getDevNullFd

fn getDevNullFd(t: *Threaded) !posix.fd_t {
    {
        mutexLock(&t.mutex);
        defer mutexUnlock(&t.mutex);
        if (t.null_file.fd != -1) return t.null_file.fd;
    }
    const mode: u32 = 0;
    const syscall: Syscall = try .start();
    while (true) {
        const rc = open_sym("/dev/null", .{ .ACCMODE = .RDWR }, mode);
        switch (posix.errno(rc)) {
            .SUCCESS => {
                syscall.finish();
                const fresh_fd: posix.fd_t = @intCast(rc);
                mutexLock(&t.mutex); // Another thread might have won the race.
                defer mutexUnlock(&t.mutex);
                if (t.null_file.fd != -1) {
                    closeFd(fresh_fd);
                    return t.null_file.fd;
                } else {
                    t.null_file.fd = fresh_fd;
                    return fresh_fd;
                }
            },
            .INTR => {
                try syscall.checkCancel();
                continue;
            },
            .ACCES => return syscall.fail(error.AccessDenied),
            .MFILE => return syscall.fail(error.ProcessFdQuotaExceeded),
            .NFILE => return syscall.fail(error.SystemFdQuotaExceeded),
            .NODEV => return syscall.fail(error.NoDevice),
            .NOENT => return syscall.fail(error.FileNotFound),
            .NOMEM => return syscall.fail(error.SystemResources),
            .PERM => return syscall.fail(error.PermissionDenied),
            else => |err| return syscall.unexpectedErrno(err),
        }
    }
}