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Propagate xactStartTimestamp and stmtStartTimestamp to parallel workers.

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Previously, a worker process would establish values for these based on
its own start time.  In v10 and up, this can trivially be shown to cause
misbehavior of transaction_timestamp(), timestamp_in(), and related
functions which are (perhaps unwisely?) marked parallel-safe.  It seems
likely that other behaviors might diverge from what happens in the parent
as well.

It's not as trivial to demonstrate problems in 9.6 or 9.5, but I'm sure
it's still possible, so back-patch to all branches containing parallel
worker infrastructure.

In HEAD only, mark now() and statement_timestamp() as parallel-safe
(other affected functions already were).  While in theory we could
still squeeze that change into v11, it doesn't seem important enough
to force a last-minute catversion bump.

Konstantin Knizhnik, whacked around a bit by me

Discussion: https://postgr.es/m/6406dbd2-5d37-4cb6-6eb2-9c44172c7e7c@postgrespro.ru
This commit is contained in:
Tom Lane
2018-10-06 12:00:09 -04:00
parent e954a727f0
commit 07ee62ce9e
5 changed files with 52 additions and 10 deletions
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11
src/backend/access/transam/parallel.c
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@ -87,6 +87,8 @@ typedef struct FixedParallelState
PGPROC *parallel_master_pgproc;
pid_t parallel_master_pid;
BackendId parallel_master_backend_id;
TimestampTz xact_ts;
TimestampTz stmt_ts;
/* Mutex protects remaining fields. */
slock_t mutex;
@ -318,6 +320,8 @@ InitializeParallelDSM(ParallelContext *pcxt)
fps->parallel_master_pgproc = MyProc;
fps->parallel_master_pid = MyProcPid;
fps->parallel_master_backend_id = MyBackendId;
fps->xact_ts = GetCurrentTransactionStartTimestamp();
fps->stmt_ts = GetCurrentStatementStartTimestamp();
SpinLockInit(&fps->mutex);
fps->last_xlog_end = 0;
shm_toc_insert(pcxt->toc, PARALLEL_KEY_FIXED, fps);
@ -1311,6 +1315,13 @@ ParallelWorkerMain(Datum main_arg)
fps->parallel_master_pid))
return;
/*
* Restore transaction and statement start-time timestamps. This must
* happen before anything that would start a transaction, else asserts in
* xact.c will fire.
*/
SetParallelStartTimestamps(fps->xact_ts, fps->stmt_ts);
/*
* Identify the entry point to be called. In theory this could result in
* loading an additional library, though most likely the entry point is in