Async rust has a few parts that doesn’t feel ‘rusty’ at all. Rust is pretty good at “forcing” local reasoning but async cancellation (drop of Future etc.) leads to non-local reasoning which leads to hard to follow sequence of events that leads to subtle bugs. I recently learnt about futurelock from this excellent blog post.

The RFD (Request For Discussion) from Oxide that describe futurelock (https://rfd.shared.oxide.computer/rfd/0609) is easy to read by an intermediate Rust programmer. Reading this RFD made me a little bit nervous about async which I though I knew decently well.

I created this diagram that summarizes the sequence of events in the RFD that eventually leads to the deadlock/futurelock. You can refer to this diagram when re-reading the RFD. It helps a lot.

I also dug a little deeper into the mechanism of Mutex waking up the relevant tasks when it is unlocked. In the past, I’ve written state machines with callbacks and I think about async in state-machine terms. Future and Waker works together to implment state-machine with callback.

Here is another diagram which shows how Waker is used to implement callback like mechanism for the example in the RFD.