Make XLOG_FPI_FOR_HINT records honor full_page_writes setting.

Commit 2c03216d83 changed XLOG_FPI_FOR_HINT records so that they always
included full-page images even when full_page_writes was disabled. However,
in this setting, they don't need to do that because hint bit updates don't
need to be protected from torn writes.

Therefore, this commit makes XLOG_FPI_FOR_HINT records honor full_page_writes
setting. That is, XLOG_FPI_FOR_HINT records may include no full-page images
if full_page_writes is disabled, and WAL replay of them does nothing.

Reported-by: Zhang Wenjie
Author: Kyotaro Horiguchi
Reviewed-by: Fujii Masao
Discussion: https://postgr.es/m/tencent_60F11973A111EED97A8596FFECC4A91ED405@qq.com

src/backend/access/transam/xlog.c

@@ -10148,7 +10148,10 @@ xlog_redo(XLogReaderState *record)
 	uint8		info = XLogRecGetInfo(record) & ~XLR_INFO_MASK;
 	XLogRecPtr	lsn = record->EndRecPtr;
 
-	/* in XLOG rmgr, backup blocks are only used by XLOG_FPI records */
+	/*
+	 * In XLOG rmgr, backup blocks are only used by XLOG_FPI and
+	 * XLOG_FPI_FOR_HINT records.
+	 */
 	Assert(info == XLOG_FPI || info == XLOG_FPI_FOR_HINT ||
 		   !XLogRecHasAnyBlockRefs(record));
 
@@ -10356,25 +10359,32 @@ xlog_redo(XLogReaderState *record)
 	else if (info == XLOG_FPI || info == XLOG_FPI_FOR_HINT)
 	{
 		/*
-		 * Full-page image (FPI) records contain nothing else but a backup
+		 * XLOG_FPI records contain nothing else but one or more block
-		 * block (or multiple backup blocks). Every block reference must
+		 * references. Every block reference must include a full-page image
-		 * include a full-page image - otherwise there would be no point in
+		 * even if full_page_writes was disabled when the record was generated
-		 * this record.
+		 * - otherwise there would be no point in this record.
+		 *
+		 * XLOG_FPI_FOR_HINT records are generated when a page needs to be
+		 * WAL-logged because of a hint bit update. They are only generated
+		 * when checksums and/or wal_log_hints are enabled. They may include
+		 * no full-page images if full_page_writes was disabled when they were
+		 * generated. In this case there is nothing to do here.
 		 *
 		 * No recovery conflicts are generated by these generic records - if a
 		 * resource manager needs to generate conflicts, it has to define a
 		 * separate WAL record type and redo routine.
-		 *
-		 * XLOG_FPI_FOR_HINT records are generated when a page needs to be
-		 * WAL- logged because of a hint bit update. They are only generated
-		 * when checksums are enabled. There is no difference in handling
-		 * XLOG_FPI and XLOG_FPI_FOR_HINT records, they use a different info
-		 * code just to distinguish them for statistics purposes.
 		 */
 		for (uint8 block_id = 0; block_id <= record->max_block_id; block_id++)
 		{
 			Buffer		buffer;
 
+			if (!XLogRecHasBlockImage(record, block_id))
+			{
+				if (info == XLOG_FPI)
+					elog(ERROR, "XLOG_FPI record did not contain a full-page image");
+				continue;
+			}
+
 			if (XLogReadBufferForRedo(record, block_id, &buffer) != BLK_RESTORED)
 				elog(ERROR, "unexpected XLogReadBufferForRedo result when restoring backup block");
 			UnlockReleaseBuffer(buffer);

src/backend/access/transam/xloginsert.c

@@ -287,8 +287,6 @@ XLogRegisterBlock(uint8 block_id, RelFileNode *rnode, ForkNumber forknum,
 {
 	registered_buffer *regbuf;
 
-	/* This is currently only used to WAL-log a full-page image of a page */
-	Assert(flags & REGBUF_FORCE_IMAGE);
 	Assert(begininsert_called);
 
 	if (block_id >= max_registered_block_id)
@@ -995,7 +993,7 @@ XLogSaveBufferForHint(Buffer buffer, bool buffer_std)
 
 	if (lsn <= RedoRecPtr)
 	{
-		int			flags;
+		int			flags = 0;
 		PGAlignedBlock copied_buffer;
 		char	   *origdata = (char *) BufferGetBlock(buffer);
 		RelFileNode rnode;
@@ -1022,7 +1020,6 @@ XLogSaveBufferForHint(Buffer buffer, bool buffer_std)
 
 		XLogBeginInsert();
 
-		flags = REGBUF_FORCE_IMAGE;
 		if (buffer_std)
 			flags |= REGBUF_STANDARD;