Spawning async methods – Am I doing it right?

[jbe]

@steffahn

AFAIK, usage of Arc is a very common approach for solving this kind of problem.

Okay, then I will not feel bad using it

@steffahn

Those kinds of bounds are normal in generic code that works with multiple threads. Also Unpin on Async[Read|Write] is also not uncommon and not exceedingly prohibitive (because things like Pin<Box<R>> always work out to be Unpin even when R: !Unpin ), and also most actual implementations of these traits are satisfying Unpin ; though an Unpin -free approach is also possible: you’d need to use something like pin_mut! for e.g. the implementation of handler then.

I do remember your posts here. For now, simply requiring Unpin works fine, except it's a bit verbose.

@steffahn

It’s related to async_trait . See its documentation .

Yeah thanks. The next subsection also explains how to make the methods non-threadsafe by using #[async_trait(?Send)]. I had to actually use this because I need my async method to hold an RwLockReadGuard and that is !Send. However, I ran into the problems that you already pointed out:

@steffahn

But note that tokio::task::spawn also needs Send , so it’s quite important that handler returns a future that does fulfill a Send bound

The only way out was to use tokio::task::spawn_local, which – in turn – required me to create a tokio::task::LocalSet in my program's main function.

So async works, but I lost multithreading support.

Is it somehow possible to hold an RwLockReadGuard or MutexGuard in an async function or method that gets spawn'ed? … Nevermind, while writing this, I just found the answer: Yes. Tokio provides several Send'able synchronization locks/guards in its tokio::sync module. That should allow me to make all futures Send again despite using locks/guards. Will try that tomorrow, but wanted to share this, in case any other people have problems with locks and async functions.

tokio::sync::MutexGuard
A handle to a held Mutex. The guard can be held across any .await point as it is Send.

Thanks for helping and encouraging me to keep on!