Bug#36835161: mysql_command_services does not work with x-protocol

The command_services suite uses the sql server sessions as an underlying
mechanism.
This was never supposed to work recursively: i.e. a command using
sessions executed in a session should fail.

Added a check to the command service to ensure it's not usable in such
a context.
Fixed some error handling in the test UDF.

Change-Id: Idb8e4793a3363a1aed6c6b8789445dec8ac462fd

Author: gkodinov

components/test/test_mysql_command_services.cc

@@ -90,6 +90,9 @@ static char *test_mysql_command_services_udf(UDF_INIT *, UDF_ARGS *args,
   unsigned int err_no;
   char *sqlstate_errmsg[50];
 
+  /* reset to empty as a start */
+  *result = 0;
+
   //  Execute the SQL specified in the argument.
   if (cmd_factory_srv->init(&mysql_h)) {
     return nullptr;

share/messages_to_clients.txt

@@ -9966,6 +9966,9 @@ ER_VALIDATE_PASSWORD_INSUFFICIENT_CHANGED_CHARACTERS
 ER_WARN_DEPRECATED_WITH_NOTE
   eng "'%s' is deprecated and will be removed in a future release. %s"
 
+ER_CANT_RUN_COMMAND_SERVICES_RECURSIVELY
+  eng "Running recursive calls to SQL execution service is not supported"
+
 #
 #  End of 8.0 error messages (server-to-client).
 #  Do NOT add messages intended for the error log above!

sql/server_component/mysql_command_services_imp.cc

@@ -61,6 +61,11 @@ struct Error_handler {
 DEFINE_BOOL_METHOD(mysql_command_services_imp::init, (MYSQL_H * mysql_h)) {
   bool ret = false;
   try {
+    /* Not supposed to work recursively */
+    if (current_thd->is_a_srv_session()) {
+      my_error(ER_CANT_RUN_COMMAND_SERVICES_RECURSIVELY, MYF(0));
+      return true;
+    }
     Mysql_handle *mysql_handle = (Mysql_handle *)my_malloc(
         key_memory_query_service, sizeof(Mysql_handle),
         MYF(MY_WME | MY_ZEROFILL));