demux: add a way to block reading after seeks

Preparation for a future commit. The demuxer queues might be read from
other threads than the one to issue the seek, and passing SEEK_BLOCK
with such a seek will provide a convenient way to synchronize this.

demux/demux.c

@@ -3775,6 +3775,9 @@ static bool queue_seek(struct demux_internal *in, double seek_pts, int flags,
     bool force_seek = flags & SEEK_FORCE;
     flags &= ~(unsigned)SEEK_FORCE;
 
+    bool block = flags & SEEK_BLOCK;
+    flags &= ~(unsigned)SEEK_BLOCK;
+
     struct demux_cached_range *cache_target =
         find_cache_seek_range(in, seek_pts, flags);
 
@@ -3795,6 +3798,8 @@ static bool queue_seek(struct demux_internal *in, double seek_pts, int flags,
 
     clear_reader_state(in, clear_back_state);
 
+    in->blocked = block;
+
     if (cache_target) {
         execute_cache_seek(in, cache_target, seek_pts, flags);
     } else {

demux/demux.h

@@ -63,6 +63,8 @@ struct demux_reader_state {
 #define SEEK_SATAN    (1 << 4)      // enable backward demuxing
 #define SEEK_HR       (1 << 5)      // hr-seek (this is a weak hint only)
 #define SEEK_FORCE    (1 << 6)      // ignore unseekable flag
+#define SEEK_BLOCK    (1 << 7)      // upon successfully queued seek, block readers
+                                    // (simplifies syncing multiple reader threads)
 
 // Strictness of the demuxer open format check.
 // demux.c will try by default: NORMAL, UNSAFE (in this order)