CancelRegion

const CancelRegion = struct {
    fiber: *Fiber,
    status: Fiber.CancelStatus,
    fn init() CancelRegion {
        const fiber = Thread.current().currentFiber();
        return .{
            .fiber = fiber,
            .status = .{
                .requested = fiber.cancel_protection.check() == .unblocked,
                .awaiting = .nothing,
            },
        };
    }
    fn initBlocked() CancelRegion {
        return .{
            .fiber = Thread.current().currentFiber(),
            .status = .{ .requested = false, .awaiting = .nothing },
        };
    }
    fn deinit(cancel_region: *CancelRegion) void {
        if (cancel_region.status.requested) {
            @branchHint(.likely);
            _ = cancel_region.fiber.cancel_status.changeAwaiting(
                cancel_region.status.awaiting,
                .nothing,
            );
        }
        cancel_region.* = undefined;
    }
    fn await(cancel_region: *CancelRegion, awaiting: Fiber.CancelStatus.Awaiting) Io.Cancelable!void {
        if (!cancel_region.status.requested) {
            @branchHint(.unlikely);
            return;
        }
        const status: Fiber.CancelStatus = .{ .requested = true, .awaiting = awaiting };
        if (cancel_region.fiber.cancel_status.changeAwaiting(
            cancel_region.status.awaiting,
            status.awaiting,
        )) {
            @branchHint(.unlikely);
            cancel_region.fiber.cancel_protection.acknowledge();
            cancel_region.status = .unrequested;
            return error.Canceled;
        }
        cancel_region.status = status;
    }
    fn awaitIoUring(cancel_region: *CancelRegion) Io.Cancelable!*Thread {
        const thread: *Thread = .current();
        try cancel_region.await(.fromIoUringFd(thread.io_uring.fd));
        return thread;
    }
    fn completion(cancel_region: *const CancelRegion) Completion {
        return cancel_region.fiber.resultPointer(Completion).*;
    }
    fn errno(cancel_region: *const CancelRegion) linux.E {
        return cancel_region.completion().errno();
    }

    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,
                };
            }
        };
    };
}