Bug#22225549 MYSQL_CHANGE_USER/MYSQL_RESET_CONNECTION+SET INNODB_*:LOGIN NOT POSSIBLE

Update of plugin-defined session variable triggers resync with global
variables and deadlocks on THD::LOCK_thd_sysvar in
alloc_and_copy_thd_dynamic_variables().

In alloc_and_copy_thd_dynamic_variables(), removed LOCK_thd_sysvar and
extended the coverage of LOCK_global_system_variables

(cherry picked from commit d02eed42ce96d52c28a007105bd056003bbff315)

Author: Christopher Powers

mysql-test/suite/perfschema/r/show_plugin.result

@@ -841,6 +841,33 @@ COUNT(*)
 # Connect con2 - reap
 connection con2;
 
+================================================================================
+TEST 5 - BUG#22225549 MYSQL_CHANGE_USER/MYSQL_RESET_CONNECTION + SET INNODB...
+Update to plugin-defined session variable triggers resync with global
+variables and deadlocks on THD::LOCK_thd_sysvar.
+================================================================================
+SELECT @@global.show_compatibility_56;
+@@global.show_compatibility_56
+0
+select @@session.innodb_strict_mode;
+@@session.innodb_strict_mode
+1
+select user(), current_user();
+user()	current_user()
+root@localhost	root@localhost
+change_user root
+#
+# Trigger a resync of session variables with global variables.
+#
+set @@session.innodb_strict_mode=off;
+
+# Restore
+
+set @@session.innodb_strict_mode=1;
+select @@session.innodb_strict_mode;
+@@session.innodb_strict_mode
+1
+
 ================================================================================
 CLEAN UP
 ================================================================================

mysql-test/suite/perfschema/t/show_plugin.test

@@ -404,6 +404,37 @@ connection con2;
 --reap
 --enable_warnings
 
+--echo
+--echo ================================================================================
+--echo TEST 5 - BUG#22225549 MYSQL_CHANGE_USER/MYSQL_RESET_CONNECTION + SET INNODB...
+--echo          Update to plugin-defined session variable triggers resync with global
+--echo          variables and deadlocks on THD::LOCK_thd_sysvar.
+--echo ================================================================================
+#
+# THD:LOCK_thd_sysvar is only used when SHOW_COMPATIBILITY_56 = OFF
+#
+SELECT @@global.show_compatibility_56;
+#
+# The deadlock occurs when plugin-defined session variables are resynced with the global
+# variables. To force a resync, change the user and update a session variable from a
+# plugin, in this case InnoDB.
+#
+select @@session.innodb_strict_mode;
+let $innodb_strict_mode_save = `select @@session.innodb_strict_mode`;
+select user(), current_user();
+--echo change_user root
+--change_user root
+--echo #
+--echo # Trigger a resync of session variables with global variables.
+--echo #
+set @@session.innodb_strict_mode=off;
+
+--echo
+--echo # Restore
+--echo
+eval set @@session.innodb_strict_mode=$innodb_strict_mode_save;
+select @@session.innodb_strict_mode;
+
 --echo
 --echo ================================================================================
 --echo CLEAN UP

sql/sql_plugin.cc

@@ -2807,8 +2807,10 @@ void alloc_and_copy_thd_dynamic_variables(THD *thd, bool global_lock)
 
   mysql_rwlock_rdlock(&LOCK_system_variables_hash);
 
-  /* Block system variable reads from other threads. */
-  mysql_mutex_lock(&thd->LOCK_thd_sysvar);
+  if (global_lock)
+    mysql_mutex_lock(&LOCK_global_system_variables);
+
+  mysql_mutex_assert_owner(&LOCK_global_system_variables);
 
   /*
     MAINTAINER:
@@ -2823,10 +2825,6 @@ void alloc_and_copy_thd_dynamic_variables(THD *thd, bool global_lock)
                global_variables_dynamic_size,
                MYF(MY_WME | MY_FAE | MY_ALLOW_ZERO_PTR));
 
-  if (global_lock)
-    mysql_mutex_lock(&LOCK_global_system_variables);
-
-  mysql_mutex_assert_owner(&LOCK_global_system_variables);
   /*
     Debug hook which allows tests to check that this code is not
     called for InnoDB after connection was created.
@@ -2879,7 +2877,6 @@ void alloc_and_copy_thd_dynamic_variables(THD *thd, bool global_lock)
   thd->variables.dynamic_variables_size=
     global_system_variables.dynamic_variables_size;
 
-  mysql_mutex_unlock(&thd->LOCK_thd_sysvar);
   mysql_rwlock_unlock(&LOCK_system_variables_hash);
 }
 
@@ -2904,7 +2901,8 @@ static uchar *intern_sys_var_ptr(THD* thd, int offset, bool global_lock)
   if (!thd->variables.dynamic_variables_ptr ||
       (uint)offset > thd->variables.dynamic_variables_head)
   {
-    if (current_thd == thd) /* TODO WL#6629: Supported for current_thd only. */
+    /* Current THD only. Don't trigger resync on remote THD. */
+    if (current_thd == thd)
       alloc_and_copy_thd_dynamic_variables(thd, global_lock);
     else
       return (uchar*) global_system_variables.dynamic_variables_ptr + offset;