Runtime: Share HTTP/2 parsing code between Kestrel and HttpClient

[anurse]

Right now, HttpClient and Kestrel share some HTTP/2 parsing code (including HPACK codec) via a set of copied code that had drifted a bit. In the 5.0 timeline, we should work on sharing this code in a way that ensures we have only one authoritative code-base.

Given the complexity and performance-sensitive nature of this code, a public API is not likely to be feasible (at least not in the near-term). Probably the easiest way to do this moving forward would be to build a "shared-source" NuGet package (this is something System.Text.Json did for a while) which would allow the code to flow from corefx in to ASP.NET Core without requiring a public API or publishing packages to NuGet.

cc @Tratcher @karelz

[karelz]

Besides HPACK (which we planned to share better in 5.0) what else could we share?

[anurse]

Probably just HPACK. @Tratcher mentioned he thought there was some frame parsing code that was copy-pasted.

[anurse]

which we planned to share better in 5.0

Is there an issue tracking that already? I didn't see one it a brief search.

[geoffkizer]

HPACK is the easiest and highest-value code to share. We had agreed to try to do this a long time ago, but we never really got off the ground with it. If we want to get serious about this (and I do think would be good), then we need to actually put together a plan here and work toward it.

As far as other code we could share:
(1) There's a small amount of frame parsing code that could in theory be shared, but it's such a minimal amount of code that it's probably more trouble than it's worth.
(2) There's a lot of frame processing and connection/stream management code, but I think this is pretty hard to share for a variety of reasons.

There's no downside to investigating either of this further, but it seems like we should actually get the HPACK code to a point where it's shared first and then we can discuss whether there is anything else that makes sense to share.

[geoffkizer]

For the future, i.e. HTTP3 and QUIC, we should look at how to share other code as well, particularly QPACK, which is the HTTP3 version of HPACK.

[karelz]

Triage: We need some plan - none of the options is clearly best.

Note (personal): CoreFX should not depend on code higher up in ASP.NET --> the flow should IMO go from bottom to up.

[karelz]

@scalablecory please kick off email discussion about how to make it happen. We may need a meeting later on.

[Tratcher]

@karelz we've had several email threads without any conclusions, it's time to meet and hash this out.

[karelz]

Yep, that's why we need list of options clarified and then a meeting :).

[scalablecory]

Will compile a list of options and results of discussions, and send new email to prep for meeting.

[karelz]

Triage: We believe that majority of the sharing was done.
If there are ideas to share specific code more, please file new specific issues, otherwise we will sit on this one indefinitely.