dirRealPathFile

fn dirRealPathFile(
    userdata: ?*anyopaque,
    dir: Dir,
    sub_path: []const u8,
    out_buffer: []u8,
) Dir.RealPathFileError!usize {
    const ev: *Evented = @ptrCast(@alignCast(userdata));

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

    if (dir.handle == c.AT.FDCWD) {
        if (out_buffer.len < c.PATH_MAX) return error.NameTooLong;
        while (true) {
            if (c.realpath(sub_path_posix, out_buffer.ptr)) |redundant_pointer| {
                assert(redundant_pointer == out_buffer.ptr);
                return std.mem.findScalar(u8, out_buffer, 0) orelse out_buffer.len;
            }
            const err: c.E = @fromBackingInt(@intCast(c._errno().*));
            switch (err) {
                .INTR => {},
                .INVAL => return errnoBug(err),
                .BADF => return errnoBug(err),
                .FAULT => return errnoBug(err),
                .ACCES => return error.AccessDenied,
                .NOENT => return error.FileNotFound,
                .OPNOTSUPP => return error.OperationUnsupported,
                .NOTDIR => return error.NotDir,
                .NAMETOOLONG => return error.NameTooLong,
                .LOOP => return error.SymLinkLoop,
                .IO => return error.InputOutput,
                else => return unexpectedErrno(err),
            }
        }
    }

    const os_flags: c.O = .{
        .NONBLOCK = true,
        .CLOEXEC = true,
    };

    const fd: c.fd_t = while (true) {
        const rc = c.openat(dir.handle, sub_path_posix, os_flags);
        switch (c.errno(rc)) {
            .SUCCESS => break @intCast(rc),
            .INTR => {},
            .FAULT => |err| return errnoBug(err),
            .INVAL => return error.BadPathName,
            .BADF => |err| return errnoBug(err), // File descriptor used after closed.
            .ACCES => return error.AccessDenied,
            .FBIG => return error.FileTooBig,
            .OVERFLOW => return error.FileTooBig,
            .ISDIR => return error.IsDir,
            .LOOP => return error.SymLinkLoop,
            .MFILE => return error.ProcessFdQuotaExceeded,
            .NAMETOOLONG => return error.NameTooLong,
            .NFILE => return error.SystemFdQuotaExceeded,
            .NODEV => return error.NoDevice,
            .NOENT => return error.FileNotFound,
            .NOMEM => return error.SystemResources,
            .NOSPC => return error.NoSpaceLeft,
            .NOTDIR => return error.NotDir,
            .PERM => return error.PermissionDenied,
            .EXIST => return error.PathAlreadyExists,
            .BUSY => return error.DeviceBusy,
            .NXIO => return error.NoDevice,
            .ILSEQ => return error.BadPathName,
            else => |err| return unexpectedErrno(err),
        }
    };
    defer closeFd(fd);
    return ev.realPath(fd, out_buffer);
}