scripting: make socket FD number for subprocesses dynamic

Before this, we pretty much guaranteed that --mpv-ipc-fd=3 would be
passed. The FD  was hardcoded, so scripts started by this mechanism
didn't need to actually parse the argument. Change this to using a
mostly random FD number instead.

I decided to do this because posix_spawnp() and the current replacement
cannot "guarantee" a FD layout. posix_spawn_file_actions_adddup2() just
runs dup2() calls, so it may be hard to set FD 3/4 if they are already
used by something else. For example imagine trying to map:

    {.fd = 3, .src_fd = 4},
    {.fd = 4, .src_fd = 3},

Then it'd do dup2(4, 3), dup2(3, 4) (reminder: dup2(src, dst)), and the
end result is that FD 4 really maps to the original FD 4.

While this was not a problem in the present code, it's too messy that I
don't want to pretend it can always done this way without an unholy
mess. So my assumption is that FDs 0-2 can be freely assigned because
they're never closed (probably...), while for all other FDs only
pass-through is reasonable.

1 files changed, 0 insertions, 5 deletions

diff --git a/DOCS/man/ipc.rst b/DOCS/man/ipc.rst
index fdf5daf9b6..4b808b9ff9 100644
--- a/DOCS/man/ipc.rst
+++ b/DOCS/man/ipc.rst
@@ -380,11 +380,6 @@ shebang and have the executable bit set.
 When executed, a socket (the IPC connection) is passed to them through file
 descriptor inheritance. The file descriptor is indicated as the special command
 line argument ``--mpv-ipc-fd=N``, where ``N`` is the numeric file descriptor.
-Currently, this is hardcoded as ``--mpv-ipc-fd=3``, and the intention is that
-it will always be ``3``. (This was a compromise between keeping it as simple as
-possible, and not doing too much implicitly. Also, since there is a chance that
-this will change anyway, you should at least validate that you got the expected
-argument.)
 
 The rest is the same as with a normal ``--input-ipc-server`` IPC connection. mpv
 does not attempt to observe or other interact with the started script process.