ffmpeg: notify when the thread message queue blocks.

This can help finding the source of A-V desync with live input.

ffmpeg.c

@@ -3356,6 +3356,7 @@ static int check_keyboard_interaction(int64_t cur_time)
 static void *input_thread(void *arg)
 {
     InputFile *f = arg;
+    unsigned flags = f->non_blocking ? AV_THREAD_MESSAGE_NONBLOCK : 0;
     int ret = 0;
 
     while (1) {
@@ -3371,7 +3372,15 @@ static void *input_thread(void *arg)
             break;
         }
         av_dup_packet(&pkt);
-        ret = av_thread_message_queue_send(f->in_thread_queue, &pkt, 0);
+        ret = av_thread_message_queue_send(f->in_thread_queue, &pkt, flags);
+        if (flags && ret == AVERROR(EAGAIN)) {
+            flags = 0;
+            ret = av_thread_message_queue_send(f->in_thread_queue, &pkt, flags);
+            av_log(f->ctx, AV_LOG_WARNING,
+                   "Thread message queue blocking; consider raising the "
+                   "thread_queue_size option (current value: %d)\n",
+                   f->thread_queue_size);
+        }
         if (ret < 0) {
             if (ret != AVERROR_EOF)
                 av_log(f->ctx, AV_LOG_ERROR,