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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 12:00:10 -04:00
parent 0dc6bf633a
commit 3c9dd963ce
3 changed files with 44 additions and 4 deletions
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11
src/backend/access/transam/parallel.c
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@@ -71,6 +71,8 @@ typedef struct FixedParallelState
PGPROC *parallel_master_pgproc;
pid_t parallel_master_pid;
BackendId parallel_master_backend_id;
TimestampTz xact_ts;
TimestampTz stmt_ts;
/* Entrypoint for parallel workers. */
parallel_worker_main_type entrypoint;
@@ -282,6 +284,8 @@ 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();
fps->entrypoint = pcxt->entrypoint;
SpinLockInit(&fps->mutex);
fps->last_xlog_end = 0;
@@ -924,6 +928,13 @@ ParallelWorkerMain(Datum main_arg)
* backend-local state to match the original backend.
*/
/*
* 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);
/*
* Load libraries that were loaded by original backend. We want to do
* this before restoring GUCs, because the libraries might define custom