Don't try to fetch database name when SetTransactionIdLimit() is executed

outside a transaction.

This repairs brain fade in my patch of 2009-08-30: the reason we had been
storing oldest-database name, not OID, in ShmemVariableCache was of course
to avoid having to do a catalog lookup at times when it might be unsafe.

This error explains why Aleksandr Dushein is having trouble getting out of
an XID wraparound state in bug #5718, though not how he got into that state
in the first place.  I suspect pg_upgrade is at fault there.

diff --git a/src/backend/access/transam/varsup.c b/src/backend/access/transam/varsup.c
index 4f3c0ae..8886897 100644 (file)
--- a/src/backend/access/transam/varsup.c
+++ b/src/backend/access/transam/varsup.c
@@ -16,6 +16,7 @@
 #include "access/clog.h"
 #include "access/subtrans.h"
 #include "access/transam.h"
+#include "access/xact.h"
 #include "commands/dbcommands.h"
 #include "miscadmin.h"
 #include "postmaster/autovacuum.h"
@@ -346,13 +347,22 @@ SetTransactionIdLimit(TransactionId oldest_datfrozenxid, Oid oldest_datoid)
        /* Give an immediate warning if past the wrap warn point */
        if (TransactionIdFollowsOrEquals(curXid, xidWarnLimit) && !InRecovery)
        {
-               char       *oldest_datname = get_database_name(oldest_datoid);
+               char       *oldest_datname;
 
                /*
-                * Note: it's possible that get_database_name fails and returns NULL,
-                * for example because the database just got dropped.  We'll still
-                * warn, even though the warning might now be unnecessary.
+                * We can be called when not inside a transaction, for example
+                * during StartupXLOG().  In such a case we cannot do database
+                * access, so we must just report the oldest DB's OID.
+                *
+                * Note: it's also possible that get_database_name fails and returns
+                * NULL, for example because the database just got dropped.  We'll
+                * still warn, even though the warning might now be unnecessary.
                 */
+               if (IsTransactionState())
+                       oldest_datname = get_database_name(oldest_datoid);
+               else
+                       oldest_datname = NULL;
+
                if (oldest_datname)
                        ereport(WARNING,
                        (errmsg("database \"%s\" must be vacuumed within %u transactions",