Consider generalising middleware by passing state into every call

[jsdw]

Currently the middleware is quite restrictive in what it enables with its passing of a copyable Instant from one method to another.

Is it worth generalising the way that we pass state between middleware calls to give people more flexibility?

Some thoughts on things one might want to do:

• Log the number of requests per connection (logs on disconnect)
• Log the average time taken per request on a connection
• Log the time that the connection was active for.

It might be that we don't need to extend middleware to support this kind of thing though?

See #793 (comment)

[jsdw]

I added this to the 1.0 milestone because it's a breaking API change, but we may decide we don't want to do anything, and that's ok too!

[niklasad1]

So in general I like the idea but it's quite involved to implement as it requires major refactoring.

AFAIU it will work like this for each connection

// one state per connection
let state = <M as Middleware>::State::default();

middleware.on_connect(remote_addr, request.headers(), &mut state);

while active_connection {
   let single_or_batch = rx();

   // calls on `Middlware::on_call` and `Middleware::on_response`
   // this won't work because we can't borrow state mutable more than once
   // so we would need a mutex for that
   spawn(execute_call_async(middleware, &mut state, single_or_batch));
}

middleware.on_disconnect(remote_addr, &mut state);

The neat thing about the current design is that each "method call/request" gets it's own state and it's not mutated so we don't
need to synchronize it.

Also State in the design can't be clone anymore as with different state if it's cloned before or after it's mutated as each method requires mut State in the "new design suggestion".

Thus, I'm a bit skeptical about this approach as Mutex would be quite expensive and impl<A, B> HttpMiddleware for (A, B) won't work as well if State: !Clone

Can't we just provide a ConnectionInstant and RequestInstant instead or other ideas?

[jsdw]

Going to close this; I don't think it's worth the time to dig into it if the current approach does what we need :)