Bug#21935106 SHOW SESSION VARIABLES RECENTLY HAVING LOCK ISSUES

The function alloc_and_copy_thd_dynamic_variables() synchronizes
session variables with global variables. As of Bug#21840950, it uses
the mutex THD::LOCK_thd_sysvar to block remote threads from accessing
session vars during updates.

With SHOW_COMPATIBILITY_56=OFF, SHOW VARIABLES can cause
LOCK_thd_sysvar to be taken twice, first in System_variable::init()
and later in alloc_and_copy_thd_dynamic_variables().

System_variables::init() now only acquires LOCK_thd_sysvar if the
target thread is not the current thread.

perfschema.show_plugin.test is now two tests so that
alloc_and_copy_thd_dynamic_variables() can be tested separately
with SHOW_COMPATIBILITY_56 = ON and OFF.

(cherry picked from commit 8d91476c0b2bc510729a786ab3ba98ef6d28c2aa)

Author: Christopher Powers

mysql-test/suite/perfschema/include/show_plugin_verifier.inc

@@ -1,30 +1,5 @@
 --disable_warnings
 
---echo #
---echo # SET COMPATIBILITY MODE = ON
---echo #
-SET @@global.show_compatibility_56 = ON;
-
---echo
-SHOW GLOBAL STATUS LIKE "example_%";
---echo
-SHOW GLOBAL VARIABLES LIKE "example_%";
---echo
-SHOW SESSION VARIABLES LIKE "example_%";
---echo
-SELECT variable_name, variable_value FROM information_schema.global_status WHERE variable_name LIKE "example_%";
---echo
-SELECT variable_name, variable_value FROM information_schema.session_status WHERE variable_name LIKE "example_%";
---echo
-SELECT variable_name, variable_value FROM information_schema.global_variables WHERE variable_name LIKE "example_%";
---echo
-SELECT variable_name, variable_value FROM information_schema.session_variables WHERE variable_name LIKE "example_%";
-
---echo #
---echo # SET COMPATIBILITY MODE = OFF
---echo #
-SET @@global.show_compatibility_56 = OFF;
-
 --echo
 SHOW GLOBAL STATUS LIKE "example_%";
 --echo
@@ -41,6 +16,7 @@ SELECT variable_name, variable_value FROM performance_schema.session_status WHER
 SELECT variable_name, variable_value FROM performance_schema.global_variables WHERE variable_name LIKE "example_%";
 --echo
 SELECT variable_name, variable_value FROM performance_schema.session_variables WHERE variable_name LIKE "example_%";
+
 #
 # Force sync of local and global system variables.
 #

mysql-test/suite/perfschema/include/show_plugin_verifier_56.inc

@@ -0,0 +1,29 @@
+--disable_warnings
+
+--echo
+SHOW GLOBAL STATUS LIKE "example_%";
+--echo
+SHOW SESSION STATUS LIKE "example_%";
+--echo
+SHOW GLOBAL VARIABLES LIKE "example_%";
+--echo
+SHOW SESSION VARIABLES LIKE "example_%";
+--echo
+SELECT variable_name, variable_value FROM information_schema.global_status WHERE variable_name LIKE "example_%";
+--echo
+SELECT variable_name, variable_value FROM information_schema.session_status WHERE variable_name LIKE "example_%";
+--echo
+SELECT variable_name, variable_value FROM information_schema.global_variables WHERE variable_name LIKE "example_%";
+--echo
+SELECT variable_name, variable_value FROM information_schema.session_variables WHERE variable_name LIKE "example_%";
+
+#
+# Force sync of local and global system variables.
+#
+--disable_result_log
+--disable_query_log
+SELECT variable_name, variable_value FROM performance_schema.variables_by_thread WHERE variable_name LIKE "example_%";
+--enable_query_log
+--enable_result_log
+
+--enable_warnings