InnoDB: test case to exploit a purge thread shutdown race condition

execute queries after purge thread has exited, but the connection
thread hasn't

before c720e68f it used to crash

mysql-test/suite/innodb/r/purge_thread_shutdown.result

+create table t1 (a int) engine=innodb;
+insert t1 values (1),(2),(3),(4);
+delete from t1 where a=1;
+select user,state from information_schema.processlist order by 2;
+user	state
+system user	
+system user	
+system user	
+system user	
+root	Filling schema table
+system user	InnoDB shutdown handler
+set global debug_dbug='+d,only_kill_system_threads';
+connect con1, localhost, root;
+shutdown;
+connection default;
+disconnect con1;
+delete from t1 where a=3;
+kill ID;
+ERROR 70100: Connection was killed
+drop table t1;

mysql-test/suite/innodb/t/purge_thread_shutdown.test

+source include/have_innodb.inc;
+source include/not_embedded.inc;
+source include/have_debug.inc;
+
+create table t1 (a int) engine=innodb;
+insert t1 values (1),(2),(3),(4);
+delete from t1 where a=1;
+
+select user,state from information_schema.processlist order by 2;
+
+set global debug_dbug='+d,only_kill_system_threads';
+
+connect con1, localhost, root;
+
+let $_server_id= `SELECT @@server_id`;
+let $_expect_file_name= $MYSQLTEST_VARDIR/tmp/mysqld.$_server_id.expect;
+exec echo "wait" > $_expect_file_name;
+send shutdown;
+
+connection default;
+disconnect con1;
+
+let $wait_condition=select count(*) = 0 from information_schema.processlist where user='system user';
+source include/wait_condition.inc;
+
+delete from t1 where a=3;
+
+let $me=`select connection_id()`;
+replace_result $me ID;
+error ER_CONNECTION_KILLED;
+eval kill $me;
+
+source include/start_mysqld.inc;
+drop table t1;

sql/mysqld.cc

@@ -1695,6 +1695,10 @@ static void close_connections(void)
     if (tmp->slave_thread)
       continue;
 
+    /* cannot use 'continue' inside DBUG_EXECUTE_IF()... */
+    if (DBUG_EVALUATE_IF("only_kill_system_threads", !tmp->system_thread, 0))
+      continue;
+
 #ifdef WITH_WSREP
     /* skip wsrep system threads as well */
     if (WSREP(tmp) && (tmp->wsrep_exec_mode==REPL_RECV || tmp->wsrep_applier))