Bug#20788853 MUTEX ISSUE IN SQL/SQL_SHOW.CC RESULTING IN SIG6. SOURCE LIKELY

FILL_VARIABLES

Prevent mutexes used in SHOW VARIABLES from being locked twice.

sql/sql_class.cc

@@ -802,6 +802,7 @@ THD::THD()
    rli_fake(0), rli_slave(NULL),
    user_time(0), in_sub_stmt(0),
    fill_status_recursion_level(0),
+   fill_variables_recursion_level(0),
    binlog_unsafe_warning_flags(0),
    binlog_table_maps(0),
    table_map_for_update(0),

sql/sql_class.h

@@ -1603,6 +1603,7 @@ public:
     decremented each time before it returns from the function.
   */
   uint fill_status_recursion_level;
+  uint fill_variables_recursion_level;
 
   /* container for handler's private per-connection data */
   Ha_data ha_data[MAX_HA];

sql/sql_show.cc

@@ -6317,7 +6317,11 @@ int fill_variables(THD *thd, TABLE_LIST *tables, COND *cond)
     Lock LOCK_plugin_delete to avoid deletion of any plugins while creating
     SHOW_VAR array and hold it until all variables are stored in the table.
   */
-  mysql_mutex_lock(&LOCK_plugin_delete);
+  if (thd->fill_variables_recursion_level++ == 0)
+  {
+    mysql_mutex_lock(&LOCK_plugin_delete);
+  }
+
   // Lock LOCK_system_variables_hash to prepare SHOW_VARs array.
   mysql_rwlock_rdlock(&LOCK_system_variables_hash);
   DEBUG_SYNC(thd, "acquired_LOCK_system_variables_hash");
@@ -6327,7 +6331,11 @@ int fill_variables(THD *thd, TABLE_LIST *tables, COND *cond)
   res= show_status_array(thd, wild, sys_var_array, option_type, NULL, "",
                          tables->table, upper_case_names, cond);
 
-  mysql_mutex_unlock(&LOCK_plugin_delete);
+  if (thd->fill_variables_recursion_level-- == 1)
+  {
+    mysql_mutex_unlock(&LOCK_plugin_delete);
+  }
+
   DBUG_RETURN(res);
 }