Still more code review for single-page hash vacuuming.

Most seriously, fix use of incorrect block ID, per a report from
Jeff Janes that it causes a crash and a diagnosis from Amit Kapila.

Improve consistency between the hash and btree versions of this
code by adding back a PANIC that btree has, and by registering
data in the xlog record in the same way, per complaints from
Jeff Janes and Amit Kapila.

Tidy up some minor cosmetic points, per complaints from Amit
Kapila.

Patch by Ashutosh Sharma, reviewed by Amit Kapila, and tested by
Jeff Janes.

Discussion: http://postgr.es/m/CAMkU=1w-9Qe=Ff1o6bSaXpNO9wqpo7_9GL8_CVhw4BoVVHasqg@mail.gmail.com

src/backend/access/hash/hash_xlog.c

@@ -957,8 +957,6 @@ hash_xlog_vacuum_get_latestRemovedXid(XLogReaderState *record)
 	OffsetNumber	hoffnum;
 	TransactionId	latestRemovedXid = InvalidTransactionId;
 	int		i;
-	char *ptr;
-	Size len;
 
 	xlrec = (xl_hash_vacuum_one_page *) XLogRecGetData(record);
 
@@ -976,13 +974,21 @@ hash_xlog_vacuum_get_latestRemovedXid(XLogReaderState *record)
 	if (CountDBBackends(InvalidOid) == 0)
 		return latestRemovedXid;
 
+	/*
+	 * Check if WAL replay has reached a consistent database state. If not,
+	 * we must PANIC. See the definition of btree_xlog_delete_get_latestRemovedXid
+	 * for more details.
+	 */
+	if (!reachedConsistency)
+		elog(PANIC, "hash_xlog_vacuum_get_latestRemovedXid: cannot operate with inconsistent data");
+
 	/*
 	 * Get index page.  If the DB is consistent, this should not fail, nor
 	 * should any of the heap page fetches below.  If one does, we return
 	 * InvalidTransactionId to cancel all HS transactions.  That's probably
 	 * overkill, but it's safe, and certainly better than panicking here.
 	 */
-	XLogRecGetBlockTag(record, 1, &rnode, NULL, &blkno);
+	XLogRecGetBlockTag(record, 0, &rnode, NULL, &blkno);
 	ibuffer = XLogReadBufferExtended(rnode, MAIN_FORKNUM, blkno, RBM_NORMAL);
 
 	if (!BufferIsValid(ibuffer))
@@ -994,9 +1000,7 @@ hash_xlog_vacuum_get_latestRemovedXid(XLogReaderState *record)
 	 * Loop through the deleted index items to obtain the TransactionId from
 	 * the heap items they point to.
 	 */
-	ptr = XLogRecGetBlockData(record, 1, &len);
-
-	unused = (OffsetNumber *) ptr;
+	unused = (OffsetNumber *) ((char *) xlrec + SizeOfHashVacuumOnePage);
 
 	for (i = 0; i < xlrec->ntuples; i++)
 	{
@@ -1121,23 +1125,15 @@ hash_xlog_vacuum_one_page(XLogReaderState *record)
 
 	if (action == BLK_NEEDS_REDO)
 	{
-		char *ptr;
-		Size len;
-
-		ptr = XLogRecGetBlockData(record, 0, &len);
-
 		page = (Page) BufferGetPage(buffer);
 
-		if (len > 0)
+		if (XLogRecGetDataLen(record) > SizeOfHashVacuumOnePage)
 		{
 			OffsetNumber *unused;
-			OffsetNumber *unend;
 
-			unused = (OffsetNumber *) ptr;
-			unend = (OffsetNumber *) ((char *) ptr + len);
+			unused = (OffsetNumber *) ((char *) xldata + SizeOfHashVacuumOnePage);
 
-			if ((unend - unused) > 0)
-				PageIndexMultiDelete(page, unused, unend - unused);
+			PageIndexMultiDelete(page, unused, xldata->ntuples);
 		}
 
 		/*

src/backend/access/hash/hashinsert.c

@@ -344,7 +344,6 @@ _hash_vacuum_one_page(Relation rel, Buffer metabuf, Buffer buf,
 	Page	page = BufferGetPage(buf);
 	HashPageOpaque	pageopaque;
 	HashMetaPage	metap;
-	double tuples_removed = 0;
 
 	/* Scan each tuple in page to see if it is marked as LP_DEAD */
 	maxoff = PageGetMaxOffsetNumber(page);
@@ -355,10 +354,7 @@ _hash_vacuum_one_page(Relation rel, Buffer metabuf, Buffer buf,
 		ItemId	itemId = PageGetItemId(page, offnum);
 
 		if (ItemIdIsDead(itemId))
-		{
 			deletable[ndeletable++] = offnum;
-			tuples_removed += 1;
-		}
 	}
 
 	if (ndeletable > 0)
@@ -386,7 +382,7 @@ _hash_vacuum_one_page(Relation rel, Buffer metabuf, Buffer buf,
 		pageopaque->hasho_flag &= ~LH_PAGE_HAS_DEAD_TUPLES;
 
 		metap = HashPageGetMeta(BufferGetPage(metabuf));
-		metap->hashm_ntuples -= tuples_removed;
+		metap->hashm_ntuples -= ndeletable;
 
 		MarkBufferDirty(buf);
 		MarkBufferDirty(metabuf);
@@ -398,13 +394,18 @@ _hash_vacuum_one_page(Relation rel, Buffer metabuf, Buffer buf,
 			XLogRecPtr	recptr;
 
 			xlrec.hnode = hnode;
-			xlrec.ntuples = tuples_removed;
+			xlrec.ntuples = ndeletable;
 
 			XLogBeginInsert();
+			XLogRegisterBuffer(0, buf, REGBUF_STANDARD);
 			XLogRegisterData((char *) &xlrec, SizeOfHashVacuumOnePage);
 
-			XLogRegisterBuffer(0, buf, REGBUF_STANDARD);
-			XLogRegisterBufData(0, (char *) deletable,
+			/*
+			 * We need the target-offsets array whether or not we store the whole
+			 * buffer, to allow us to find the latestRemovedXid on a standby
+			 * server.
+			 */
+			XLogRegisterData((char *) deletable,
 						ndeletable * sizeof(OffsetNumber));
 
 			XLogRegisterBuffer(1, metabuf, REGBUF_STANDARD);

src/backend/access/rmgrdesc/hashdesc.c

@@ -113,7 +113,7 @@ hash_desc(StringInfo buf, XLogReaderState *record)
 			{
 				xl_hash_vacuum_one_page *xlrec = (xl_hash_vacuum_one_page *) rec;
 
-				appendStringInfo(buf, "ntuples %g",
+				appendStringInfo(buf, "ntuples %d",
 								 xlrec->ntuples);
 				break;
 			}

src/include/access/hash_xlog.h

@@ -265,11 +265,13 @@ typedef struct xl_hash_init_bitmap_page
 typedef struct xl_hash_vacuum_one_page
 {
 	RelFileNode	hnode;
-	double		ntuples;
+	int		ntuples;
+
+	/* TARGET OFFSET NUMBERS FOLLOW AT THE END */
 }	xl_hash_vacuum_one_page;
 
 #define SizeOfHashVacuumOnePage	\
-	(offsetof(xl_hash_vacuum_one_page, ntuples) + sizeof(double))
+	(offsetof(xl_hash_vacuum_one_page, ntuples) + sizeof(int))
 
 extern void hash_redo(XLogReaderState *record);
 extern void hash_desc(StringInfo buf, XLogReaderState *record);