Change Service::call to take &self

[seanmonstar]

Now that hyper defines its own Service trait (#2853), there's a key part we should clarify before hitting 1.0 Final: Should it take &self or &mut self? Whichever the decision, the why should be documented (not necessarily in the API docs, but in code comments or in the docs/ folder).

The usual arguments are here:

• &self
  • It prepares the way for async fn, since then the future only borrows &self, and thus a Service can concurrently handle multiple outstanding requests at once.
  • It's clearer that Services can likely be cloned.
• &mut self
  • If a Service has state (usually per-connection), it's cheaper to mutate it on each connection. Not being &mut would require internal synchronization.

I'm happy to open it up to discussion here. I'd recommend that comments are arguments be well-formed and reasoned, and try to provide something new, so that we keep the signal high. Chat is a fine place to bounce an idea around, and then come back and write it up so we can keep the history (decision history is extremely valuable).

[davidpdrsn]

My experience is that most services need to be Clone for two reasons:

1. To get an owned reference that can be moved into the response future.
2. When using tower::make::Shared (or similar)

Therefore to share state across clones you generally need Arc<Mutex<_>>. That means you're not really using the &mut self and could do with a &self. So while I haven't done much experimentation with writing &self services my gut feeling is that &self is better.

[LucioFranco]

I will write up more once I have explored what the async fn in traits feature provides us. But from my experience tonic has used async fn handler(&self, Req) -> Res setup and had no issues. In fact, I think it expresses very well what the api does and allows users to add a mutex or atomic ops or static read only types etc. That said, we need this api to be future proof for what tower might be and does that mean we need to wait for GATs?

[asonix]

This may have been brought up elsewhere, but actix-web's Service trait takes &self, but is otherwise very similar to Tower's trait

[fundon]

This is one of my practices Handler in Viz. &self should be sufficient for most scenarios.