Whoops, I was a tad too enthusiastic about using shared lock mode for

SInvalLock.  GetSnapshotData(true) has to use exclusive lock, since
it sets MyProc->xmin.

diff --git a/src/backend/storage/ipc/sinval.c b/src/backend/storage/ipc/sinval.c
index 24506b9..4a3b111 100644 (file)
--- a/src/backend/storage/ipc/sinval.c
+++ b/src/backend/storage/ipc/sinval.c
@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *       $Header: /cvsroot/pgsql/src/backend/storage/ipc/sinval.c,v 1.41 2001/09/29 04:02:24 tgl Exp $
+ *       $Header: /cvsroot/pgsql/src/backend/storage/ipc/sinval.c,v 1.42 2001/09/29 15:29:48 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -310,19 +310,24 @@ GetSnapshotData(bool serializable)
        if (snapshot == NULL)
                elog(ERROR, "Memory exhausted in GetSnapshotData");
 
-       snapshot->xmin = GetCurrentTransactionId();
-
-       LWLockAcquire(SInvalLock, LW_SHARED);
-
        /*
-        * There can be no more than lastBackend active transactions, so this
-        * is enough space:
+        * Allocating space for MaxBackends xids is usually overkill;
+        * lastBackend would be sufficient.  But it seems better to do the
+        * malloc while not holding the lock, so we can't look at lastBackend.
         */
        snapshot->xip = (TransactionId *)
-               malloc(segP->lastBackend * sizeof(TransactionId));
+               malloc(MaxBackends * sizeof(TransactionId));
        if (snapshot->xip == NULL)
                elog(ERROR, "Memory exhausted in GetSnapshotData");
 
+       snapshot->xmin = GetCurrentTransactionId();
+
+       /*
+        * If we are going to set MyProc->xmin then we'd better get exclusive
+        * lock; if not, this is a read-only operation so it can be shared.
+        */
+       LWLockAcquire(SInvalLock, serializable ? LW_EXCLUSIVE : LW_SHARED);
+
        /*--------------------
         * Unfortunately, we have to call ReadNewTransactionId() after acquiring
         * SInvalLock above.  It's not good because ReadNewTransactionId() does