Fix off-by-one error in calculating subtrans/multixact truncation point.

If there were no subtransactions (or multixacts) active, we would calculate
the oldestxid == next xid. That's correct, but if next XID happens to be
on the next pg_subtrans (pg_multixact) page, the page does not exist yet,
and SimpleLruTruncate will produce an "apparent wraparound" warning. The
warning is harmless in this case, but looks very alarming to users.

Backpatch to all supported versions. Patch and analysis by Thomas Munro.

diff --git a/src/backend/access/transam/multixact.c b/src/backend/access/transam/multixact.c
index 377d0842bdd9d0f77588ba58b4fdeb4bc00f6922..1933a87d6568e68a6a7466ee442f95c16bb549b8 100644 (file)
--- a/src/backend/access/transam/multixact.c
+++ b/src/backend/access/transam/multixact.c
@@ -173,6 +173,8 @@
 #define MULTIXACT_MEMBER_DANGER_THRESHOLD      \
        (MaxMultiXactOffset - MaxMultiXactOffset / 4)
 
+#define PreviousMultiXactId(xid) \
+       ((xid) == FirstMultiXactId ? MaxMultiXactId : (xid) - 1)
 
 /*
  * Links to shared-memory data structures for MultiXact control
@@ -3057,10 +3059,15 @@ TruncateMultiXact(void)
 
        SlruScanDirectory(MultiXactMemberCtl, SlruScanDirCbRemoveMembers, &range);
 
-       /* Now we can truncate MultiXactOffset */
+       /*
+        * Now we can truncate MultiXactOffset.  We step back one multixact to
+        * avoid passing a cutoff page that hasn't been created yet in the rare
+        * case that oldestMXact would be the first item on a page and oldestMXact
+        * == nextMXact.  In that case, if we didn't subtract one, we'd trigger
+        * SimpleLruTruncate's wraparound detection.
+        */
        SimpleLruTruncate(MultiXactOffsetCtl,
-                                         MultiXactIdToOffsetPage(oldestMXact));
-
+                                 MultiXactIdToOffsetPage(PreviousMultiXactId(oldestMXact)));
 
        /*
         * Now, and only now, we can advance the stop point for multixact members.

diff --git a/src/backend/access/transam/subtrans.c b/src/backend/access/transam/subtrans.c
index 4bc24d9bbcbc4fbdb9660f237d730d6f77432b27..6b709823227803b6975c5a962e2dced57a8e0f97 100644 (file)
--- a/src/backend/access/transam/subtrans.c
+++ b/src/backend/access/transam/subtrans.c
@@ -340,8 +340,13 @@ TruncateSUBTRANS(TransactionId oldestXact)
 
        /*
         * The cutoff point is the start of the segment containing oldestXact. We
-        * pass the *page* containing oldestXact to SimpleLruTruncate.
+        * pass the *page* containing oldestXact to SimpleLruTruncate.  We step
+        * back one transaction to avoid passing a cutoff page that hasn't been
+        * created yet in the rare case that oldestXact would be the first item on
+        * a page and oldestXact == next XID.  In that case, if we didn't subtract
+        * one, we'd trigger SimpleLruTruncate's wraparound detection.
         */
+       TransactionIdRetreat(oldestXact);
        cutoffPage = TransactionIdToPage(oldestXact);
 
        SimpleLruTruncate(SubTransCtl, cutoffPage);