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.

3 files changed, 6 insertions, 5 deletions

diff --git a/demux/demux_mkv.c b/demux/demux_mkv.c
index bd36edb784..4c5f376137 100644
--- a/demux/demux_mkv.c
+++ b/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;
 }
 
diff --git a/demux/demux_playlist.c b/demux/demux_playlist.c
index a7650b2b69..b51edad86c 100644
--- a/demux/demux_playlist.c
+++ b/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;
     }
 }
diff --git a/demux/ebml.c b/demux/ebml.c
index 44fa0c0793..143a5fe57b 100644
--- a/demux/ebml.c
+++ b/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);