dirRealPathFilePosix

fn dirRealPathFilePosix(userdata: ?*anyopaque, dir: Dir, sub_path: []const u8, out_buffer: []u8) Dir.RealPathFileError!usize {
    if (native_os == .wasi) return error.OperationUnsupported;

    const t: *Threaded = @ptrCast(@alignCast(userdata));
    _ = t;

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

    if (builtin.link_libc and dir.handle == posix.AT.FDCWD) {
        if (out_buffer.len < posix.PATH_MAX) return error.NameTooLong;
        const syscall: Syscall = try .start();
        while (true) {
            if (std.c.realpath(sub_path_posix, out_buffer.ptr)) |redundant_pointer| {
                syscall.finish();
                assert(redundant_pointer == out_buffer.ptr);
                return std.mem.findScalar(u8, out_buffer, 0) orelse out_buffer.len;
            }
            const err: posix.E = @fromBackingInt(@intCast(std.c._errno().*));
            if (err == .INTR) {
                try syscall.checkCancel();
                continue;
            }
            syscall.finish();
            switch (err) {
                .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 posix.unexpectedErrno(err),
            }
        }
    }

    var flags: posix.O = .{};
    if (@hasField(posix.O, "NONBLOCK")) flags.NONBLOCK = true;
    if (@hasField(posix.O, "CLOEXEC")) flags.CLOEXEC = true;
    if (@hasField(posix.O, "PATH")) flags.PATH = true;

    const mode: posix.mode_t = 0;

    const syscall: Syscall = try .start();
    const fd: posix.fd_t = while (true) {
        const rc = openat_sym(dir.handle, sub_path_posix, flags, mode);
        switch (posix.errno(rc)) {
            .SUCCESS => {
                syscall.finish();
                break @intCast(rc);
            },
            .INTR => {
                try syscall.checkCancel();
                continue;
            },
            else => |e| {
                syscall.finish();
                switch (e) {
                    .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,
                    .SRCH => return error.FileNotFound, // Linux when accessing procfs.
                    .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 posix.unexpectedErrno(err),
                }
            },
        }
    };
    defer closeFd(fd);
    return realPathPosix(fd, out_buffer);
}