openFileZ

pub fn openFileZ(self: Dir, sub_path: [*:0]const u8, flags: File.OpenFlags) File.OpenError!File {
    switch (native_os) {
        .windows => {
            const path_w = try windows.cStrToPrefixedFileW(self.fd, sub_path);
            return self.openFileW(path_w.span(), flags);
        },
        // Use the libc API when libc is linked because it implements things
        // such as opening absolute file paths.
        .wasi => if (!builtin.link_libc) {
            return openFile(self, mem.sliceTo(sub_path, 0), flags);
        },
        else => {},
    }

    var os_flags: posix.O = switch (native_os) {
        .wasi => .{
            .read = flags.mode != .write_only,
            .write = flags.mode != .read_only,
        },
        else => .{
            .ACCMODE = switch (flags.mode) {
                .read_only => .RDONLY,
                .write_only => .WRONLY,
                .read_write => .RDWR,
            },
        },
    };
    if (@hasField(posix.O, "CLOEXEC")) os_flags.CLOEXEC = true;
    if (@hasField(posix.O, "LARGEFILE")) os_flags.LARGEFILE = true;
    if (@hasField(posix.O, "NOCTTY")) os_flags.NOCTTY = !flags.allow_ctty;

    // Use the O locking flags if the os supports them to acquire the lock
    // atomically.
    const has_flock_open_flags = @hasField(posix.O, "EXLOCK");
    if (has_flock_open_flags) {
        // Note that the NONBLOCK flag is removed after the openat() call
        // is successful.
        switch (flags.lock) {
            .none => {},
            .shared => {
                os_flags.SHLOCK = true;
                os_flags.NONBLOCK = flags.lock_nonblocking;
            },
            .exclusive => {
                os_flags.EXLOCK = true;
                os_flags.NONBLOCK = flags.lock_nonblocking;
            },
        }
    }
    const fd = try posix.openatZ(self.fd, sub_path, os_flags, 0);
    errdefer posix.close(fd);

    if (have_flock and !has_flock_open_flags and flags.lock != .none) {
        // TODO: integrate async I/O
        const lock_nonblocking: i32 = if (flags.lock_nonblocking) posix.LOCK.NB else 0;
        try posix.flock(fd, switch (flags.lock) {
            .none => unreachable,
            .shared => posix.LOCK.SH | lock_nonblocking,
            .exclusive => posix.LOCK.EX | lock_nonblocking,
        });
    }

    if (has_flock_open_flags and flags.lock_nonblocking) {
        var fl_flags = posix.fcntl(fd, posix.F.GETFL, 0) catch |err| switch (err) {
            error.FileBusy => unreachable,
            error.Locked => unreachable,
            error.PermissionDenied => unreachable,
            error.DeadLock => unreachable,
            error.LockedRegionLimitExceeded => unreachable,
            else => |e| return e,
        };
        fl_flags &= ~@as(usize, 1 << @bitOffsetOf(posix.O, "NONBLOCK"));
        _ = posix.fcntl(fd, posix.F.SETFL, fl_flags) catch |err| switch (err) {
            error.FileBusy => unreachable,
            error.Locked => unreachable,
            error.PermissionDenied => unreachable,
            error.DeadLock => unreachable,
            error.LockedRegionLimitExceeded => unreachable,
            else => |e| return e,
        };
    }

    return .{ .handle = fd };
}