movdec: Restart parsing root-level atoms at the right spot

If parsing moov+mdat in a non-seekable file, we currently
abort parsing directly after parsing the header of the mdat
atom. If we want to continue parsing later (if looking to
parse later fragments), we need to skip past the content of the
mdat atom, otherwise we end up parsing the content of the mdat
atom as root level atoms.
Signed-off-by: NMartin Storsjö <martin@martin.st>

libavformat/isom.h

@@ -143,6 +143,7 @@ typedef struct MOVContext {
     unsigned trex_count;
     int itunes_metadata;  ///< metadata are itunes style
     int chapter_track;
+    int64_t next_root_atom; ///< offset of the next root atom
 } MOVContext;
 
 int ff_mp4_read_descr_len(AVIOContext *pb);

libavformat/mov.c

@@ -350,8 +350,11 @@ static int mov_read_default(MOVContext *c, AVIOContext *pb, MOVAtom atom)
             if (err < 0)
                 return err;
             if (c->found_moov && c->found_mdat &&
-                (!pb->seekable || start_pos + a.size == avio_size(pb)))
+                (!pb->seekable || start_pos + a.size == avio_size(pb))) {
+                if (!pb->seekable)
+                    c->next_root_atom = start_pos + a.size;
                 return 0;
+            }
             left = a.size - avio_tell(pb) + start_pos;
             if (left > 0) /* skip garbage at atom end */
                 avio_skip(pb, left);
@@ -2667,8 +2670,11 @@ static int mov_read_packet(AVFormatContext *s, AVPacket *pkt)
     sample = mov_find_next_sample(s, &st);
     if (!sample) {
         mov->found_mdat = 0;
-        if (s->pb->seekable||
-            mov_read_default(mov, s->pb, (MOVAtom){ AV_RL32("root"), INT64_MAX }) < 0 ||
+        if (!mov->next_root_atom)
+            return AVERROR_EOF;
+        avio_seek(s->pb, mov->next_root_atom, SEEK_SET);
+        mov->next_root_atom = 0;
+        if (mov_read_default(mov, s->pb, (MOVAtom){ AV_RL32("root"), INT64_MAX }) < 0 ||
             s->pb->eof_reached)
             return AVERROR_EOF;
         av_dlog(s, "read fragments, offset 0x%"PRIx64"\n", avio_tell(s->pb));