fix: when kill a connection, it have a change make the killed thread wait forever

the killer thread:
   shutdown the killed thread vio, this action will close the connection fd and remove the fd from the red-black tree of the epoll_wait fd
the killed thread:
   first run the start_io function, this function will bind the connection fd to the epoll_wait fd.
   second the connection wait from the epoll_wait

when worker thread process a connection, first bind the connection fd to the epoll_wait fd, then killer thread close the connection fd, which
will make the killed connection have no change to run any more.

Author: pkullj

include/violite.h

@@ -407,6 +407,10 @@ struct Vio {
   std::atomic_flag kevent_wakeup_flag = ATOMIC_FLAG_INIT;
 #endif
 
+#ifdef HAVE_POOL_OF_THREADS
+  std::atomic_flag epoll_shutdown_flag = ATOMIC_FLAG_INIT;
+#endif
+
 #ifdef HAVE_SETNS
   /**
     Socket network namespace.

sql/threadpool_unix.cc

@@ -712,6 +712,8 @@ static connection_t *listener(thread_group_t *thread_group) {
     */
     for (int i = (listener_picks_event) ? 1 : 0; i < cnt; i++) {
       connection_t *c = (connection_t *)native_event_get_userdata(&ev[i]);
+      Vio* vio = c->thd->get_protocol_classic()->get_vio();
+      vio->epoll_shutdown_flag.clear();
       if (connection_is_high_prio(*c)) {
         c->tickets--;
         thread_group->high_prio_queue.push_back(c);
@@ -724,6 +726,8 @@ static connection_t *listener(thread_group_t *thread_group) {
     if (listener_picks_event) {
       /* Handle the first event. */
       retval = (connection_t *)native_event_get_userdata(&ev[0]);
+      Vio* vio = retval->thd->get_protocol_classic()->get_vio();
+      vio->epoll_shutdown_flag.clear();
       mysql_mutex_unlock(&thread_group->mutex);
       break;
     }
@@ -1403,6 +1407,14 @@ static int start_io(connection_t *connection) {
     Bind to poll descriptor if not yet done.
   */
   Vio *vio = connection->thd->get_protocol_classic()->get_vio();
+  if (vio && vio->epoll_shutdown_flag.test_and_set()) {
+    /*
+      connection in the shutdown progress,
+      return -1 make sure abort this connection
+     */
+    vio->epoll_shutdown_flag.clear();
+    return -1;
+  }
   int fd = mysql_socket_getfd(vio->mysql_socket);
   if (!connection->bound_to_poll_descriptor) {
     connection->bound_to_poll_descriptor = true;

vio/viosocket.cc

@@ -527,6 +527,16 @@ static void vio_wait_until_woken(Vio *vio) {
 }
 #endif
 
+
+#ifdef HAVE_POOL_OF_THREADS
+static void vio_wait_until_epoll_woken(Vio *vio) {
+  while (vio->epoll_shutdown_flag.test_and_set()) {
+    // make connection wake up from epoll_wait in the worker thread.
+    vio_cancel(vio, SHUT_RD);
+  }
+}
+#endif
+
 int vio_shutdown(Vio *vio, int how) {
   DBUG_TRACE;
 
@@ -553,6 +563,12 @@ int vio_shutdown(Vio *vio, int how) {
       vio_wait_until_woken(vio);
 #endif
 
+#ifdef HAVE_POOL_OF_THREADS
+    if (vio->thread_id.value() != 0 && vio->epoll_shutdown_flag.test_and_set()) {
+      vio_wait_until_epoll_woken(vio);
+    }
+#endif
+
     if (mysql_socket_close(vio->mysql_socket)) r = -1;
 #ifdef HAVE_KQUEUE
     if (vio->kq_fd == -1 || close(vio->kq_fd)) r = -1;