player: allow seeking in cached parts of unseekable streams

Before this change and before the seekable stream cache became a thing,
we could possibly seek using the stream cache. But we couldn't know
whether the seek would succeed. We knew the available byte range, but
could in general not tell whether a demuxer would stay within the range
when trying to seek to a specific time position. We preferred to have
safe defaults, so seeking in streams that were detected as unseekable
were not honored. We allowed overriding this via --force-seekable=yes,
in which case it depended on your luck whether the seek would work, or
the player crapped its pants.

With the demuxer packet cache, we can tell exactly whether a seek will
work (at least if there's only 1 seek range). We can just let seeks go
through. Everything to allow this is already in place, and this commit
just moves around some minor things.

Note that the demux_seek() return value was not used before, because low
level (i.e. network level) seeks are usually asynchronous, and if they
fail, the state is pretty much undefined. We simply repurpose the return
value to signal whether cache seeking worked. If it didn't, we can just
resume playback normally, because demuxing continues unaffected, and no
decoder are reset.

This should be particularly helpful to people who for some reason stream
data into stdin via streamlink and such.

1 files changed, 10 insertions, 7 deletions

diff --git a/player/playloop.c b/player/playloop.c
index bf1de03db5..245b49217e 100644
--- a/player/playloop.c
+++ b/player/playloop.c
@@ -253,12 +253,6 @@ static void mp_seek(MPContext *mpctx, struct seek_params seek)
     if (!mpctx->demuxer || seek.type == MPSEEK_NONE || seek.amount == MP_NOPTS_VALUE)
         return;
 
-    if (!mpctx->demuxer->seekable) {
-        MP_ERR(mpctx, "Cannot seek in this file.\n");
-        MP_ERR(mpctx, "You can forcibly enable it with '--force-seekable=yes'.\n");
-        return;
-    }
-
     bool hr_seek_very_exact = seek.exact == MPSEEK_VERY_EXACT;
     double current_time = get_current_time(mpctx);
     if (current_time == MP_NOPTS_VALUE && seek.type == MPSEEK_RELATIVE)
@@ -325,7 +319,16 @@ static void mp_seek(MPContext *mpctx, struct seek_params seek)
         demux_flags = (demux_flags | SEEK_HR) & ~SEEK_FORWARD;
     }
 
-    demux_seek(mpctx->demuxer, demux_pts, demux_flags);
+    if (!mpctx->demuxer->seekable)
+        demux_flags |= SEEK_CACHED;
+
+    if (!demux_seek(mpctx->demuxer, demux_pts, demux_flags)) {
+        if (!mpctx->demuxer->seekable) {
+            MP_ERR(mpctx, "Cannot seek in this file.\n");
+            MP_ERR(mpctx, "You can force it with '--force-seekable=yes'.\n");
+        }
+        return;
+    }
 
     // Seek external, extra files too:
     for (int t = 0; t < mpctx->num_tracks; t++) {