamp();
758+
if (!IsParallelWorker())
759+
stmtStartTimestamp=GetCurrentTimestamp();
760+
else
761+
Assert(stmtStartTimestamp!=0);
740762
}
741763

742764
/*
@@ -1889,10 +1911,16 @@ StartTransaction(void)
18891911
/*
18901912
* set transaction_timestamp() (a/k/a now()). We want this to be the same
18911913
* as the first command's statement_timestamp(), so don't do a fresh
1892-
* GetCurrentTimestamp() call (which'd be expensive anyway). Also, mark
1893-
* xactStopTimestamp as unset.
1914+
* GetCurrentTimestamp() call (which'd be expensive anyway). In a
1915+
* parallel worker, this should already have been provided by a call to
1916+
* SetParallelStartTimestamps().
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*
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* Also, mark xactStopTimestamp as unset.
18941919
*/
1895-
xactStartTimestamp=stmtStartTimestamp;
1920+
if (!IsParallelWorker())
1921+
xactStartTimestamp=stmtStartTimestamp;
1922+
else
1923+
Assert(xactStartTimestamp!=0);
18961924
xactStopTimestamp=0;
18971925
pgstat_report_xact_timestamp(xactStartTimestamp);
18981926

‎src/include/access/xact.h

Lines changed: 1 addition & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -318,6 +318,7 @@ extern SubTransactionId GetCurrentSubTransactionId(void);
318318
externvoidMarkCurrentTransactionIdLoggedIfAny(void);
319319
externboolSubTransactionIsActive(SubTransactionIdsubxid);
320320
externCommandIdGetCurrentCommandId(boolused);
321+
externvoidSetParallelStartTimestamps(TimestampTzxact_ts,TimestampTzstmt_ts);
321322
externTimestampTzGetCurrentTransactionStartTimestamp(void);
322323
externTimestampTzGetCurrentStatementStartTimestamp(void);
323324
externTimestampTzGetCurrentTransactionStopTimestamp(void);

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