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.
wm4
5ebb498b7a