Add WAL consistency checking facility.

When the new GUC wal_consistency_checking is set to a non-empty value,
it triggers recording of additional full-page images, which are
compared on the standby against the results of applying the WAL record
(without regard to those full-page images).  Allowable differences
such as hints are masked out, and the resulting pages are compared;
any difference results in a FATAL error on the standby.

Kuntal Ghosh, based on earlier patches by Michael Paquier and Heikki
Linnakangas.  Extensively reviewed and revised by Michael Paquier and
by me, with additional reviews and comments from Amit Kapila, Álvaro
Herrera, Simon Riggs, and Peter Eisentraut.

1 files changed, 6 insertions, 5 deletions

diff --git a/src/backend/access/transam/xlogutils.c b/src/backend/access/transam/xlogutils.c
index 0de2419e54b..6627f5498b9 100644
--- a/src/backend/access/transam/xlogutils.c
+++ b/src/backend/access/transam/xlogutils.c
@@ -275,9 +275,9 @@ XLogCheckInvalidPages(void)
  * will complain if we don't have the lock.  In hot standby mode it's
  * definitely necessary.)
  *
- * Note: when a backup block is available in XLOG, we restore it
- * unconditionally, even if the page in the database appears newer.  This is
- * to protect ourselves against database pages that were partially or
+ * Note: when a backup block is available in XLOG with the BKPIMAGE_APPLY flag
+ * set, we restore it, even if the page in the database appears newer.  This
+ * is to protect ourselves against database pages that were partially or
  * incorrectly written during a crash.  We assume that the XLOG data must be
  * good because it has passed a CRC check, while the database page might not
  * be.  This will force us to replay all subsequent modifications of the page
@@ -352,9 +352,10 @@ XLogReadBufferForRedoExtended(XLogReaderState *record,
 	if (!willinit && zeromode)
 		elog(PANIC, "block to be initialized in redo routine must be marked with WILL_INIT flag in the WAL record");
 
-	/* If it's a full-page image, restore it. */
-	if (XLogRecHasBlockImage(record, block_id))
+	/* If it has a full-page image and it should be restored, do it. */
+	if (XLogRecBlockImageApply(record, block_id))
 	{
+		Assert(XLogRecHasBlockImage(record, block_id));
 		*buf = XLogReadBufferExtended(rnode, forknum, blkno,
 		   get_cleanup_lock ? RBM_ZERO_AND_CLEANUP_LOCK : RBM_ZERO_AND_LOCK);
 		page = BufferGetPage(*buf);