avformat/udp: cancel pending IO on win32 manually

recvfrom() is not a cancellation point in pthreads-win32, see
https://sourceware.org/pthreads-win32/manual/pthread_cancel.html

In order to be able to cancel the reader thread on Win32 properly we first
shutdown the socket then call CancelIoEx to abort pending IO. Subsequent
recvfrom() calls will fail with WSAESHUTDOWN causing the thread to exit.

Fixes ticket #5717.
Signed-off-by: Marton Balint <cus@passwd.hu>

libavformat/udp.c

@@ -1069,8 +1069,17 @@ static int udp_close(URLContext *h)
     if (s->thread_started) {
         int ret;
         // Cancel only read, as write has been signaled as success to the user
-        if (h->flags & AVIO_FLAG_READ)
+        if (h->flags & AVIO_FLAG_READ) {
+#ifdef _WIN32
+            /* recvfrom() is not a cancellation point for win32, so we shutdown
+             * the socket and abort pending IO, subsequent recvfrom() calls
+             * will fail with WSAESHUTDOWN causing the thread to exit. */
+            shutdown(s->udp_fd, SD_RECEIVE);
+            CancelIoEx((HANDLE)(SOCKET)s->udp_fd, NULL);
+#else
             pthread_cancel(s->circular_buffer_thread);
+#endif
+        }
         ret = pthread_join(s->circular_buffer_thread, NULL);
         if (ret != 0)
             av_log(h, AV_LOG_ERROR, "pthread_join(): %s\n", strerror(ret));