Fixed MDEV-4394 Sporadic failures in multi_source tests

Fixed MDEV-4033 Unable to use slave's temporary directory /tmp - Can't create/write to file '/tmp/SQL_LOAD-' (Errcode: 17 "File exists")
- Cache value of check_temp_dir() to avoid race condition
- Set $rpl_server_count to avoid error in show_rpl_debug_info.inc

mysql-test/suite/multi_source/info_logs.test:
  Set $rpl_server_count
mysql-test/suite/multi_source/multisource.test:
  Set $rpl_server_count
mysql-test/suite/multi_source/relaylog_events.test:
  Set $rpl_server_count
mysql-test/suite/multi_source/reset_slave.test:
  Set $rpl_server_count
mysql-test/suite/multi_source/simple.test:
  Set $rpl_server_count
mysql-test/suite/multi_source/skip_counter.test:
  Set $rpl_server_count
mysql-test/suite/multi_source/status_vars.test:
  Set $rpl_server_count
sql/slave.cc:
  Cache value of check_temp_dir() to avoid race condition

mysql-test/suite/multi_source/info_logs.test

@@ -3,6 +3,7 @@
 #
 
 --source include/not_embedded.inc
+--let $rpl_server_count= 0
 
 --connect (slave,127.0.0.1,root,,,$SERVER_MYPORT_3)
 

mysql-test/suite/multi_source/multisource.test

@@ -5,6 +5,7 @@
 
 --source include/not_embedded.inc
 --source include/have_innodb.inc
+--let $rpl_server_count= 0
 
 --connect (slave,127.0.0.1,root,,,$SERVER_MYPORT_3)
 

mysql-test/suite/multi_source/relaylog_events.test

@@ -4,6 +4,7 @@
 #
 
 --source include/not_embedded.inc
+--let $rpl_server_count= 0
 
 --connect (slave,127.0.0.1,root,,,$SERVER_MYPORT_3)
 

mysql-test/suite/multi_source/reset_slave.test

@@ -3,6 +3,7 @@
 #
 
 --source include/not_embedded.inc
+--let $rpl_server_count= 0
 
 --connect (slave,127.0.0.1,root,,,$SERVER_MYPORT_3)
 

mysql-test/suite/multi_source/simple.test

@@ -3,6 +3,7 @@
 #
 
 --source include/not_embedded.inc
+--let $rpl_server_count= 0
 
 --connect (slave,127.0.0.1,root,,,$SERVER_MYPORT_3)
 --connect (master1,127.0.0.1,root,,,$SERVER_MYPORT_1)

mysql-test/suite/multi_source/skip_counter.test

@@ -6,6 +6,7 @@
 # on the 1st master
 
 --source include/not_embedded.inc
+--let $rpl_server_count= 0
 
 --connect (master1,127.0.0.1,root,,,$SERVER_MYPORT_1)
 

mysql-test/suite/multi_source/status_vars.test

@@ -3,6 +3,7 @@
 #
 
 --source include/not_embedded.inc
+--let $rpl_server_count= 0
 
 --connect (slave,127.0.0.1,root,,,$SERVER_MYPORT_3)
 

sql/slave.cc

@@ -3526,17 +3526,34 @@ err_during_init:
 /*
   Check the temporary directory used by commands like
   LOAD DATA INFILE.
+
+  As the directory never changes during a mysqld run, we only
+  test this once and cache the result. This also resolve a race condition
+  when this can be run by multiple threads at the same time.
  */
+
+static bool check_temp_dir_run= 0;
+static int check_temp_dir_result= 0;
+
 static 
 int check_temp_dir(char* tmp_file)
 {
-  int fd;
+  File fd;
+  int result= 1;                                // Assume failure
   MY_DIR *dirp;
   char tmp_dir[FN_REFLEN];
   size_t tmp_dir_size;
 
   DBUG_ENTER("check_temp_dir");
 
+  mysql_mutex_lock(&LOCK_thread_count);
+  if (check_temp_dir_run)
+  {
+    result= check_temp_dir_result;
+    goto end;
+  }
+  check_temp_dir_run= 1;
+
   /*
     Get the directory from the temporary file.
   */
@@ -3546,27 +3563,33 @@ int check_temp_dir(char* tmp_file)
     Check if the directory exists.
    */
   if (!(dirp=my_dir(tmp_dir,MYF(MY_WME))))
-    DBUG_RETURN(1);
+    goto end;
   my_dirend(dirp);
 
   /*
-    Check permissions to create a file.
+    Check permissions to create a file. We use O_TRUNC to ensure that
+    things works even if we happen to have and old file laying around.
    */
   if ((fd= mysql_file_create(key_file_misc,
                              tmp_file, CREATE_MODE,
-                             O_WRONLY | O_BINARY | O_EXCL | O_NOFOLLOW,
+                             O_WRONLY | O_BINARY | O_TRUNC | O_NOFOLLOW,
                              MYF(MY_WME))) < 0)
-  DBUG_RETURN(1);
+    goto end;
 
+  result= 0;                                    // Directory name ok
   /*
     Clean up.
    */
   mysql_file_close(fd, MYF(0));
   mysql_file_delete(key_file_misc, tmp_file, MYF(0));
 
-  DBUG_RETURN(0);
+end:
+  check_temp_dir_result= result;
+  mysql_mutex_unlock(&LOCK_thread_count);
+  DBUG_RETURN(result);
 }
 
+
 /**
   Slave SQL thread entry point.