demux: make determining seek capability generic

Instead of having each demuxer do it (only demux_mkv actually did...),
let generic code determine whether the file is seekable. This requires
adding exceptions to demuxers where the stream is not seekable, but the
demuxer is.

Sort-of try to improve handling of unseekable files in the player. Exit
early if the file is determined to be unseekable, instead of resetting
all decoders and then performing a pointless seek.

Add an exception to allow seeking if the file is not seekable, but the
stream cache is enabled. Print a warning in this case, because seeking
outside the cache (which we can't prevent since the demuxer is not aware
of this problem) still messes everything up.

demux/demux.c

@@ -533,7 +533,8 @@ static struct demuxer *open_given_type(struct MPOpts *opts,
         .type = desc->type,
         .stream = stream,
         .stream_pts = MP_NOPTS_VALUE,
-        .seekable = 1,
+        .seekable = (stream->flags & MP_STREAM_SEEK) == MP_STREAM_SEEK &&
+                    stream->end_pos > 0,
         .accurate_seek = true,
         .filepos = -1,
         .opts = opts,
@@ -564,6 +565,12 @@ static struct demuxer *open_given_type(struct MPOpts *opts,
         add_stream_chapters(demuxer);
         demuxer_sort_chapters(demuxer);
         demux_info_update(demuxer);
+        // Pretend we can seek if we can't seek, but there's a cache.
+        if (!demuxer->seekable && stream->uncached_stream) {
+            mp_msg(MSGT_DEMUXER, MSGL_WARN,
+                   "File is not seekable, but there's a cache: enabling seeking.\n");
+            demuxer->seekable = true;
+        }
         return demuxer;
     }
 
@@ -632,7 +639,7 @@ void demux_flush(demuxer_t *demuxer)
 int demux_seek(demuxer_t *demuxer, float rel_seek_secs, int flags)
 {
     if (!demuxer->seekable) {
-        mp_tmsg(MSGT_SEEK, MSGL_WARN, "Cannot seek in this file.\n");
+        mp_tmsg(MSGT_DEMUXER, MSGL_WARN, "Cannot seek in this file.\n");
         return 0;
     }
 

demux/demux_lavf.c

@@ -584,9 +584,12 @@ static int demux_open_lavf(demuxer_t *demuxer, enum demux_check check)
         }
     }
 
-    if (!(priv->avif->flags & AVFMT_NOFILE) &&
-        demuxer->stream->type != STREAMTYPE_AVDEVICE)
+    if ((priv->avif->flags & AVFMT_NOFILE) ||
+        demuxer->stream->type == STREAMTYPE_AVDEVICE)
     {
+        // This might be incorrect.
+        demuxer->seekable = true;
+    } else {
         void *buffer = av_malloc(lavfdopts->buffersize);
         if (!buffer)
             return -1;
@@ -597,9 +600,7 @@ static int demux_open_lavf(demuxer_t *demuxer, enum demux_check check)
             return -1;
         }
         priv->pb->read_seek = mp_read_seek;
-        priv->pb->seekable = demuxer->stream->end_pos
-                 && (demuxer->stream->flags & MP_STREAM_SEEK) == MP_STREAM_SEEK
-                ? AVIO_SEEKABLE_NORMAL : 0;
+        priv->pb->seekable = demuxer->seekable ? AVIO_SEEKABLE_NORMAL : 0;
         avfc->pb = priv->pb;
     }
 

demux/demux_libass.c

@@ -100,6 +100,8 @@ static int d_check_file(struct demuxer *demuxer, enum demux_check check)
     sh->sub->track = track;
     sh->codec = "ass";
 
+    demuxer->seekable = true;
+
     return 0;
 }
 

demux/demux_mf.c

@@ -210,6 +210,7 @@ static int demux_open_mf(demuxer_t* demuxer, enum demux_check check)
 
   mf->sh = sh_video;
   demuxer->priv=(void*)mf;
+  demuxer->seekable = true;
 
   return 0;
 

demux/demux_mkv.c

@@ -1844,12 +1844,6 @@ static int demux_mkv_open(demuxer_t *demuxer, enum demux_check check)
 
     display_create_tracks(demuxer);
 
-    if (s->end_pos == 0) {
-        demuxer->seekable = 0;
-    } else {
-        demuxer->seekable = 1;
-    }
-
     return 0;
 }
 

demux/demux_subreader.c

@@ -1358,6 +1358,8 @@ static int d_open_file(struct demuxer *demuxer, enum demux_check check)
     add_sub_data(demuxer, sd);
     subdata_free(sd);
 
+    demuxer->seekable = true;
+
     return 0;
 }
 

mpvcore/player/playloop.c

@@ -215,6 +215,11 @@ static int seek(MPContext *mpctx, struct seek_params seek,
     if (!mpctx->demuxer)
         return -1;
 
+    if (!mpctx->demuxer->seekable) {
+        MP_ERR(mpctx, "Can't seek in this file.\n");
+        return -1;
+    }
+
     if (mpctx->stop_play == AT_END_OF_FILE)
         mpctx->stop_play = KEEP_PLAYING;
     bool hr_seek = mpctx->demuxer->accurate_seek && opts->correct_pts;