Correct StartupSUBTRANS for page wraparound

StartupSUBTRANS() incorrectly handled cases near the max pageid in the subtrans
data structure, which in some cases could lead to errors in startup for Hot
Standby.
This patch wraps the pageids correctly, avoiding any such errors.
Identified by exhaustive crash testing by Jeff Janes.

Jeff Janes

diff --git a/src/backend/access/transam/subtrans.c b/src/backend/access/transam/subtrans.c
index 6b709823227803b6975c5a962e2dced57a8e0f97..dde72b7d2e1cd073a2b043f8f6f7d5470da9cce8 100644 (file)
--- a/src/backend/access/transam/subtrans.c
+++ b/src/backend/access/transam/subtrans.c
@@ -44,7 +44,8 @@
  * 0xFFFFFFFF/SUBTRANS_XACTS_PER_PAGE, and segment numbering at
  * 0xFFFFFFFF/SUBTRANS_XACTS_PER_PAGE/SLRU_PAGES_PER_SEGMENT.  We need take no
  * explicit notice of that fact in this module, except when comparing segment
- * and page numbers in TruncateSUBTRANS (see SubTransPagePrecedes).
+ * and page numbers in TruncateSUBTRANS (see SubTransPagePrecedes) and zeroing
+ * them in StartupSUBTRANS.
  */
 
 /* We need four bytes per xact */
@@ -253,6 +254,9 @@ StartupSUBTRANS(TransactionId oldestActiveXID)
    {
        (void) ZeroSUBTRANSPage(startPage);
        startPage++;
+       /* must account for wraparound */
+       if (startPage > TransactionIdToPage(MaxTransactionId))
+           startPage=0;
    }
    (void) ZeroSUBTRANSPage(startPage);