Context Managed Transports

[tomchristie]

We need to ensure that the Client __enter__/__exit__ and __aenter__/__aexit__ methods also call into the transport dunder methods, for all installed transports/proxies.

A good proof of having resolved the issue here would be streaming responses in the ASGITransport, which require nursery functionality in order to work properly. See notes here... #998 (comment)

[florimondmanca]

Hey,

Agreed we want clients to call into context manager dunder methods wherever appropriate.

While looking over #998 I also think we have currently a problem re: .aclose() vs __aexit__(*exc_info) in clients (and perhaps transports?).

Right now the client calls .aclose() in its __aexit__() method, which calls into the .aclose() of transports, but this is not correct. If an exception occurs, then we want it to propagate in the __aexit__() of transports. Right now it won't, because we're calling .aclose() on transports which does nothing. It's a bit hard to explain in words but I might submit a PR to demonstrate this point…

(Edit^: this doesn't actually seem to be a problem for this particular case. Hmm…)

Also, as I think I've said already, I really don't think this issue applies much to ASGITransport. The nursery object we need for ASGITransport is request-level, not client-level (since you want the app to stop when the request is finished), so whether clients call into __aenter__()/__aexit__() is not really relevant.