Fix WAL-logging of FSM and VM truncation.

When a relation is truncated, it is important that the FSM is truncated as
well. Otherwise, after recovery, the FSM can return a page that has been
truncated away, leading to errors like:

ERROR:  could not read block 28991 in file "base/16390/572026": read only 0
of 8192 bytes

We were using MarkBufferDirtyHint() to dirty the buffer holding the last
remaining page of the FSM, but during recovery, that might in fact not
dirty the page, and the FSM update might be lost.

To fix, use the stronger MarkBufferDirty() function. MarkBufferDirty()
requires us to do WAL-logging ourselves, to protect from a torn page, if
checksumming is enabled.

Also fix an oversight in visibilitymap_truncate: it also needs to WAL-log
when checksumming is enabled.

Analysis by Pavan Deolasee.

Discussion: 

Backpatch to 9.3, where we got data checksums.

diff --git a/src/backend/access/heap/visibilitymap.c b/src/backend/access/heap/visibilitymap.c
index a0c0c7f2a6bbab00627c3d8e6c8495a9be959461..ed44a8c0d12947248940454005fa34ec53a37ba9 100644 (file)
--- a/src/backend/access/heap/visibilitymap.c
+++ b/src/backend/access/heap/visibilitymap.c
@@ -473,6 +473,9 @@ visibilitymap_truncate(Relation rel, BlockNumber nheapblocks)
 
        LockBuffer(mapBuffer, BUFFER_LOCK_EXCLUSIVE);
 
+       /* NO EREPORT(ERROR) from here till changes are logged */
+       START_CRIT_SECTION();
+
        /* Clear out the unwanted bytes. */
        MemSet(&map[truncByte + 1], 0, MAPSIZE - (truncByte + 1));
 
@@ -488,7 +491,20 @@ visibilitymap_truncate(Relation rel, BlockNumber nheapblocks)
         */
        map[truncByte] &= (1 << truncBit) - 1;
 
+       /*
+        * Truncation of a relation is WAL-logged at a higher-level, and we
+        * will be called at WAL replay. But if checksums are enabled, we need
+        * to still write a WAL record to protect against a torn page, if the
+        * page is flushed to disk before the truncation WAL record. We cannot
+        * use MarkBufferDirtyHint here, because that will not dirty the page
+        * during recovery.
+        */
        MarkBufferDirty(mapBuffer);
+       if (!InRecovery && RelationNeedsWAL(rel) && XLogHintBitIsNeeded())
+           log_newpage_buffer(mapBuffer, false);
+
+       END_CRIT_SECTION();
+
        UnlockReleaseBuffer(mapBuffer);
    }
    else

diff --git a/src/backend/storage/freespace/freespace.c b/src/backend/storage/freespace/freespace.c
index 8eee0ce80e639a595606ccc683e4e5e4a61f36e7..7362263e4186af78c1225fada692cdd6d073a8b6 100644 (file)
--- a/src/backend/storage/freespace/freespace.c
+++ b/src/backend/storage/freespace/freespace.c
@@ -23,7 +23,9 @@
  */
 #include "postgres.h"
 
+#include "access/heapam_xlog.h"
 #include "access/htup_details.h"
+#include "access/xlog.h"
 #include "access/xlogutils.h"
 #include "miscadmin.h"
 #include "storage/freespace.h"
@@ -285,8 +287,26 @@ FreeSpaceMapTruncateRel(Relation rel, BlockNumber nblocks)
        if (!BufferIsValid(buf))
            return;             /* nothing to do; the FSM was already smaller */
        LockBuffer(buf, BUFFER_LOCK_EXCLUSIVE);
+
+       /* NO EREPORT(ERROR) from here till changes are logged */
+       START_CRIT_SECTION();
+
        fsm_truncate_avail(BufferGetPage(buf), first_removed_slot);
-       MarkBufferDirtyHint(buf, false);
+
+       /*
+        * Truncation of a relation is WAL-logged at a higher-level, and we
+        * will be called at WAL replay. But if checksums are enabled, we need
+        * to still write a WAL record to protect against a torn page, if the
+        * page is flushed to disk before the truncation WAL record. We cannot
+        * use MarkBufferDirtyHint here, because that will not dirty the page
+        * during recovery.
+        */
+       MarkBufferDirty(buf);
+       if (!InRecovery && RelationNeedsWAL(rel) && XLogHintBitIsNeeded())
+           log_newpage_buffer(buf, false);
+
+       END_CRIT_SECTION();
+
        UnlockReleaseBuffer(buf);
 
        new_nfsmblocks = fsm_logical_to_physical(first_removed_address) + 1;