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 a9edf7b9e882f5ccb09745cf046e10d8e0bbff9b..2f12680a88062dba2ea0fc2b5044bbc5a2b6736f 100644 (file)
--- a/src/backend/access/transam/parallel.c
+++ b/src/backend/access/transam/parallel.c
@@ -86,6 +86,8 @@ 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;
@@ -321,6 +323,8 @@ 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);
@@ -1303,6 +1307,13 @@ 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 9aa63c8792be56e4186ef2bbfc12198c00d3b6c3..88bf1f435ab3b56674729181099e3284a218b5ed 100644 (file)
--- a/src/backend/access/transam/xact.c
+++ b/src/backend/access/transam/xact.c
@@ -693,6 +693,22 @@ 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
  */
@@ -727,11 +743,17 @@ 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);
 }
 
 /*
@@ -1886,10 +1908,16 @@ 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 083e879d5c33641ccb62f930b1550863e910709f..689c57c59269263da84f918300f10571cd5afb61 100644 (file)
--- a/src/include/access/xact.h
+++ b/src/include/access/xact.h
@@ -359,6 +359,7 @@ 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);