provide access to raw hyper::Request via an extractor #555
Description
Currently we provide every request handler with an rqctx: Arc<RequestContext>, which contains rqctx.request: Arc<Mutex<hyper::Request>>.
(Why? Access to the raw hyper::Request is useful as an escape hatch for stuff that Dropshot currently doesn't support. The two use cases I know of are websockets (now supported) and streaming bodies (work in progress). Although we expect these both to be supported in the not-too-distant future, this escape hatch remains useful.)
But we've always known that this way of exposing it is janky. There's only one thing with access to the request, so it's annoying that it's wrapped in an Arc+Mutex. Worse: under #542 and related PRs, we're considering support for streaming request bodies. These also need access to the hyper::Request (well, specifically, the body, which can only be gotten by consuming the request). By allowing access to all handlers via the Arc+Mutex, it's possible to wind up with a deadlock, which means imposing implicit locking rules on consumers that we have no way to enforce.
Instead, let's remove the raw hyper::Request from the RequestContext and create a new extractor for getting it. This way, if we can also ensure that no endpoint has two extractors that want access to the request (#556), there's no Arc+Mutex, so less awkwardness and no locking issues.
Note that there have been two attempts at this:
- Allow Raw Hyper Context to be Extracted as
&mut Request<Body>#106 kind of ran out of steam after I dropped the ball. - [draft] straw proposal to remove the request from RequestContext #548 prototypes this as well. After some discussion, I'm working on a similar prototype that would enforce the new constraints at compile-time rather than runtime.
I've got a prototype of shared/exclusive extractors in #556 that's looking promising. With that, I think we can make TypedBody, UntypedBody, and a new RawRequest into ExclusiveExtractors, which means you can only have one of these as an argument to your endpoint function.