[swift-evolution] [Concurrency] async/await + actors

Adam Kemp adam.kemp at apple.com
Fri Aug 25 12:07:59 CDT 2017

> On Aug 25, 2017, at 9:54 AM, Thomas <tclementdev at free.fr> wrote:
> I'd tend to think non-FIFO actor messaging will cause more trouble than potential deadlocks. I'm re-reading the proposal and it seems to go this way as well:
> "An await on an actor method suspends the current task, and since you can get circular waits, you can end up with deadlock. This is because only one message is processed by the actor at a time. The trivial case like this can also be trivially diagnosed by the compiler. The complex case would ideally be diagnosed at runtime with a trap, depending on the runtime implementation model."

I understand what you’re saying, but I just think trying to make synchronous, blocking actor methods goes against the fundamental ideal of the actor model, and it’s a recipe for disaster. When actors communicate with each other that communication needs to be asynchronous or you will get deadlocks. It’s not just going to be a corner case. It’s going to be a very frequent occurrence.

One of the general rules of multithreaded programming is “don’t call unknown code while holding a lock”. Blocking a queue is effectively the same as holding a lock, and calling another actor is calling unknown code. So if the model works that way then the language itself will be encouraging people to call unknown code while holding locks. That is not going to go well.
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