smol-rs / parking Goto Github PK

Unparker is an Arc<Inner>. If Inner implements std::task::Wake, that Arc<Inner> can be converted into a Waker without any additional allocations. This would be useful for the implementation of futures_lite::future::block_on, whose Waker implementation just calls Unparker::unpark.

It is unclear from looking through the docs and the readme of this crate what advantages it has over using the standard library std::thread::park() and std::thread::Thread::unpark() functions. A cursory glance over the codebase seems to indicate that it uses the same code that the generic thread-parking function uses, which means that it allocates memory for something threads already have built-in. The main advantage from what I can glean is that this algorithm defeats spurious wakeups, which the standard library doesn't guarantee. However, all three implementations already have guards against spurious wakeups.

The caller of Unparker::unpark briefly has the lock on the Mutex before notifying the condvar. Would it be feasible to pass a Send value through the Mutex to the unparked thread for it to consume? Perhaps, say, a async_task runnable? If the threads already unparked or never was to begin with,unpark can just return back an error containing the value.

In WebAssembly targets, blocking is unimplemented in the standard library except for target_feature = "atomics". Therefore blocking cannot happen until that feature is used. But, we can compile conditionally for that feature.

On web browsers, there is an Atomics API that can be used to block in Web Workers, but not on the main thread. So we could conditionally select that on web (like getrandom does), and then try to get that API. Then, if we're in a web worker, block, and if we're in the main thread, panic.

We could even have a try_park() method that allows users to handle the case of if they cannot actually park, because they're on the main thread. This would allow users to handle the case of not parking.

It would be nice to implement this in parking, that way it can be bubbled up to other primitives in smol.

See this comment for more info.