Bug#33728209: set variable to function value cause double locking mutex & deadlock/hang

Assignment to the @@session_track_system_variables variable caused
a recursive locking of the LOCK_plugin mutex in corner cases.

The fix removes an ad-hoc processing of the
@@session_track_system_variables from the System_variable_tracker
class.

Change-Id: I38748f3a6f767ef7023ea40dbfd0c46322c864bf
(cherry picked from commit e5b2f18f360ef1ea99ef75a5033d1f4e72c7c088)
(cherry picked from commit 053b58cf5c7459aeb47019974ede11609fa54b48)

Author: GlebShchepa

mysql-test/r/session_tracker.result

@@ -843,5 +843,19 @@ session_track_state_change	ON
 SHOW VARIABLES like 'session_track_state_change';
 Variable_name	Value
 session_track_state_change	OFF
+#
+# Bug#33728209: set variable to function value cause double locking mutex & deadlock/hang
+#
+create function f() returns longtext no sql
+begin
+declare continue handler for sqlexception begin end;
+replace into asd values();
+return 'a';
+end $
+set session_track_system_variables=f();
+Warnings:
+Warning	1231	a is not a valid system variable and will be ignored.
+set session_track_system_variables=DEFAULT;
+drop function f;
 
 # End of tests

mysql-test/t/session_tracker.test

@@ -532,5 +532,23 @@ SHOW VARIABLES like 'session_track_state_change';
 
 connection default;
 
+--echo #
+--echo # Bug#33728209: set variable to function value cause double locking mutex & deadlock/hang
+--echo #
+
+delimiter $;
+create function f() returns longtext no sql
+begin
+  declare continue handler for sqlexception begin end;
+  replace into asd values();
+  return 'a';
+end $
+delimiter ;$
+
+set session_track_system_variables=f();
+set session_track_system_variables=DEFAULT;
+
+drop function f;
+
 --echo
 --echo # End of tests

sql/set_var.cc

@@ -768,11 +768,7 @@ bool System_variable_tracker::access_system_variable(
               thd, suppress_not_found_error == Suppress_not_found_error::YES))
         return true;
       if (f) {
-        extern sys_var *Sys_track_session_sys_vars_ptr;
-        if (m_static.m_static_var == Sys_track_session_sys_vars_ptr)
-          visit_session_track_system_variables(thd, f);
-        else
-          f(*this, m_static.m_static_var);
+        f(*this, m_static.m_static_var);
       }
       return false;
     case KEYCACHE:
@@ -1132,43 +1128,6 @@ bool System_variable_tracker::enumerate_sys_vars(
   return false;
 }
 
-void System_variable_tracker::visit_session_track_system_variables(
-    THD *thd,
-    std::function<void(const System_variable_tracker &, sys_var *)> f) const {
-  assert(m_tag == STATIC);
-  extern sys_var *Sys_track_session_sys_vars_ptr;
-  if (m_static.m_static_var != Sys_track_session_sys_vars_ptr)
-    assert(false);  // should never happen
-  if (f == nullptr) {
-    return;
-  }
-  if (thd != nullptr) {
-    if (thd->plugin_lock_recursion_depth++ == 0) {
-      /*
-        Historically, we acquire LOCK_plugin *before*
-        LOCK_system_variables_hash, so, we are not allowed to "upgrade" an
-        existent LOCK_system_variables_hash
-        to LOCK_plugin. Thus, even if LOCK_plugin is not needed at some point
-        where we're acquiring LOCK_system_variables_hash, we still have to
-        hold LOCK_plugin because of possible indirect visit_plugin_variable()
-        call. So, let lock LOCK_plugin "in advance":
-      */
-      mysql_mutex_lock(&LOCK_plugin);
-    }
-    if (thd->system_variable_hash_lock_recursion_depth++ == 0)
-      mysql_rwlock_rdlock(&LOCK_system_variables_hash);
-  }
-
-  f(*this, m_static.m_static_var);
-
-  if (thd != nullptr) {
-    if (--thd->system_variable_hash_lock_recursion_depth == 0)
-      mysql_rwlock_unlock(&LOCK_system_variables_hash);
-    if (--thd->plugin_lock_recursion_depth == 0)
-      mysql_mutex_unlock(&LOCK_plugin);
-  }
-}
-
 bool System_variable_tracker::visit_plugin_variable(
     THD *thd, std::function<bool(sys_var *)> function,
     Suppress_not_found_error suppress_not_found_error) const {

sql/set_var.h

@@ -851,10 +851,6 @@ class System_variable_tracker final {
   }
 
  private:
-  void visit_session_track_system_variables(
-      THD *,
-      std::function<void(const System_variable_tracker &, sys_var *)>) const;
-
   bool visit_plugin_variable(THD *, std::function<bool(sys_var *)>,
                              Suppress_not_found_error) const;