`linkerd-app-core` includes an error recovery body middleware. this middleware will gracefully catch and report errors encountered when polling an inner body, and via an `R`-typed recovery strategy provided by the caller, will attempt to map the error to a gRPC status code denoting an error.

before we upgrade to hyper 1.0 in service of linkerd/linkerd2#8733, we add some test coverage to ensure that we preserve the behavior of this middleware.

see:
* linkerd/linkerd2#8733
* #3614.

for historical context on this tower layer, see:
* #222
* #1246
* #1282

---

* refactor(http/retry): outline `ForwardCompatibleBody<B>`

in #3559 (4b53081), we introduced a backported `Frame<T>` type, and a
`ForwardCompatibleBody<B>` type that allows us to interact with a
`http_body::Body` circa 0.4.6 in terms of frame-based interfaces that
match those of the 1.0 interface.

see linkerd/linkerd2#8733 for more information on upgrading hyper.

in #3559, we narrowly added this as an internal submodule of the
`linkerd-http-retry` library. these facilities however, would have
utility in other places such as `linkerd-app-core`.

this commit pulls these compatibility shims out into a
`linkerd-http-body-compat` library so that they can be imported and
reused elsewhere.

Signed-off-by: katelyn martin <[email protected]>

* nit(http/body-compat): tidy `combinators` imports

Signed-off-by: katelyn martin <[email protected]>

* refactor(app/core): hoist `errors::code_header` helper

Signed-off-by: katelyn martin <[email protected]>

* refactor(app/core): `l5d-*` constants are headers

these are header values. `http::HeaderName` has a const fn constructor,
so let's use that.

Signed-off-by: katelyn martin <[email protected]>

* refactor(app/core): grpc constants are headers

Signed-off-by: katelyn martin <[email protected]>

* refactor(app/core): hoist `l5d-` and `grpc-` constants

Signed-off-by: katelyn martin <[email protected]>

* refactor(app/core): outline `ResponseBody` middleware

we'll add a few tests for this middleware shortly.

this commit moves this middleware out into its own submodule.

Signed-off-by: katelyn martin <[email protected]>

* refactor(app/core): encapsulate `ResponseBody` enum

for other body middleware, we hide inner enum variants and their
constituent members by using the "inner" pattern.

this commit tweaks `ResponseBody` to follow suit, such that it now holds
an `Inner`, but does not expose its passthrough and rescue variants to
callers.

Signed-off-by: katelyn martin <[email protected]>

* docs(app/core): document `ResponseBody<R, B>`

this adds a small documentation comment describing what this type does.

Signed-off-by: katelyn martin <[email protected]>

* refactor(app/core): a unit test suite for rescue body

this commit introduces a test suite for our error recovery middleware.

this body middleware provides a mechanism to "rescue" errors, gracefully
mapping an error encountered when polling a gRPC body into e.g. trailers
with a gRPC status code.

before we upgrade this middleware in service of linkerd/linkerd2#8733,
we add some test coverage to ensure that we preserve this middleware.

Signed-off-by: katelyn martin <[email protected]>

---------

Signed-off-by: katelyn martin <[email protected]>