xwayland: Unlink X11 socket before binding to it

When starting xwayland with smithay, the first time I would get the
following output (do not ask what happens with displays 0, 1, 2. That's
not important right now):

Dec 29 14:13:31.031 DEBG Attempting to aquire an X11 display lock, D: 3, smithay_module: XWayland
Dec 29 14:13:31.032 DEBG X11 lock aquired, D: 3, smithay_module: XWayland
Dec 29 14:13:31.032 INFO Initialization completed, starting the main loop.

When killing the process with ctrl-c and starting it again, this
happened:

Dec 29 14:13:29.138 DEBG Attempting to aquire an X11 display lock, D: 3, smithay_module: XWayland
Dec 29 14:13:29.138 DEBG Failed to acquire lock, D: 3, smithay_module: XWayland
Dec 29 14:13:29.138 DEBG Lock was blocked by a defunct X11 server, trying again, D: 3, smithay_module: XWayland
Dec 29 14:13:29.139 DEBG Attempting to aquire an X11 display lock, D: 3, smithay_module: XWayland
Dec 29 14:13:29.139 DEBG X11 lock aquired, D: 3, smithay_module: XWayland
Dec 29 14:13:29.139 INFO Cleaning up X11 lock., smithay_module: XWayland
Dec 29 14:13:29.139 DEBG Attempting to aquire an X11 display lock, D: 4, smithay_module: XWayland
Dec 29 14:13:29.139 DEBG X11 lock aquired, D: 4, smithay_module: XWayland
Dec 29 14:13:29.139 INFO Initialization completed, starting the main loop.

The reason for the above behaviour is the
smithay::xwayland::x11_sockets::open_x11_sockets_for_display() failed.
The code successfully acquired the lock file, but then could not bind
the sockets. More specifically, the concrete socket in /tmp/.X11-unix/
already existed and thus bind() failed with EADDRINUSE.

(The code in X11Lock::grab() would then drop the already acquired
X11Lock and its Drop impl deleted the socket. Thus, when I started
things again, this time it successfully acquired display 3.)

Fix this removing the socket before trying to bind it.

This is also done by wlroots: In xwayland/sockets.x, the function
open_socket() has the same job as smithay's open_socket() function.
However, for non-abstract sockets, it would first unlink the target
before trying to bind:

        if (addr->sun_path[0]) {
                unlink(addr->sun_path);
        }

Signed-off-by: Uli Schlachter <psychon@znc.in>

src/xwayland/x11_sockets.rs

@@ -108,6 +108,7 @@ impl Drop for X11Lock {
 /// Should only be done after the associated lockfile is aquired!
 fn open_x11_sockets_for_display(display: u32) -> NixResult<[UnixStream; 2]> {
     let path = format!("/tmp/.X11-unix/X{}", display);
+    let _ = ::std::fs::remove_file(&path);
     // We know this path is not to long, these unwrap cannot fail
     let fs_addr = socket::UnixAddr::new(path.as_bytes()).unwrap();
     let abs_addr = socket::UnixAddr::new_abstract(path.as_bytes()).unwrap();