Sync

    const Sync = struct {
        cancel_region: CancelRegion,
        fn init(ev: *Evented) Io.Cancelable!Sync {
            if (ev.sync_limit) |*sync_limit| try sync_limit.wait(ev.io());
            return .{ .cancel_region = .init() };
        }
        fn initBlocked(ev: *Evented) Sync {
            if (ev.sync_limit) |*sync_limit| sync_limit.waitUncancelable(ev.io());
            return .{ .cancel_region = .initBlocked() };
        }
        fn deinit(sync: *Sync, ev: *Evented) void {
            sync.cancel_region.deinit();
            if (ev.sync_limit) |*sync_limit| sync_limit.post(ev.io());
        }

        const Maybe = union(enum) {
            cancel_region: CancelRegion,
            sync: Sync,

            fn deinit(maybe: *Maybe, ev: *Evented) void {
                switch (maybe.*) {
                    .cancel_region => |*cancel_region| cancel_region.deinit(),
                    .sync => |*sync| sync.deinit(ev),
                }
            }

            fn enterSync(maybe: *Maybe, ev: *Evented) Io.Cancelable!*Sync {
                switch (maybe.*) {
                    .cancel_region => |cancel_region| {
                        if (ev.sync_limit) |*sync_limit| try sync_limit.wait(ev.io());
                        maybe.* = .{ .sync = .{ .cancel_region = cancel_region } };
                    },
                    .sync => {},
                }
                return &maybe.sync;
            }

            fn leaveSync(maybe: *Maybe, ev: *Evented) void {
                switch (maybe.*) {
                    .cancel_region => {},
                    .sync => |sync| {
                        if (ev.sync_limit) |*sync_limit| sync_limit.post(ev.io());
                        maybe.* = .{ .cancel_region = sync.cancel_region };
                    },
                }
            }

            fn cancelRegion(maybe: *Maybe) *CancelRegion {
                return switch (maybe.*) {
                    .cancel_region => |*cancel_region| cancel_region,
                    .sync => |*sync| &sync.cancel_region,
                };
            }
        };
    }