demux_mkv, stream: attempt to improve behavior in unseekable streams

stream_skip() semantics were kind of bad, especially after the recent
change to the stream code. Forward stream_skip() calls could still
trigger a seek and fail, even if it was supposed to actually skip data.
(Maybe the idea that stream_skip() should try to seek is worthless in
the first place.)

Rename it to stream_seek_skip() (takes absolute position now because I
think that's better), and make it always skip if the stream is marked as
forward.

While we're at it, make EOF detection more robust. I guess s->eof
shouldn't exist at all, since it's valid only "sometimes". It should be
removed... but not today. A 1-byte stream_read_peek() call is good to
get the s->eof flag set to a correct value.

demux/demux_mkv.c

@@ -1954,7 +1954,7 @@ static int read_mkv_segment_header(demuxer_t *demuxer, int64_t *segment_end)
     if (demuxer->params)
         num_skip = demuxer->params->matroska_wanted_segment;
 
-    while (!s->eof) {
+    while (stream_read_peek(s, &(char){0}, 1)) {
         if (ebml_read_id(s) != MATROSKA_ID_SEGMENT) {
             MP_VERBOSE(demuxer, "segment not found\n");
             return 0;
@@ -2580,7 +2580,7 @@ static int read_block(demuxer_t *demuxer, int64_t end, struct block_info *block)
 exit:
     if (res <= 0)
         free_block(block);
-    stream_seek(s, endpos);
+    stream_seek_skip(s, endpos);
     return res;
 }
 

demux/demux_playlist.c

@@ -99,7 +99,7 @@ static int read_characters(stream_t *s, uint8_t *dst, int dstsize, int utf16)
         if (len > dstsize)
             return -1; // line too long
         memcpy(dst, buf, len);
-        stream_skip(s, len);
+        stream_seek_skip(s, len);
         return len;
     }
 }

demux/ebml.c

@@ -180,14 +180,14 @@ int ebml_read_skip(struct mp_log *log, int64_t end, stream_t *s)
     if (len >= INT64_MAX - pos2 || (end > 0 && pos2 + len > end))
         goto invalid;
 
-    if (!stream_skip(s, len))
+    if (!stream_seek_skip(s, pos2 + len))
         goto invalid;
 
     return 0;
 
 invalid:
     mp_err(log, "Invalid EBML length at position %"PRId64"\n", pos);
-    stream_seek(s, pos);
+    stream_seek_skip(s, pos);
     return 1;
 }
 
@@ -198,6 +198,7 @@ int ebml_resync_cluster(struct mp_log *log, stream_t *s)
 {
     int64_t pos = stream_tell(s);
     uint32_t last_4_bytes = 0;
+    stream_read_peek(s, &(char){0}, 1);
     if (!s->eof) {
         mp_err(log, "Corrupt file detected. "
                "Trying to resync starting from position %"PRId64"...\n", pos);

stream/stream.c

@@ -692,13 +692,13 @@ bool stream_seek(stream_t *s, int64_t pos)
     return stream_seek_unbuffered(s, pos);
 }
 
-bool stream_skip(stream_t *s, int64_t len)
+// Like stream_seek(), but strictly prefer skipping data instead of failing, if
+// it's a forward-seek.
+bool stream_seek_skip(stream_t *s, int64_t pos)
 {
-    if (!stream_seek(s, stream_tell(s) + len))
+    return !s->seekable && pos > stream_tell(s)
-        return false;
+        ? stream_skip_read(s, pos - stream_tell(s))
-    // Make sure s->eof is set correctly, even if a seek happened.
+        : stream_seek(s, pos);
-    stream_read_more(s, 1);
-    return true;
 }
 
 int stream_control(stream_t *s, int cmd, void *arg)
@@ -732,7 +732,7 @@ int stream_skip_bom(struct stream *s)
     bstr data = {buf, len};
     for (int n = 0; n < 3; n++) {
         if (bstr_startswith0(data, bom[n])) {
-            stream_skip(s, strlen(bom[n]));
+            stream_seek_skip(s, stream_tell(s) + strlen(bom[n]));
             return n;
         }
     }

stream/stream.h

@@ -121,7 +121,7 @@ typedef struct stream {
     void (*close)(struct stream *s);
 
     int64_t pos;
-    int eof;
+    int eof; // valid only after read calls that returned a short result
     int mode; //STREAM_READ or STREAM_WRITE
     void *priv; // used for DVD, TV, RTSP etc
     char *url;  // filename/url (possibly including protocol prefix)
@@ -194,7 +194,7 @@ inline static int64_t stream_tell(stream_t *s)
     return s->pos + s->buf_cur - s->buf_end;
 }
 
-bool stream_skip(stream_t *s, int64_t len);
+bool stream_seek_skip(stream_t *s, int64_t pos);
 bool stream_seek(stream_t *s, int64_t pos);
 int stream_read(stream_t *s, void *mem, int total);
 int stream_read_partial(stream_t *s, void *buf, int buf_size);

stream/stream_libarchive.c

@@ -84,7 +84,7 @@ static int64_t skip_cb(struct archive *arch, void *priv, int64_t request)
     if (!volume_seek(vol))
         return -1;
     int64_t old = stream_tell(vol->src);
-    stream_skip(vol->src, request);
+    stream_seek_skip(vol->src, old + request);
     return stream_tell(vol->src) - old;
 }