MDEV-14192: mariabackup.incremental_backup failed in buildbot with Failing assertion: byte_offset % OS_FILE_LOG_BLOCK_SIZE == 0

In some cases it's possible that InnoDB redo log file header is re-written so,
that checkpoint lsn and checkpoint lsn offset are updated, but checkpoint
number stays the same. The fix is to re-read redo log header if at least one
of those three parametes is changed at backup start.

Repeat the logic of log_group_checkpoint() on choosing InnoDB checkpoint info
field on backup start. This does not influence backup correctness, but
simplifies bugs analysis.

extra/mariabackup/xtrabackup.cc

@@ -4187,9 +4187,11 @@ reread_log_header:
 
 	log_group_header_read(&log_sys->log, max_cp_field);
 
-	if (checkpoint_no_start != mach_read_from_8(buf + LOG_CHECKPOINT_NO)) {
+	if (checkpoint_no_start != mach_read_from_8(buf + LOG_CHECKPOINT_NO)
+		|| checkpoint_lsn_start != mach_read_from_8(buf + LOG_CHECKPOINT_LSN)
+		|| log_sys->log.lsn_offset
+			!= mach_read_from_8(buf + LOG_CHECKPOINT_OFFSET))
 		goto reread_log_header;
-	}
 
 	log_mutex_exit();
 
@@ -4209,43 +4211,39 @@ reread_log_header:
 	}
 
 	/* label it */
-	byte MY_ALIGNED(OS_FILE_LOG_BLOCK_SIZE) log_hdr[OS_FILE_LOG_BLOCK_SIZE];
-	memset(log_hdr, 0, sizeof log_hdr);
-	mach_write_to_4(LOG_HEADER_FORMAT + log_hdr, log_sys->log.format);
-	mach_write_to_4(LOG_HEADER_SUBFORMAT + log_hdr,
-			log_sys->log.subformat);
-	mach_write_to_8(LOG_HEADER_START_LSN + log_hdr, checkpoint_lsn_start);
-	strcpy(reinterpret_cast<char*>(LOG_HEADER_CREATOR + log_hdr),
-	       "Backup " MYSQL_SERVER_VERSION);
-	log_block_set_checksum(log_hdr,
-			       log_block_calc_checksum_crc32(log_hdr));
-
-	/* Write the log header. */
-	if (ds_write(dst_log_file, log_hdr, sizeof log_hdr)) {
-	log_write_fail:
-		msg("error: write to logfile failed");
-		goto fail;
-	}
-	/* Adjust the checkpoint page. */
-	memcpy(log_hdr, buf, OS_FILE_LOG_BLOCK_SIZE);
-	mach_write_to_8(log_hdr + LOG_CHECKPOINT_OFFSET,
-			(checkpoint_lsn_start & (OS_FILE_LOG_BLOCK_SIZE - 1))
-			| LOG_FILE_HDR_SIZE);
+	byte MY_ALIGNED(OS_FILE_LOG_BLOCK_SIZE) log_hdr_buf[LOG_FILE_HDR_SIZE];
+	memset(log_hdr_buf, 0, sizeof log_hdr_buf);
+
+	byte *log_hdr_field = log_hdr_buf;
+	mach_write_to_4(LOG_HEADER_FORMAT + log_hdr_field, log_sys->log.format);
+	mach_write_to_4(LOG_HEADER_SUBFORMAT + log_hdr_field, log_sys->log.subformat);
+	mach_write_to_8(LOG_HEADER_START_LSN + log_hdr_field, checkpoint_lsn_start);
+	strcpy(reinterpret_cast<char*>(LOG_HEADER_CREATOR + log_hdr_field),
+		"Backup " MYSQL_SERVER_VERSION);
+	log_block_set_checksum(log_hdr_field,
+		log_block_calc_checksum_crc32(log_hdr_field));
+
+	/* copied from log_group_checkpoint() */
+	log_hdr_field +=
+		(log_sys->next_checkpoint_no & 1) ? LOG_CHECKPOINT_2 : LOG_CHECKPOINT_1;
 	/* The least significant bits of LOG_CHECKPOINT_OFFSET must be
 	stored correctly in the copy of the ib_logfile. The most significant
 	bits, which identify the start offset of the log block in the file,
 	we did choose freely, as LOG_FILE_HDR_SIZE. */
 	ut_ad(!((log_sys->log.lsn ^ checkpoint_lsn_start)
 		& (OS_FILE_LOG_BLOCK_SIZE - 1)));
-	log_block_set_checksum(log_hdr,
-			       log_block_calc_checksum_crc32(log_hdr));
-	/* Write checkpoint page 1 and two empty log pages before the
-	payload. */
-	if (ds_write(dst_log_file, log_hdr, OS_FILE_LOG_BLOCK_SIZE)
-	    || !memset(log_hdr, 0, sizeof log_hdr)
-	    || ds_write(dst_log_file, log_hdr, sizeof log_hdr)
-	    || ds_write(dst_log_file, log_hdr, sizeof log_hdr)) {
-		goto log_write_fail;
+	/* Adjust the checkpoint page. */
+	memcpy(log_hdr_field, log_sys->checkpoint_buf, OS_FILE_LOG_BLOCK_SIZE);
+	mach_write_to_8(log_hdr_field + LOG_CHECKPOINT_OFFSET,
+		(checkpoint_lsn_start & (OS_FILE_LOG_BLOCK_SIZE - 1))
+		| LOG_FILE_HDR_SIZE);
+	log_block_set_checksum(log_hdr_field,
+			log_block_calc_checksum_crc32(log_hdr_field));
+
+	/* Write log header*/
+	if (ds_write(dst_log_file, log_hdr_buf, sizeof(log_hdr_buf))) {
+		msg("error: write to logfile failed");
+		goto fail;
 	}
 
 	log_copying_running = true;