Derive oldestActiveXid at correct time for Hot Standby.

There was a timing window between when oldestActiveXid was derived
and when it should have been derived that only shows itself under
heavy load. Move code around to ensure correct timing of derivation.
No change to StartupSUBTRANS() code, which is where this failed.

Bug report by Chris Redekop

diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index bf57b3bf49e08635340d5e7b278d5d76e4460bb5..0944aa4162d4032e5c3cade1aa430030eee921ef 100644 (file)
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -7636,6 +7636,16 @@ CreateCheckPoint(int flags)
        MemSet(&checkPoint, 0, sizeof(checkPoint));
        checkPoint.time = (pg_time_t) time(NULL);
 
+       /*
+        * For Hot Standby, derive the oldestActiveXid before we fix the redo pointer.
+        * This allows us to begin accumulating changes to assemble our starting
+        * snapshot of locks and transactions.
+        */
+       if (!shutdown && XLogStandbyInfoActive())
+               checkPoint.oldestActiveXid = GetOldestActiveTransactionId();
+       else
+               checkPoint.oldestActiveXid = InvalidTransactionId;
+
        /*
         * We must hold WALInsertLock while examining insert state to determine
         * the checkpoint REDO pointer.
@@ -7822,9 +7832,7 @@ CreateCheckPoint(int flags)
         * Update checkPoint.nextXid since we have a later value
         */
        if (!shutdown && XLogStandbyInfoActive())
-               LogStandbySnapshot(&checkPoint.oldestActiveXid, &checkPoint.nextXid);
-       else
-               checkPoint.oldestActiveXid = InvalidTransactionId;
+               LogStandbySnapshot(&checkPoint.nextXid);
 
        START_CRIT_SECTION();
 

diff --git a/src/backend/storage/ipc/procarray.c b/src/backend/storage/ipc/procarray.c
index 0284ac4082395d5e1184a557ca3c60e5e717a95c..1a48485f970b6a3427cc59b446e693b52e577725 100644 (file)
--- a/src/backend/storage/ipc/procarray.c
+++ b/src/backend/storage/ipc/procarray.c
@@ -1620,6 +1620,63 @@ GetRunningTransactionData(void)
        return CurrentRunningXacts;
 }
 
+/*
+ * GetOldestActiveTransactionId()
+ *
+ * Similar to GetSnapshotData but returns just oldestActiveXid. We include
+ * all PGPROCs with an assigned TransactionId, even VACUUM processes.
+ * We look at all databases, though there is no need to include WALSender
+ * since this has no effect on hot standby conflicts.
+ *
+ * This is never executed during recovery so there is no need to look at
+ * KnownAssignedXids.
+ *
+ * We don't worry about updating other counters, we want to keep this as
+ * simple as possible and leave GetSnapshotData() as the primary code for
+ * that bookkeeping.
+ */
+TransactionId
+GetOldestActiveTransactionId(void)
+{
+       ProcArrayStruct *arrayP = procArray;
+       TransactionId oldestRunningXid;
+       int                     index;
+
+       Assert(!RecoveryInProgress());
+
+       LWLockAcquire(ProcArrayLock, LW_SHARED);
+
+       oldestRunningXid = ShmemVariableCache->nextXid;
+
+       /*
+        * Spin over procArray collecting all xids and subxids.
+        */
+       for (index = 0; index < arrayP->numProcs; index++)
+       {
+               volatile PGPROC *proc = arrayP->procs[index];
+               TransactionId xid;
+
+               /* Fetch xid just once - see GetNewTransactionId */
+               xid = proc->xid;
+
+               if (!TransactionIdIsNormal(xid))
+                       continue;
+
+               if (TransactionIdPrecedes(xid, oldestRunningXid))
+                       oldestRunningXid = xid;
+
+               /*
+                * Top-level XID of a transaction is always less than any of its
+                * subxids, so we don't need to check if any of the subxids are
+                * smaller than oldestRunningXid
+                */
+       }
+
+       LWLockRelease(ProcArrayLock);
+
+       return oldestRunningXid;
+}
+
 /*
  * GetTransactionsInCommit -- Get the XIDs of transactions that are committing
  *

diff --git a/src/backend/storage/ipc/standby.c b/src/backend/storage/ipc/standby.c
index 72c6b97b225854fd6572415be13421f53922671e..3f2fc8136811eb894f01543c7825202756f34e75 100644 (file)
--- a/src/backend/storage/ipc/standby.c
+++ b/src/backend/storage/ipc/standby.c
@@ -815,7 +815,7 @@ standby_desc(StringInfo buf, uint8 xl_info, char *rec)
  * making WAL entries.
  */
 void
-LogStandbySnapshot(TransactionId *oldestActiveXid, TransactionId *nextXid)
+LogStandbySnapshot(TransactionId *nextXid)
 {
        RunningTransactions running;
        xl_standby_lock *locks;
@@ -845,7 +845,6 @@ LogStandbySnapshot(TransactionId *oldestActiveXid, TransactionId *nextXid)
        /* GetRunningTransactionData() acquired XidGenLock, we must release it */
        LWLockRelease(XidGenLock);
 
-       *oldestActiveXid = running->oldestRunningXid;
        *nextXid = running->nextXid;
 }
 

diff --git a/src/include/storage/procarray.h b/src/include/storage/procarray.h
index 71c82437cdf84322e4d743109d5b6a403ed806bf..3e80cc5c075cd97e400dc35b77ba2a2d49e3454e 100644 (file)
--- a/src/include/storage/procarray.h
+++ b/src/include/storage/procarray.h
@@ -50,6 +50,7 @@ extern RunningTransactions GetRunningTransactionData(void);
 extern bool TransactionIdIsInProgress(TransactionId xid);
 extern bool TransactionIdIsActive(TransactionId xid);
 extern TransactionId GetOldestXmin(bool allDbs, bool ignoreVacuum);
+extern TransactionId GetOldestActiveTransactionId(void);
 
 extern int     GetTransactionsInCommit(TransactionId **xids_p);
 extern bool HaveTransactionsInCommit(TransactionId *xids, int nxids);

diff --git a/src/include/storage/standby.h b/src/include/storage/standby.h
index 6ebac62db5413d2fcaf26d658808656c90451b6d..e587a5c4a4dcb48d7c2d15c031590788ce2912b0 100644 (file)
--- a/src/include/storage/standby.h
+++ b/src/include/storage/standby.h
@@ -111,6 +111,6 @@ typedef RunningTransactionsData *RunningTransactions;
 extern void LogAccessExclusiveLock(Oid dbOid, Oid relOid);
 extern void LogAccessExclusiveLockPrepare(void);
 
-extern void LogStandbySnapshot(TransactionId *oldestActiveXid, TransactionId *nextXid);
+extern void LogStandbySnapshot(TransactionId *nextXid);
 
 #endif   /* STANDBY_H */