Fix record length computation in pg_waldump/xlogdump.

The current method of computing the record length (excluding the
lenght of full-page images) has been wrong since the WAL format has
been revamped in 2c03216d83.  Only the
main record's length was counted, but that can be significantly too
little if there's data associated with further blocks.

Fix by computing the record length as total_lenght - fpi_length.

Reported-By: Chen Huajun
Bug: #14687
Reviewed-By: Heikki Linnakangas
Discussion: https://postgr.es/m/20170603165939.1436.58887@wrigleys.postgresql.org
Backpatch: 9.5-

src/bin/pg_waldump/pg_waldump.c

@@ -363,23 +363,13 @@ XLogDumpReadPage(XLogReaderState *state, XLogRecPtr targetPagePtr, int reqLen,
 }
 
 /*
- * Store per-rmgr and per-record statistics for a given record.
+ * Calculate the size of a record, split into !FPI and FPI parts.
  */
 static void
-XLogDumpCountRecord(XLogDumpConfig *config, XLogDumpStats *stats,
-					XLogReaderState *record)
+XLogDumpRecordLen(XLogReaderState *record, uint32 *rec_len, uint32 *fpi_len)
 {
-	RmgrId		rmid;
-	uint8		recid;
-	uint32		rec_len;
-	uint32		fpi_len;
 	int			block_id;
 
-	stats->count++;
-
-	rmid = XLogRecGetRmid(record);
-	rec_len = XLogRecGetDataLen(record) + SizeOfXLogRecord;
-
 	/*
 	 * Calculate the amount of FPI data in the record.
 	 *
@@ -387,13 +377,38 @@ XLogDumpCountRecord(XLogDumpConfig *config, XLogDumpStats *stats,
 	 * bimg_len indicating the length of FPI data. It doesn't seem worth it to
 	 * add an accessor macro for this.
 	 */
-	fpi_len = 0;
+	*fpi_len = 0;
 	for (block_id = 0; block_id <= record->max_block_id; block_id++)
 	{
 		if (XLogRecHasBlockImage(record, block_id))
-			fpi_len += record->blocks[block_id].bimg_len;
+			*fpi_len += record->blocks[block_id].bimg_len;
 	}
 
+	/*
+	 * Calculate the length of the record as the total length - the length of
+	 * all the block images.
+	 */
+	*rec_len = XLogRecGetTotalLen(record) - *fpi_len;
+}
+
+/*
+ * Store per-rmgr and per-record statistics for a given record.
+ */
+static void
+XLogDumpCountRecord(XLogDumpConfig *config, XLogDumpStats *stats,
+					XLogReaderState *record)
+{
+	RmgrId		rmid;
+	uint8		recid;
+	uint32		rec_len;
+	uint32		fpi_len;
+
+	stats->count++;
+
+	rmid = XLogRecGetRmid(record);
+
+	XLogDumpRecordLen(record, &rec_len, &fpi_len);
+
 	/* Update per-rmgr statistics */
 
 	stats->rmgr_stats[rmid].count++;
@@ -422,6 +437,8 @@ XLogDumpDisplayRecord(XLogDumpConfig *config, XLogReaderState *record)
 {
 	const char *id;
 	const RmgrDescData *desc = &RmgrDescTable[XLogRecGetRmid(record)];
+	uint32		rec_len;
+	uint32		fpi_len;
 	RelFileNode rnode;
 	ForkNumber	forknum;
 	BlockNumber blk;
@@ -429,13 +446,15 @@ XLogDumpDisplayRecord(XLogDumpConfig *config, XLogReaderState *record)
 	uint8		info = XLogRecGetInfo(record);
 	XLogRecPtr	xl_prev = XLogRecGetPrev(record);
 
+	XLogDumpRecordLen(record, &rec_len, &fpi_len);
+
 	id = desc->rm_identify(info);
 	if (id == NULL)
 		id = psprintf("UNKNOWN (%x)", info & ~XLR_INFO_MASK);
 
 	printf("rmgr: %-11s len (rec/tot): %6u/%6u, tx: %10u, lsn: %X/%08X, prev %X/%08X, ",
 		   desc->rm_name,
-		   XLogRecGetDataLen(record), XLogRecGetTotalLen(record),
+		   rec_len, XLogRecGetTotalLen(record),
 		   XLogRecGetXid(record),
 		   (uint32) (record->ReadRecPtr >> 32), (uint32) record->ReadRecPtr,
 		   (uint32) (xl_prev >> 32), (uint32) xl_prev);