MDEV-14192 Mariabackup assertion failure: byte_offset % OS_FILE_LOG_BLOCK_SIZE == 0

xtrabackup_backup_func(): If the log checkpoint header changed
since we last read it, search for the most recent checkpoint again.
Otherwise, we could corrupt the backup of the redo log, because the
least significant bits of checkpoint_lsn_start would not match
log_sys->log.lsn.

extra/mariabackup/xtrabackup.cc

@@ -4003,9 +4003,7 @@ static bool xtrabackup_backup_low()
 
 /** Implement --backup
 @return	whether the operation succeeded */
-static
-bool
-xtrabackup_backup_func()
+static bool xtrabackup_backup_func()
 {
 	MY_STAT			 stat_info;
 	uint			 i;
@@ -4175,38 +4173,25 @@ xtrabackup_backup_func()
 
 	log_mutex_enter();
 
+reread_log_header:
 	dberr_t err = recv_find_max_checkpoint(&max_cp_field);
 
 	if (err != DB_SUCCESS) {
-log_fail:
+		msg("Error: cannot read redo log header");
 		log_mutex_exit();
 		goto fail;
 	}
 
 	if (log_sys->log.format == 0) {
-old_format:
-		msg("Error: cannot process redo log"
-		    " before MariaDB 10.2.2");
+		msg("Error: cannot process redo log before MariaDB 10.2.2");
 		log_mutex_exit();
-		goto log_fail;
+		goto fail;
 	}
 
 	const byte* buf = log_sys->checkpoint_buf;
-
-reread_log_header:
 	checkpoint_lsn_start = log_sys->log.lsn;
 	checkpoint_no_start = log_sys->next_checkpoint_no;
 
-	err = recv_find_max_checkpoint(&max_cp_field);
-
-	if (err != DB_SUCCESS) {
-		goto log_fail;
-	}
-
-	if (log_sys->log.format == 0) {
-		goto old_format;
-	}
-
 	log_group_header_read(&log_sys->log, max_cp_field);
 
 	if (checkpoint_no_start != mach_read_from_8(buf + LOG_CHECKPOINT_NO)) {
@@ -4253,6 +4238,12 @@ xtrabackup_backup_func()
 	mach_write_to_8(log_hdr + LOG_CHECKPOINT_OFFSET,
 			(checkpoint_lsn_start & (OS_FILE_LOG_BLOCK_SIZE - 1))
 			| LOG_FILE_HDR_SIZE);
+	/* 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