Forbid SLAVE STOP if the thread executing the query has locked

tables.  This removes a possible deadlock situation.

sql/sql_parse.cc

@@ -1376,6 +1376,24 @@ mysql_execute_command(void)
     start_slave(thd);
     break;
   case SQLCOM_SLAVE_STOP:
+   /*
+      if the client thread has locked tables, a deadlock is possible.
+      Assume that
+      - the client thread does LOCK TABLE t READ.
+      - then the master updates t.
+      - then the SQL slave thread wants to update t,
+      so it waits for the client thread because t is locked by it.
+      - then the client thread does SLAVE STOP.
+      SLAVE STOP waits for the SQL slave thread to terminate its
+      update t, which waits for the client thread because t is locked by it.
+      To prevent that, refuse SLAVE STOP if the
+      client thread has locked tables
+    */
+    if (thd->locked_tables || thd->active_transaction())
+    {
+      send_error(&thd->net,ER_LOCK_OR_ACTIVE_TRANSACTION);
+      break;
+    }
     stop_slave(thd);
     break;