Add implementation of h2c into x/net/http2/h2c.

[misterwilliam]

I would like to add an implementation of h2c into x/net/http2/h2c.

The proposed implementation strategy is to implement h2c by leveraging the existing http2.Server by implementing the 2 ways to start an h2c connection as described in RFC 7540, which are: (1)
create a connection starting with HTTP/1 and then upgrading to h2c [Section 3.2] and (2) starting a connection directly speaking h2c (aka starting with prior knowledge) [Section 3.4].

For both of the above connection methods the implementation strategy is to hijack a HTTP/1 connection, perform the h2c connection on the hijacked net.Conn, and create a suitably configured net.Conn to pass into http2.Server.ServeConn.

For h2c with prior knowledge this is relatively simple. For that we just have to verify the HTTP/2 client preface has been written to the net.Conn, and then reforward the client preface to the hijacked connection.

For h2c upgraded from HTTP/1, this is a bit more involved. First we validate the HTTP/1 Upgrade request, and respond to the client with 101 Switching Protocols. Then we write a HTTP/2 client preface on behalf of the client, and a settings frame and a headers frame which correspond to what was in
the upgrade request. Then since http2.Server is going respond with a settings ACK, we swallow it so that it is not forwarded to the client since for h2c upgrade from HTTP/1 the 101 Switching Protocols response replaces the settings ACK.

I have a working example of this strategy submitted to gerrit as: 112997

The code I have will obviously need some work before submission, but I think I am far enough that I would like to start the conversation on what the next steps should be. (This is my first time trying to contribute, so I'm a little confused what the process is. I am under the impression that I should get some feedback on my proposal before trying to produce the final solution.)

[gopherbot]

Change https://golang.org/cl/112997 mentions this issue: Add implementation of h2c into x/net/http2/h2c.

[odeke-em]

/cc @tombergan @bradfitz

[agnivade]

Seems like a straight dupe of #14141. Any reason this needs to be opened as a new issue again ?

[bradfitz]

Duplicate of #14141 (magic phrase for GitHub)

Yes, let's use #14141 instead.