MDEV-23857: replication master password length

After MDEV-4013, the maximum length of replication passwords was extended to
96 ASCII characters. After a restart, however, slaves only read the first 41
characters of MASTER_PASSWORD from the master.info file. This lead to slaves
unable to reconnect to the master after a restart.

After a slave restart, if a master.info file is detected, use the full
allowable length of the password rather than 41 characters.

Reviewed By:
============
Sergei Golubchik <serg@mariadb.com>

mysql-test/suite/rpl/r/rpl_slave_restart_long_password.result

+include/master-slave.inc
+[connection master]
+include/assert.inc [Password length is 96]
+connection master;
+SET SQL_LOG_BIN=0;
+GRANT REPLICATION SLAVE ON *.* TO rpl@127.0.0.1 IDENTIFIED BY '123456789X12141618202224262830323436384042444648505254565860626466687072747678808284868890929496';
+SET SQL_LOG_BIN=1;
+connection slave;
+include/stop_slave.inc
+CHANGE MASTER TO MASTER_HOST='127.0.0.1', master_user='rpl', master_password='123456789X12141618202224262830323436384042444648505254565860626466687072747678808284868890929496';
+include/start_slave.inc
+include/check_slave_param.inc [Slave_IO_Running]
+connection master;
+include/rpl_restart_server.inc [server_number=2]
+connection slave;
+include/start_slave.inc
+include/check_slave_param.inc [Slave_IO_Running]
+connection master;
+SET SQL_LOG_BIN=0;
+DROP USER rpl@127.0.0.1;
+FLUSH PRIVILEGES;
+SET SQL_LOG_BIN=1;
+connection slave;
+include/stop_slave.inc
+CHANGE MASTER TO MASTER_USER = 'root', MASTER_PASSWORD = '';
+include/start_slave.inc
+connection master;
+include/rpl_end.inc

mysql-test/suite/rpl/t/rpl_slave_restart_long_password.test

+#
+#  This test validates a fix for a bug where slaves only read the
+# first 41 characters of MASTER_PASSWORD from the master.info file
+# after restarts.
+#
+#  The test ensures that passwords up to the maximum allowable
+# length (96 ASCII characters) will be read from the master.info
+# file after slave restarts
+#
+# References:
+#  MDEV-23857: replication master password length
+#
+
+# Test is format independent, so only run with one format
+--source include/have_binlog_format_mixed.inc
+--source include/master-slave.inc
+
+
+#####
+# Setup
+#
+--let $passwd=123456789X12141618202224262830323436384042444648505254565860626466687072747678808284868890929496
+--let $expected_pwlen=96
+--let assert_cond=CHAR_LENGTH("$passwd")=$expected_pwlen
+--let assert_text=Password length is $expected_pwlen
+--source include/assert.inc
+
+connection master;
+SET SQL_LOG_BIN=0;
+--eval GRANT REPLICATION SLAVE ON *.* TO rpl@127.0.0.1 IDENTIFIED BY '$passwd'
+SET SQL_LOG_BIN=1;
+#####
+
+
+#####
+# Change master to new user/password combination
+#
+connection slave;
+--source include/stop_slave.inc
+--eval CHANGE MASTER TO MASTER_HOST='127.0.0.1', master_user='rpl', master_password='$passwd'
+
+--source include/start_slave.inc
+--let $slave_param= Slave_IO_Running
+--let $slave_param_value= Yes
+--source include/check_slave_param.inc
+#####
+
+
+#####
+#  Ensure slave can re-connect to master after restart
+#
+connection master;
+--let $rpl_server_number= 2
+--source include/rpl_restart_server.inc
+
+connection slave;
+--source include/start_slave.inc
+--let $slave_param= Slave_IO_Running
+--let $slave_param_value= Yes
+--source include/check_slave_param.inc
+#####
+
+
+#####
+# Cleanup
+#
+connection master;
+SET SQL_LOG_BIN=0;
+DROP USER rpl@127.0.0.1;
+FLUSH PRIVILEGES;
+SET SQL_LOG_BIN=1;
+
+connection slave;
+--source include/stop_slave.inc
+CHANGE MASTER TO MASTER_USER = 'root', MASTER_PASSWORD = '';
+--source include/start_slave.inc
+
+connection master;
+-- source include/rpl_end.inc
+#####

sql/rpl_mi.cc

@@ -482,7 +482,7 @@ file '%s')", fname);
     if (init_intvar_from_file(&master_log_pos, &mi->file, 4) ||
         init_strvar_from_file(mi->host, sizeof(mi->host), &mi->file, 0) ||
         init_strvar_from_file(mi->user, sizeof(mi->user), &mi->file, "test") ||
-        init_strvar_from_file(mi->password, SCRAMBLED_PASSWORD_CHAR_LENGTH+1,
+        init_strvar_from_file(mi->password, sizeof(mi->password),
                               &mi->file, 0) ||
         init_intvar_from_file(&port, &mi->file, MYSQL_PORT) ||
         init_intvar_from_file(&connect_retry, &mi->file,