stream: turn into a ring buffer, make size configurable

In some corner cases (see #6802), it can be beneficial to use a larger
stream buffer size. Use this as argument to rewrite everything for no
reason.

Turn stream.c itself into a ring buffer, with configurable size. The
latter would have been easily achievable with minimal changes, and the
ring buffer is the hard part. There is no reason to have a ring buffer
at all, except possibly if ffmpeg don't fix their awful mp4 demuxer, and
some subtle issues with demux_mkv.c wanting to seek back by small
offsets (the latter was handled with small stream_peek() calls, which
are unneeded now).

In addition, this turns small forward seeks into reads (where data is
simply skipped). Before this commit, only stream_skip() did this (which
also mean that stream_skip() simply calls stream_seek() now).

Replace all stream_peek() calls with something else (usually
stream_read_peek()). The function was a problem, because it returned a
pointer to the internal buffer, which is now a ring buffer with
wrapping. The new function just copies the data into a buffer, and in
some cases requires callers to dynamically allocate memory. (The most
common case, demux_lavf.c, required a separate buffer allocation anyway
due to FFmpeg "idiosyncrasies".) This is the bulk of the demuxer_*
changes.

I'm not happy with this. There still isn't a good reason why there
should be a ring buffer, that is complex, and most of the time just
wastes half of the available memory. Maybe another rewrite soon.

It also contains bugs; you're an alpha tester now.

1 files changed, 3 insertions, 4 deletions

diff --git a/demux/demux_lavf.c b/demux/demux_lavf.c
index 670b611375..9ac6495f15 100644
--- a/demux/demux_lavf.c
+++ b/demux/demux_lavf.c
@@ -458,11 +458,10 @@ static int lavf_check_file(demuxer_t *demuxer, enum demux_check check)
         } else {
             int nsize = av_clip(avpd.buf_size * 2, INITIAL_PROBE_SIZE,
                                 PROBE_BUF_SIZE);
-            bstr buf = stream_peek(s, nsize);
-            if (buf.len <= avpd.buf_size)
+            nsize = stream_read_peek(s, avpd.buf, nsize);
+            if (nsize <= avpd.buf_size)
                 final_probe = true;
-            memcpy(avpd.buf, buf.start, buf.len);
-            avpd.buf_size = buf.len;
+            avpd.buf_size = nsize;
 
             priv->avif = av_probe_input_format2(&avpd, avpd.buf_size > 0, &score);
         }