lavf: add FFERROR_REDO to let demuxers return no packet.

libavformat/internal.h

@@ -516,4 +516,10 @@ int ff_copy_whitelists(AVFormatContext *dst, AVFormatContext *src);
 int ffio_open2_wrapper(struct AVFormatContext *s, AVIOContext **pb, const char *url, int flags,
                        const AVIOInterruptCB *int_cb, AVDictionary **options);
 
+/**
+ * Returned by demuxers to indicate that data was consumed but discarded
+ * (ignored streams or junk data). The framework will re-call the demuxer.
+ */
+#define FFERROR_REDO FFERRTAG('R','E','D','O')
+
 #endif /* AVFORMAT_INTERNAL_H */

libavformat/utils.c

@@ -680,6 +680,11 @@ int ff_read_packet(AVFormatContext *s, AVPacket *pkt)
         av_init_packet(pkt);
         ret = s->iformat->read_packet(s, pkt);
         if (ret < 0) {
+            /* Some demuxers return FFERROR_REDO when they consume
+               data and discard it (ignored streams, junk, extradata).
+               We must re-call the demuxer to get the real packet. */
+            if (ret == FFERROR_REDO)
+                continue;
             if (!pktl || ret == AVERROR(EAGAIN))
                 return ret;
             for (i = 0; i < s->nb_streams; i++) {