MDEV-18353 - minor cleanup

Do not repeat yourself.

Instead of having the same DBUG_EXECUTE_IF code in threadpool and
thread-per-connection, add this code to setup_connection_thread_globals()
which is executed in all scheduling modes.

sql/sql_connect.cc

@@ -1107,6 +1107,14 @@ static int check_connection(THD *thd)
 
 bool setup_connection_thread_globals(THD *thd)
 {
+
+  DBUG_EXECUTE_IF("CONNECT_wait", {
+    extern MYSQL_SOCKET unix_sock;
+    DBUG_ASSERT(unix_sock.fd >= 0);
+    while (unix_sock.fd >= 0)
+      my_sleep(1000);
+  });
+
   if (thd->store_globals())
   {
     close_connection(thd, ER_OUT_OF_RESOURCES);
@@ -1360,14 +1368,6 @@ void do_handle_one_connection(CONNECT *connect)
     return;
   }
 
-  DBUG_EXECUTE_IF("CONNECT_wait",
-  {
-    extern MYSQL_SOCKET unix_sock;
-    DBUG_ASSERT(unix_sock.fd >= 0);
-    while (unix_sock.fd >= 0)
-      my_sleep(1000);
-  });
-
   /*
     If a thread was created to handle this connection:
     increment slow_launch_threads counter if it took more than

sql/threadpool_common.cc

@@ -222,14 +222,6 @@ static THD* threadpool_add_connection(CONNECT *connect, void *scheduler_data)
 {
   THD *thd= NULL;
 
-  DBUG_EXECUTE_IF("CONNECT_wait",
-  {
-    extern MYSQL_SOCKET unix_sock;
-    DBUG_ASSERT(unix_sock.fd >= 0);
-    while (unix_sock.fd >= 0)
-      my_sleep(1000);
-  });
-
   /*
     Create a new connection context: mysys_thread_var and PSI thread
     Store them in THD.