player: add prefetching of the next playlist entry

Since for mpv CLI, the player state is a singleton, full prefetching is
a bit tricky. We do it only on the demuxer layer.

The implementation reuses the old "open thread". This means there is
significant potential for regressions even if the new option is not
used. This is made worse by the fact that I barely tested this code.

The generic mpctx_run_reentrant() wrapper is also removed - this was its
only user, and its remains become part of the new implementation.

1 files changed, 21 insertions, 0 deletions

diff --git a/DOCS/man/options.rst b/DOCS/man/options.rst
index a812fb088c..079672533f 100644
--- a/DOCS/man/options.rst
+++ b/DOCS/man/options.rst
@@ -2737,6 +2737,27 @@ Demuxer
     (This value tends to be fuzzy, because many file formats don't store linear
     timestamps.)
 
+``--prefetch-playlist=<yes|no>``
+    Prefetch next playlist entry while playback of the current entry is ending
+    (default: no). This merely opens the URL of the next playlist entry as soon
+    as the current URL is fully read.
+
+    This does **not** work with URLs resolved by the ``youtube-dl`` wrapper,
+    and it won't.
+
+    This does not affect HLS (``.m3u8`` URLs) - HLS prefetching depends on the
+    demuxer cache settings and is on by default.
+
+    This can give subtly wrong results if per-file options are used, or if
+    options are changed in the time window between prefetching start and next
+    file played.
+
+    This can occasionally make wrong prefetching decisions. For example, it
+    can't predict whether you go backwards in the playlist, and assumes you
+    won't edit the playlist.
+
+    Highly experimental.
+
 ``--force-seekable=<yes|no>``
     If the player thinks that the media is not seekable (e.g. playing from a
     pipe, or it's an http stream with a server that doesn't support range