demux: add missing seekpoint when cached ranges are joined

The impact was that you couldn't exactly seek to the join point with a
keyframe seek, even though there was a keyframe. This commit fixes it by
preserving the necessary metadata that got lost on cached range joining.

This is so absurdly obscure that it gets a longer code comment.

1 files changed, 8 insertions, 0 deletions

diff --git a/demux/demux.c b/demux/demux.c
index b903100b96..d904d03fbf 100644
--- a/demux/demux.c
+++ b/demux/demux.c
@@ -999,6 +999,14 @@ static void attempt_range_joining(struct demux_internal *in)
                         goto failed;
                     }
 
+                    // q1 usually meets q2 at a keyframe. q1 will end on a key-
+                    // frame (because it tries joining when reading a keyframe).
+                    // Obviously, q1 can not know the kf_seek_pts yet; it would
+                    // have to read packets after it to compute it. Ideally,
+                    // we'd remove it and use q2's packet, but the linked list
+                    // makes this hard, so copy this missing metadata instead.
+                    end->kf_seek_pts = dp->kf_seek_pts;
+
                     remove_head_packet(q2);
                     join_point_found = true;
                     break;