dirOpenFile

fn dirOpenFile(
    userdata: ?*anyopaque,
    dir: Dir,
    sub_path: []const u8,
    flags: Dir.OpenFileOptions,
) File.OpenError!File {
    const ev: *Evented = @ptrCast(@alignCast(userdata));

    var path_buffer: [PATH_MAX]u8 = undefined;
    const sub_path_posix = try pathToPosix(sub_path, &path_buffer);

    var maybe_sync: CancelRegion.Sync.Maybe = .{ .cancel_region = .init() };
    defer maybe_sync.deinit(ev);
    const fd = ev.openat(&maybe_sync.cancel_region, dir.handle, sub_path_posix, .{
        .ACCMODE = switch (flags.mode) {
            .read_only => .RDONLY,
            .write_only => .WRONLY,
            .read_write => .RDWR,
        },
        .NOCTTY = !flags.allow_ctty,
        .NOFOLLOW = !flags.follow_symlinks,
        .CLOEXEC = true,
        .PATH = flags.path_only,
    }, 0) catch |err| switch (err) {
        error.OperationUnsupported => return error.Unexpected, // TMPFILE unset.
        else => |e| return e,
    };
    errdefer ev.closeAsync(fd);

    if (!flags.allow_directory) {
        const is_dir = is_dir: {
            const s = ev.stat(&maybe_sync.cancel_region, fd) catch |err| switch (err) {
                // The directory-ness is either unknown or unknowable
                error.Streaming => break :is_dir false,
                else => |e| return e,
            };
            break :is_dir s.kind == .directory;
        };
        if (is_dir) return error.IsDir;
    }

    switch (flags.lock) {
        .none => {},
        .shared, .exclusive => try ev.flock(
            try maybe_sync.enterSync(ev),
            fd,
            flags.lock,
            if (flags.lock_nonblocking) .nonblocking else .blocking,
        ),
    }

    return .{ .handle = fd, .flags = .{ .nonblocking = false } };
}