operate

fn operate(userdata: ?*anyopaque, operation: Io.Operation) Io.Cancelable!Io.Operation.Result {
    const ev: *Evented = @ptrCast(@alignCast(userdata));
    var maybe_sync: CancelRegion.Sync.Maybe = .{ .cancel_region = .init() };
    defer maybe_sync.deinit(ev);
    return switch (operation) {
        .file_read_streaming => |o| .{
            .file_read_streaming = ev.fileReadStreaming(
                &maybe_sync.cancel_region,
                o.file,
                o.data,
            ) catch |err| switch (err) {
                error.Canceled => |e| return e,
                else => |e| e,
            },
        },
        .file_write_streaming => |o| .{
            .file_write_streaming = ev.fileWriteStreaming(
                &maybe_sync.cancel_region,
                o.file,
                o.header,
                o.data,
                o.splat,
            ) catch |err| switch (err) {
                error.Canceled => |e| return e,
                else => |e| e,
            },
        },
        .device_io_control => |o| .{
            .device_io_control = try ev.deviceIoControl(try maybe_sync.enterSync(ev), o),
        },
        .net_receive => |o| .{
            .net_receive = r: {
                const opt_err, const n = ev.netReceive(&maybe_sync.cancel_region, o.socket_handle, o.message_buffer, o.data_buffer, o.flags);
                break :r .{
                    if (opt_err) |err| switch (err) {
                        error.Canceled => |e| return e,
                        else => |e| e,
                    } else null,
                    n,
                };
            },
        },
        .net_send => |o| .{
            .net_send = r: {
                _ = o;
                break :r .{ error.NetworkDown, 0 }; // TODO
            },
        },
        .net_read => |o| .{
            .net_read = r: {
                _ = o;
                break :r error.NetworkDown; // TODO
            },
        },
        .net_write => @panic("TODO implement net_write operation"),
    };
}