Reorder code so that we don't have to hold a critical section while

reserving SLRU space for a new MultiXact.  The original coding would have
treated out-of-disk-space as a PANIC condition, which is unnecessary.

diff --git a/src/backend/access/transam/multixact.c b/src/backend/access/transam/multixact.c
index fcc2546..af254da 100644 (file)
--- a/src/backend/access/transam/multixact.c
+++ b/src/backend/access/transam/multixact.c
@@ -42,7 +42,7 @@
  * Portions Copyright (c) 1996-2005, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/backend/access/transam/multixact.c,v 1.10 2005/10/28 17:27:29 tgl Exp $
+ * $PostgreSQL: pgsql/src/backend/access/transam/multixact.c,v 1.11 2005/10/28 19:00:19 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -631,21 +631,12 @@ CreateMultiXactId(int nxids, TransactionId *xids)
        }
 
        /*
-        * Critical section from here until we've written the data; we don't
-        * want to error out with a partly written MultiXact structure.
-        * (In particular, failing to write our start offset after advancing
-        * nextMXact would effectively corrupt the previous MultiXact.)
-        */
-       START_CRIT_SECTION();
-
-       /*
         * Assign the MXID and offsets range to use, and make sure there is
-        * space in the OFFSETs and MEMBERs files.
+        * space in the OFFSETs and MEMBERs files.  NB: this routine does
+        * START_CRIT_SECTION().
         */
        multi = GetNewMultiXactId(nxids, &offset);
 
-       debug_elog4(DEBUG2, "Create: assigned id %u offset %u", multi, offset);
-
        /*
         * Make an XLOG entry describing the new MXID.
         *
@@ -768,6 +759,9 @@ RecordNewMultiXact(MultiXactId multi, MultiXactOffset offset,
  * files.  Unfortunately, we have to do that while holding MultiXactGenLock
  * to avoid race conditions --- the XLOG record for zeroing a page must appear
  * before any backend can possibly try to store data in that page!
+ *
+ * We start a critical section before advancing the shared counters.  The
+ * caller must end the critical section after writing SLRU data.
  */
 static MultiXactId
 GetNewMultiXactId(int nxids, MultiXactOffset *offset)
@@ -794,18 +788,7 @@ GetNewMultiXactId(int nxids, MultiXactOffset *offset)
        ExtendMultiXactOffset(result);
 
        /*
-        * Advance counter.  As in GetNewTransactionId(), this must not happen
-        * until after ExtendMultiXactOffset has succeeded!
-        *
-        * We don't care about MultiXactId wraparound here; it will be handled by
-        * the next iteration.  But note that nextMXact may be InvalidMultiXactId
-        * after this routine exits, so anyone else looking at the variable must
-        * be prepared to deal with that.
-        */
-       (MultiXactState->nextMXact)++;
-
-       /*
-        * Reserve the members space.  Same considerations as above.  Also, be
+        * Reserve the members space, similarly to above.  Also, be
         * careful not to return zero as the starting offset for any multixact.
         * See GetMultiXactIdMembers() for motivation.
         */
@@ -820,6 +803,27 @@ GetNewMultiXactId(int nxids, MultiXactOffset *offset)
 
        ExtendMultiXactMember(nextOffset, nxids);
 
+       /*
+        * Critical section from here until caller has written the data into
+        * the just-reserved SLRU space; we don't want to error out with a partly
+        * written MultiXact structure.  (In particular, failing to write our
+        * start offset after advancing nextMXact would effectively corrupt the
+        * previous MultiXact.)
+        */
+       START_CRIT_SECTION();
+
+       /*
+        * Advance counters.  As in GetNewTransactionId(), this must not happen
+        * until after file extension has succeeded!
+        *
+        * We don't care about MultiXactId wraparound here; it will be handled by
+        * the next iteration.  But note that nextMXact may be InvalidMultiXactId
+        * after this routine exits, so anyone else looking at the variable must
+        * be prepared to deal with that.  Similarly, nextOffset may be zero,
+        * but we won't use that as the actual start offset of the next multixact.
+        */
+       (MultiXactState->nextMXact)++;
+
        MultiXactState->nextOffset += nxids;
 
        LWLockRelease(MultiXactGenLock);