Propagate xactStartTimestamp and stmtStartTimestamp to parallel workers.

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

diff --git a/src/backend/access/transam/parallel.c b/src/backend/access/transam/parallel.c
index 82b2f3e1466c18640df8a8c22c371dd9adac2309..0121c68bd291fbf3a2c946d641e1b9cb56ed47d9 100644 (file)
--- a/src/backend/access/transam/parallel.c
+++ b/src/backend/access/transam/parallel.c
@@ -78,6 +78,8 @@ 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 (deprecated)! */
        parallel_worker_main_type entrypoint;
@@ -324,6 +326,8 @@ 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;
@@ -1148,6 +1152,13 @@ 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

diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c
index 225775ceb4aae1e2947ee94d347599470a8a9a4d..a2981f5515ce6b5fa20cbd168c6ac9f56d67102b 100644 (file)
--- a/src/backend/access/transam/xact.c
+++ b/src/backend/access/transam/xact.c
@@ -698,6 +698,22 @@ GetCurrentCommandId(bool used)
        return currentCommandId;
 }
 
+/*
+ *     SetParallelStartTimestamps
+ *
+ * In a parallel worker, we should inherit the parent transaction's
+ * timestamps rather than setting our own.  The parallel worker
+ * infrastructure must call this to provide those values before
+ * calling StartTransaction() or SetCurrentStatementStartTimestamp().
+ */
+void
+SetParallelStartTimestamps(TimestampTz xact_ts, TimestampTz stmt_ts)
+{
+       Assert(IsParallelWorker());
+       xactStartTimestamp = xact_ts;
+       stmtStartTimestamp = stmt_ts;
+}
+
 /*
  *     GetCurrentTransactionStartTimestamp
  */
@@ -732,11 +748,17 @@ GetCurrentTransactionStopTimestamp(void)
 
 /*
  *     SetCurrentStatementStartTimestamp
+ *
+ * In a parallel worker, this should already have been provided by a call
+ * to SetParallelStartTimestamps().
  */
 void
 SetCurrentStatementStartTimestamp(void)
 {
-       stmtStartTimestamp = GetCurrentTimestamp();
+       if (!IsParallelWorker())
+               stmtStartTimestamp = GetCurrentTimestamp();
+       else
+               Assert(stmtStartTimestamp != 0);
 }
 
 /*
@@ -1889,10 +1911,16 @@ StartTransaction(void)
        /*
         * set transaction_timestamp() (a/k/a now()).  We want this to be the same
         * as the first command's statement_timestamp(), so don't do a fresh
-        * GetCurrentTimestamp() call (which'd be expensive anyway).  Also, mark
-        * xactStopTimestamp as unset.
+        * GetCurrentTimestamp() call (which'd be expensive anyway).  In a
+        * parallel worker, this should already have been provided by a call to
+        * SetParallelStartTimestamps().
+        *
+        * Also, mark xactStopTimestamp as unset.
         */
-       xactStartTimestamp = stmtStartTimestamp;
+       if (!IsParallelWorker())
+               xactStartTimestamp = stmtStartTimestamp;
+       else
+               Assert(xactStartTimestamp != 0);
        xactStopTimestamp = 0;
        pgstat_report_xact_timestamp(xactStartTimestamp);
 

diff --git a/src/include/access/xact.h b/src/include/access/xact.h
index db917bb226af5efb5a03f17454dc280659ef1680..243811c96fb5f489724dd8715a623d7cb88ee700 100644 (file)
--- a/src/include/access/xact.h
+++ b/src/include/access/xact.h
@@ -318,6 +318,7 @@ extern SubTransactionId GetCurrentSubTransactionId(void);
 extern void MarkCurrentTransactionIdLoggedIfAny(void);
 extern bool SubTransactionIsActive(SubTransactionId subxid);
 extern CommandId GetCurrentCommandId(bool used);
+extern void SetParallelStartTimestamps(TimestampTz xact_ts, TimestampTz stmt_ts);
 extern TimestampTz GetCurrentTransactionStartTimestamp(void);
 extern TimestampTz GetCurrentStatementStartTimestamp(void);
 extern TimestampTz GetCurrentTransactionStopTimestamp(void);