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, 4 insertions, 4 deletions

diff --git a/player/scripting.c b/player/scripting.c
index bb4c93b60e..7b3f9afb3c 100644
--- a/player/scripting.c
+++ b/player/scripting.c
@@ -335,8 +335,8 @@ static int load_run(struct mp_script_args *args)
 
     // Hardcode them (according to opts.fds[]), because we want to allow clients
     // to hardcode them if they want. Sue me.
-    char *fdopt = fds[1] >= 0 ? "--mpv-ipc-fd=3:4"
-                              : "--mpv-ipc-fd=3";
+    char *fdopt = fds[1] >= 0 ? mp_tprintf(80, "--mpv-ipc-fd=%d:%d", fds[0], fds[1])
+                              : mp_tprintf(80, "--mpv-ipc-fd=%d", fds[0]);
 
     struct mp_subprocess_opts opts = {
         .exe = (char *)args->filename,
@@ -348,8 +348,8 @@ static int load_run(struct mp_script_args *args)
             {.fd = 2, .src_fd = 2,},
             // Just hope these don't step over each other (e.g. fds[1] is not
             // below 4, if the std FDs are missing).
-            {.fd = 3, .src_fd = fds[0], },
-            {.fd = 4, .src_fd = fds[1], },
+            {.fd = fds[0], .src_fd = fds[0], },
+            {.fd = fds[1], .src_fd = fds[1], },
         },
         .num_fds = fds[1] >= 0 ? 5 : 4,
         .detach = true,