fftools/thread_queue: add THREAD_QUEUE_FLAG_NO_BLOCK

Exactly what it says on the tin. There is some ambiguity as to whether this
should also prevent reading from *choked*, as opposed to empty queue, but
I think it makes sense to consider them equivalent, as I struggle to think
of a use case where it would be beneficial to allow draining a queue that
was explicitly choked by the upstream (to e.g. prevent further reads).

Sponsored-by: nxtedition AB
Signed-off-by: Niklas Haas <git@haasn.dev>

fftools/thread_queue.c

@@ -211,7 +211,7 @@ int tq_receive(ThreadQueue *tq, int *stream_idx, void *data, int flags)
         if (can_read != av_container_fifo_can_read(tq->fifo))
             pthread_cond_broadcast(&tq->cond);
 
-        if (ret == AVERROR(EAGAIN)) {
+        if (ret == AVERROR(EAGAIN) && !(flags & THREAD_QUEUE_FLAG_NO_BLOCK)) {
             pthread_cond_wait(&tq->cond, &tq->lock);
             continue;
         }

fftools/thread_queue.h

@@ -26,6 +26,12 @@ enum ThreadQueueType {
     THREAD_QUEUE_PACKETS,
 };
 
+enum ThreadQueueFlags {
+    /* When set, tq_receive() will return AVERROR(EAGAIN) instead of blocking
+     * when the queue is empty or choked. */
+    THREAD_QUEUE_FLAG_NO_BLOCK = (1 << 0),
+};
+
 typedef struct ThreadQueue ThreadQueue;
 
 /**
@@ -74,7 +80,7 @@ void tq_choke(ThreadQueue *tq, int choked);
  *                   written here
  * @param data the data item will be written here on success using the
  *             callback provided to tq_alloc()
- * @param flags currently unused
+ * @param flags combination of THREAD_QUEUE_FLAG_*
  *
  * @return
  * - 0 a data item was successfully read; *stream_idx contains a non-negative