Add block information in error context of WAL REDO apply loop

Providing this information can be useful for example when diagnosing
problems related to recovery conflicts or for recovery issues without
having to go through the output generated by pg_waldump to get some
information about the blocks a WAL record works on.

The block information is printed in the same format as pg_waldump.  This
already existed in xlog.c for debugging purposes with -DWAL_DEBUG, so
adding the block information in the callback has required just a small
refactoring.

Author: Bertrand Drouvot
Reviewed-by: Michael Paquier, Masahiko Sawada
Discussion: https://postgr.es/m/c31e2cba-efda-762c-f4ad-5c25e5dac3d0@amazon.com

src/backend/access/transam/xlog.c

@@ -940,6 +940,7 @@ static bool CheckForStandbyTrigger(void);
 #ifdef WAL_DEBUG
 static void xlog_outrec(StringInfo buf, XLogReaderState *record);
 #endif
+static void xlog_block_info(StringInfo buf, XLogReaderState *record);
 static void xlog_outdesc(StringInfo buf, XLogReaderState *record);
 static void pg_start_backup_callback(int code, Datum arg);
 static void pg_stop_backup_callback(int code, Datum arg);
@@ -10258,6 +10259,19 @@ xlog_outrec(StringInfo buf, XLogReaderState *record)
 	appendStringInfo(buf, "; len %u",
 					 XLogRecGetDataLen(record));
 
+	xlog_block_info(buf, record);
+}
+#endif							/* WAL_DEBUG */
+
+/*
+ * Returns a string giving information about all the blocks in an
+ * XLogRecord.
+ */
+static void
+xlog_block_info(StringInfo buf, XLogReaderState *record)
+{
+	int			block_id;
+
 	/* decode block references */
 	for (block_id = 0; block_id <= record->max_block_id; block_id++)
 	{
@@ -10284,7 +10298,6 @@ xlog_outrec(StringInfo buf, XLogReaderState *record)
 			appendStringInfoString(buf, " FPW");
 	}
 }
-#endif							/* WAL_DEBUG */
 
 /*
  * Returns a string describing an XLogRecord, consisting of its identity
@@ -11765,6 +11778,7 @@ rm_redo_error_callback(void *arg)
 
 	initStringInfo(&buf);
 	xlog_outdesc(&buf, record);
+	xlog_block_info(&buf, record);
 
 	/* translator: %s is a WAL record description */
 	errcontext("WAL redo at %X/%X for %s",