MDEV-38179 Lock system_variables_hash outside of find_sys_var in Session_sysvars_tracker::vars_list::parse_var_list

This saves 10 calls to pthread_mutex_lock when constructing the
initial OK packet for the default session_track_system_variables which
has 6 variables

Also added a testcase showing there's no need to worry about wildcard
session_track_system_variables as it causes construction of empty
string in the initial OK packet.

Author: mariadb-YuchenPei

mysql-test/suite/sys_vars/r/session_track_system_variables_basic.result

@@ -254,6 +254,16 @@ select @@session.session_track_system_variables;
 @@session.session_track_system_variables
 character_set_client,example_int_var,max_connections,sql_slave_skip_counter
 connection default;
+disconnect foo;
+set global session_track_system_variables='sql_slave_skip_counter,*';
+select @@global.session_track_system_variables;
+@@global.session_track_system_variables
+*
+connect foo,localhost,root;
+select @@session.session_track_system_variables;
+@@session.session_track_system_variables
+*
+connection default;
 SET global session_track_system_variables = @global_saved_tmp;
 disconnect foo;
 uninstall soname 'ha_example.so';

mysql-test/suite/sys_vars/t/session_track_system_variables_basic.test

@@ -209,6 +209,16 @@ enable_session_track_info;
 connect foo,localhost,root;
 select @@session.session_track_system_variables;
 
+connection default;
+disconnect foo;
+
+set global session_track_system_variables='sql_slave_skip_counter,*';
+select @@global.session_track_system_variables;
+enable_session_track_info;
+
+connect foo,localhost,root;
+select @@session.session_track_system_variables;
+
 connection default;
 
 SET global session_track_system_variables = @global_saved_tmp;

sql/session_tracker.cc

@@ -166,6 +166,7 @@ bool Session_sysvars_tracker::vars_list::parse_var_list(THD *thd,
   token= var_list.str;
 
   track_all= false;
+  mysql_prlock_rdlock(&LOCK_system_variables_hash);
   for (;;)
   {
     sys_var *svar;
@@ -189,7 +190,8 @@ bool Session_sysvars_tracker::vars_list::parse_var_list(THD *thd,
     {
       track_all= true;
     }
-    else if ((svar= find_sys_var(thd, var.str, var.length, throw_error)))
+    else if ((svar= find_sys_var(thd, var.str, var.length, throw_error,
+                                 /*hash_already_locked=*/true)))
     {
       if (insert(svar) == TRUE)
         return true;
@@ -209,6 +211,7 @@ bool Session_sysvars_tracker::vars_list::parse_var_list(THD *thd,
     else
       break;
   }
+  mysql_prlock_unlock(&LOCK_system_variables_hash);
   return false;
 }
 

sql/set_var.h

@@ -439,7 +439,7 @@ SHOW_VAR* enumerate_sys_vars(THD *thd, bool sorted, enum enum_var_type type);
 int fill_sysvars(THD *thd, TABLE_LIST *tables, COND *cond);
 
 sys_var *find_sys_var(THD *thd, const char *str, size_t length= 0,
-                      bool throw_error= false);
+                      bool throw_error= false, bool hash_already_locked= false);
 int sql_set_variables(THD *thd, List<set_var_base> *var_list, bool free);
 
 #define SYSVAR_AUTOSIZE(VAR,VAL)                        \

sql/sql_plugin.cc

@@ -2963,14 +2963,15 @@ static void update_func_double(THD *thd, struct st_mysql_sys_var *var,
 ****************************************************************************/
 
 sys_var *find_sys_var(THD *thd, const char *str, size_t length,
-                      bool throw_error)
+                      bool throw_error, bool hash_already_locked)
 {
   sys_var *var;
   sys_var_pluginvar *pi;
   DBUG_ENTER("find_sys_var");
   DBUG_PRINT("enter", ("var '%.*s'", (int)length, str));
 
-  mysql_prlock_rdlock(&LOCK_system_variables_hash);
+  if (!hash_already_locked)
+    mysql_prlock_rdlock(&LOCK_system_variables_hash);
   if ((var= intern_find_sys_var(str, length)) &&
       (pi= var->cast_pluginvar()))
   {
@@ -2980,7 +2981,8 @@ sys_var *find_sys_var(THD *thd, const char *str, size_t length,
       var= NULL; /* failed to lock it, it must be uninstalling */
     mysql_mutex_unlock(&LOCK_plugin);
   }
-  mysql_prlock_unlock(&LOCK_system_variables_hash);
+  if (!hash_already_locked)
+    mysql_prlock_unlock(&LOCK_system_variables_hash);
 
   if (unlikely(!throw_error && !var))
     my_error(ER_UNKNOWN_SYSTEM_VARIABLE, MYF(0),