Add interop tests to verify that Kestrel's HTTP/2 implementation works with HttpClient's implementation

[Tratcher]

There are not currently any functional test for the Http2 features. We accidentally disabled the entire feature when merging the https and core packages due to a missing TFM and didn't notice. aspnet/KestrelHttpServer#2224

The big blocker here is the lack of consistent client support. However, even if there's one client on one of the OS's then that would be a good start.

[linianhui]

Does client mean web browser or web server?

[Tratcher]

Browsers are clients, yes. System.Net.Http.HttpClient is the client we prefer for functional tests.

[linianhui]

@Tratcher But now System.Net.Http.HttpClient does not support http2. (￣▽￣)"

[benaadams]

HttpClient supports http2 on .NET Core for UWP, Linux dotnet/corefx#4756 and Windows dotnet/corefx#10948 (but not Full Framework as yet)

[Tratcher]

Note the Windows version restriction as well.
Windows 10 Anniversary release a.k.a Windows 10 Version 1607

[Tratcher]

I did some initial work in #2665 but only for Win10 WinHttpHandler. Linux and Mac have too many dependency issues, we may have to wait for SocketsHttpHandler.

[muratg]

The remaining test we'd like to add is using new HttpClient.

[analogrelay]

We think there's enough support to do this now.

[analogrelay]

@Tratcher to follow up on the status of HTTP/2 support in HttpClient.

[analogrelay]

@Tratcher turned the existing tests on again 🎉

We do need some additional interop tests that use Trailers.

[analogrelay]

This issue is now tracking adding tests that use Trailers to the HttpClient interop test suite.

[Tratcher]

HttpClient interop to verify:

• ALPN handshake
• H2C (via app context switch AppContext.SetSwitch("System.Net.Http.SocketsHttpHandler.Http2UnencryptedSupport", true);)
• Bidirectional streaming https://github.com/dotnet/corefx/issues/38559
• Response Trailers
• Reset
  • Reset after CompleteAsync (kill the request body)
• Multi-frame request & response headers
• Settings
  • Concurrent request limits
  • Frame size limits
  • Header table size
  • Window size
  • Max header list size
• Header validation (e.g. Custom host, url encoding, header encoding)

[analogrelay]

Moving to preview9 for now. The bar for test-only changes is super low (basically just tell mode)

[analogrelay]

We can keep churning on this in master for now. We have a lot of tests now, but @Tratcher's list has some things we still want to cover. We'll look at backporting test coverage if it would be helpful.