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.

Author: marcalff

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 @@ class THD :public Statement,
  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);
 }