MDEV-20703: mariabackup creates binlog files in server binlog directory on --prepare --export step

When "--export" mariabackup option is used, mariabackup starts the server in
bootstrap mode to generate *.cfg files for the certain innodb tables.
The started instance of the server reads options from the file, pointed
out in "--defaults-file" mariabackup option.

If the server uses the same config file as mariabackup, and binlog is
switched on in that config file, then "mariabackup --prepare --export"
will create binary log files in the server's binary log directory, what
can cause issues.

The fix is to add "--skip-log-bin" in mysld options when the server is
started to generate *.cfg files.

extra/mariabackup/xtrabackup.cc

@@ -1535,7 +1535,8 @@ static int prepare_export()
       " --defaults-extra-file=./backup-my.cnf --defaults-group-suffix=%s --datadir=."
       " --innodb --innodb-fast-shutdown=0 --loose-partition"
       " --innodb_purge_rseg_truncate_frequency=1 --innodb-buffer-pool-size=%llu"
-      " --console  --skip-log-error --bootstrap  < "  BOOTSTRAP_FILENAME IF_WIN("\"",""),
+      " --console  --skip-log-error --skip-log-bin --bootstrap  < "
+      BOOTSTRAP_FILENAME IF_WIN("\"",""),
       mariabackup_exe, 
       orig_argv1, (my_defaults_group_suffix?my_defaults_group_suffix:""),
       xtrabackup_use_memory);
@@ -1547,7 +1548,8 @@ static int prepare_export()
       " --defaults-file=./backup-my.cnf --defaults-group-suffix=%s --datadir=."
       " --innodb --innodb-fast-shutdown=0 --loose-partition"
       " --innodb_purge_rseg_truncate_frequency=1 --innodb-buffer-pool-size=%llu"
-      " --console  --log-error= --bootstrap  < "  BOOTSTRAP_FILENAME IF_WIN("\"",""),
+      " --console  --log-error= --skip-log-bin --bootstrap  < "
+      BOOTSTRAP_FILENAME IF_WIN("\"",""),
       mariabackup_exe,
       (my_defaults_group_suffix?my_defaults_group_suffix:""),
       xtrabackup_use_memory);

mysql-test/suite/mariabackup/partial.test

@@ -12,7 +12,7 @@ CREATE TABLE t2(i int) ENGINE INNODB;
 
 echo # xtrabackup backup;
 
-let $targetdir=$MYSQLTEST_VARDIR/tmp/backup;
+let targetdir=$MYSQLTEST_VARDIR/tmp/backup;
 --disable_result_log
 exec $XTRABACKUP --defaults-file=$MYSQLTEST_VARDIR/my.cnf  --backup "--tables=test.*1" --target-dir=$targetdir;
 --enable_result_log
@@ -25,13 +25,27 @@ EOF
 write_file $targetdir/test/junk.frm;
 EOF
 
+let server_cnf=$targetdir/server.cnf;
+copy_file $MYSQLTEST_VARDIR/my.cnf $server_cnf;
+
+# Emulate server config file turnes on binary logs
+perl;
+my $binlog_path="$ENV{'targetdir'}/mysqld-bin";
+my $config_path=$ENV{'server_cnf'};
+open(my $fd, '>>', "$config_path");
+print $fd "\n[mysqld]\n";
+print $fd "log-bin=$binlog_path\n";
+close $fd;
+EOF
 
 echo # xtrabackup prepare;
 --disable_result_log
-exec $XTRABACKUP --defaults-file=$MYSQLTEST_VARDIR/my.cnf --defaults-group-suffix=.1 --prepare --export --target-dir=$targetdir;
+exec $XTRABACKUP --defaults-file=$server_cnf --defaults-group-suffix=.1 --prepare --export --target-dir=$targetdir;
 --enable_result_log
 
 list_files $targetdir/test *.cfg;
+# There must not be binary logs created on --prepare step
+list_files $targetdir/ mysqld-bin.*;
 
 let $MYSQLD_DATADIR= `select @@datadir`;
 ALTER TABLE t1 DISCARD TABLESPACE;