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 e1dcac8d828caadf1705c2395909a095406d5e5d..d7964f4e180ba4b17fd2dd681b4677790e7e5966 100644 (file)
--- a/src/backend/access/transam/parallel.c
+++ b/src/backend/access/transam/parallel.c
@@ -79,6 +79,8 @@ 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;
@@ -289,6 +291,8 @@ 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);
@@ -1118,6 +1122,13 @@ 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 9004e38e6d4405f35161da3b6d55c6fb13c2220b..b028b318e5c88cc071d3cf77ef88700f94cb1af9 100644 (file)
--- a/src/backend/access/transam/xact.c
+++ b/src/backend/access/transam/xact.c
@@ -701,6 +701,22 @@ 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
  */
@@ -735,11 +751,17 @@ 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);
 }
 
 /*
@@ -1893,10 +1915,16 @@ 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 ad5aad96df0cdcc11e1d022dd07c84a9a4307545..905f42c2b1d634049d96e65c2776340e579f2992 100644 (file)
--- a/src/include/access/xact.h
+++ b/src/include/access/xact.h
@@ -337,6 +337,7 @@ 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);