Fix for bug #29976: Excessive Slave I/O errors in replication tests

Problem:

The "Slave I/O thread couldn't register on master" error sporadically
occurred in replication tests because the slave I/O thread got
killed by STOP SLAVE before or while registering on master.

Fixed by checking the state of the I/O thread, and issueing
the error only if it was not explicitely killed by a user.

sql/slave.cc

@@ -137,6 +137,7 @@ static int terminate_slave_thread(THD *thd,
                                   pthread_cond_t* term_cond,
                                   volatile uint *slave_running,
                                   bool skip_lock);
+static bool check_io_slave_killed(THD *thd, Master_info *mi, const char *info);
 
 /*
   Find out which replications threads are running
@@ -821,7 +822,7 @@ static int get_master_version_and_clock(MYSQL* mysql, Master_info* mi)
     mi->clock_diff_with_master=
       (long) (time((time_t*) 0) - strtoul(master_row[0], 0, 10));
   }
-  else
+  else if (!check_io_slave_killed(mi->io_thd, mi, NULL))
   {
     mi->clock_diff_with_master= 0; /* The "most sensible" value */
     sql_print_warning("\"SELECT UNIX_TIMESTAMP()\" failed on master, "
@@ -1223,7 +1224,7 @@ int register_slave_on_master(MYSQL* mysql, Master_info *mi,
     {
       *suppress_warnings= TRUE;                 // Suppress reconnect warning
     }
-    else
+    else if (!check_io_slave_killed(mi->io_thd, mi, NULL))
     {
       char buf[256];
       my_snprintf(buf, sizeof(buf), "%s (Errno: %d)", mysql_error(mysql), 
@@ -1985,7 +1986,7 @@ static bool check_io_slave_killed(THD *thd, Master_info *mi, const char *info)
 {
   if (io_slave_killed(thd, mi))
   {
-    if (global_system_variables.log_warnings)
+    if (info && global_system_variables.log_warnings)
       sql_print_information(info);
     return TRUE;
   }
@@ -2169,13 +2170,17 @@ pthread_handler_t handle_slave_io(void *arg)
     */
     thd->proc_info = "Registering slave on master";
     if (register_slave_on_master(mysql, mi, &suppress_warnings))
+    {
+      if (!check_io_slave_killed(thd, mi, "Slave I/O thread killed "
+                                "while registering slave on master"))
       {
         sql_print_error("Slave I/O thread couldn't register on master");
-      if (check_io_slave_killed(thd, mi, "Slave I/O thread killed while \
-registering slave on master") ||
-          try_to_reconnect(thd, mysql, mi, &retry_count, suppress_warnings,
+        if (try_to_reconnect(thd, mysql, mi, &retry_count, suppress_warnings,
                              reconnect_messages[SLAVE_RECON_ACT_REG]))
           goto err;
+      }
+      else
+        goto err;
       goto connected;
     }
     DBUG_EXECUTE_IF("FORCE_SLAVE_TO_RECONNECT_REG",