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 83b4d432728bc96c2d48b904fcfbd4161f2935ad..51685d52e02fae835e08a84df0e7ef8ce9257931 100644 (file)
--- a/src/backend/access/transam/multixact.c
+++ b/src/backend/access/transam/multixact.c
@@ -89,6 +89,8 @@
 #define MXOffsetToMemberEntry(xid) \
    ((xid) % (TransactionId) MULTIXACT_MEMBERS_PER_PAGE)
 
+#define PreviousMultiXactId(xid) \
+   ((xid) == FirstMultiXactId ? MaxMultiXactId : (xid) - 1)
 
 /*
  * Links to shared-memory data structures for MultiXact control
@@ -1907,17 +1909,21 @@ TruncateMultiXact(void)
    }
 
    /*
-    * The cutoff point is the start of the segment containing oldestMXact. We
-    * pass the *page* containing oldestMXact to SimpleLruTruncate.
+    * The cutoff point is the start of the segment containing oldestMXact.
+    * 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.
     */
-   cutoffPage = MultiXactIdToOffsetPage(oldestMXact);
+   cutoffPage = MultiXactIdToOffsetPage(PreviousMultiXactId(oldestMXact));
 
    SimpleLruTruncate(MultiXactOffsetCtl, cutoffPage);
 
    /*
     * Also truncate MultiXactMember at the previously determined offset.
     */
-   cutoffPage = MXOffsetToMemberPage(oldestOffset);
+   cutoffPage = MXOffsetToMemberPage(oldestOffset - 1);
 
    SimpleLruTruncate(MultiXactMemberCtl, cutoffPage);
 

diff --git a/src/backend/access/transam/subtrans.c b/src/backend/access/transam/subtrans.c
index 6665171a7673ddf4311b4575b8dc7d9bda7f04ef..2ad7477d430ecf2b044758b54aa5ff415231f77c 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);

diff --git a/src/include/access/multixact.h b/src/include/access/multixact.h
index 7f46c3eeea4e8fa2db817cda50570650ce73979f..22419bbf10013903a6ed539cd1f94a89fb8a42bd 100644 (file)
--- a/src/include/access/multixact.h
+++ b/src/include/access/multixact.h
@@ -15,6 +15,7 @@
 
 #define InvalidMultiXactId ((MultiXactId) 0)
 #define FirstMultiXactId   ((MultiXactId) 1)
+#define MaxMultiXactId     ((MultiXactId) 0xFFFFFFFF)
 
 #define MultiXactIdIsValid(multi) ((multi) != InvalidMultiXactId)