Set the all-visible flag on heap page before writing WAL record, not after.

If we set the all-visible flag after writing WAL record, and XLogInsert
takes a full-page image of the page, the image would not include the flag.
We will then proceed to set the VM bit, which would then be set without the
corresponding all-visible flag on the heap page.

Found by comparing page images on master and standby, after writing/replaying
each WAL record. (There is still a discrepancy: the all-visible flag won't
be set after replaying the HEAP_CLEAN record, even though it is set in the
master. However, it will be set when replaying the HEAP2_VISIBLE record and
setting the VM bit, so the all-visible flag and VM bit are always consistent
on the standby, even though they are momentarily out-of-sync with master)

Backpatch to 9.3 where this code was introduced.

diff --git a/src/backend/commands/vacuumlazy.c b/src/backend/commands/vacuumlazy.c
index ce4172cb100873a63df7223e66478966b6c7e6a4..2eec4e74d97254862752aa037eff0c3c0d4bca53 100644 (file)
--- a/src/backend/commands/vacuumlazy.c
+++ b/src/backend/commands/vacuumlazy.c
@@ -1206,6 +1206,13 @@ lazy_vacuum_page(Relation onerel, BlockNumber blkno, Buffer buffer,
 
    PageRepairFragmentation(page);
 
+   /*
+    * Now that we have removed the dead tuples from the page, once again
+    * check if the page has become all-visible.
+    */
+   if (heap_page_is_all_visible(buffer, &visibility_cutoff_xid))
+       PageSetAllVisible(page);
+
    /*
     * Mark buffer dirty before we write WAL.
     */
@@ -1224,14 +1231,13 @@ lazy_vacuum_page(Relation onerel, BlockNumber blkno, Buffer buffer,
    }
 
    /*
-    * Now that we have removed the dead tuples from the page, once again
-    * check if the page has become all-visible.
+    * All the changes to the heap page have been done. If the all-visible
+    * flag is now set, also set the VM bit.
     */
-   if (!visibilitymap_test(onerel, blkno, vmbuffer) &&
-       heap_page_is_all_visible(buffer, &visibility_cutoff_xid))
+   if (PageIsAllVisible(page) &&
+       !visibilitymap_test(onerel, blkno, vmbuffer))
    {
        Assert(BufferIsValid(*vmbuffer));
-       PageSetAllVisible(page);
        visibilitymap_set(onerel, blkno, buffer, InvalidXLogRecPtr, *vmbuffer,
                          visibility_cutoff_xid);
    }