Since the internet failed me, and the only working example of a H2C client I can find was in the actual go code test suite, I’m going to lay out what I discovered about H2C support in golang here.

First is that the standard golang code supports HTTP2 but does not directly support H2C. H2C support only exists in the golang.org/x/net/http2/h2c package. You can make your HTTP server H2C capable by wrapping your handler or mux with h2c.NewHandler() like so.

h2s := &http2.Server{}
 
handler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
    fmt.Fprintf(w, "Hello, %v, http: %v", r.URL.Path, r.TLS == nil)
})
 
server := &http.Server{
    Addr:    "0.0.0.0:1010",
    Handler: h2c.NewHandler(handler, h2s),
}
 
fmt.Printf("Listening [0.0.0.0:1010]...\n")
checkErr(server.ListenAndServe(), "while listening")

The above code allows the server to support H2C upgrade and H2C prior knowledge along with standard HTTP/2 and HTTP/1.1 that golang natively supports.

If you don’t care about supporting HTTP/1.1 then you can run this code which only supports H2C prior knowledge.

server := http2.Server{}
 
l, err := net.Listen("tcp", "0.0.0.0:1010")
checkErr(err, "while listening")
 
fmt.Printf("Listening [0.0.0.0:1010]...\n")
for {
    conn, err := l.Accept()
    checkErr(err, "during accept")
 
    server.ServeConn(conn, &http2.ServeConnOpts{
        Handler: http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
            fmt.Fprintf(w, "Hello, %v, http: %v", r.URL.Path, r.TLS == nil)
        }),
    })
}

Once you have a running server you can test your server by installing curl-openssl.

$ brew install curl-openssl
 
# Add curl-openssl to the front of your path
$ export PATH="/usr/local/opt/curl-openssl/bin:$PATH"

You can now use curl to test your H2C enabled server like so.

Connect via HTTP1.1 then upgrade to HTTP/2 (H2C)
$ curl -v --http2 http://localhost:1010
*   Trying ::1:1010...
* TCP_NODELAY set
* Connected to localhost (::1) port 1010 (#0)
> GET / HTTP/1.1
> Host: localhost:1010
> User-Agent: curl/7.65.0
> Accept: */*
> Connection: Upgrade, HTTP2-Settings
> Upgrade: h2c
> HTTP2-Settings: AAMAAABkAARAAAAAAAIAAAAA
>
* Mark bundle as not supporting multiuse
< HTTP/1.1 101 Switching Protocols
< Connection: Upgrade
< Upgrade: h2c
* Received 101
* Using HTTP2, server supports multi-use
* Connection state changed (HTTP/2 confirmed)
* Copying HTTP/2 data in stream buffer to connection buffer after upgrade: len=0
* Connection state changed (MAX_CONCURRENT_STREAMS == 250)!
< HTTP/2 200
< content-type: text/plain; charset=utf-8
< content-length: 20
< date: Wed, 05 Jun 2019 19:01:40 GMT
<
* Connection #0 to host localhost left intact
Hello, /, http: true
Connect via HTTP/2 (H2C)
$ curl -v --http2-prior-knowledge http://localhost:1010
*   Trying ::1:1010...
* TCP_NODELAY set
* Connected to localhost (::1) port 1010 (#0)
* Using HTTP2, server supports multi-use
* Connection state changed (HTTP/2 confirmed)
* Copying HTTP/2 data in stream buffer to connection buffer after upgrade: len=0
* Using Stream ID: 1 (easy handle 0x7fdab8007000)
> GET / HTTP/2
> Host: localhost:1010
> User-Agent: curl/7.65.0
> Accept: */*
>
* Connection state changed (MAX_CONCURRENT_STREAMS == 250)!
< HTTP/2 200
< content-type: text/plain; charset=utf-8
< content-length: 20
< date: Wed, 05 Jun 2019 19:00:43 GMT
<
* Connection #0 to host localhost left intact
Hello, /, http: true

Now, Remember when I said that the golang standard library does not support H2C? While that is technically correct there is a workaround to get the golang standard http2 client to connect to an H2C enabled server.

To do so you have to override DialTLS and set the super secret AllowHTTP flag.

client := http.Client{
    Transport: &http2.Transport{
        // So http2.Transport doesn't complain the URL scheme isn't 'https'
        AllowHTTP: true,
        // Pretend we are dialing a TLS endpoint. (Note, we ignore the
        // passed tls.Config)
        DialTLS: func(network, addr string, cfg *tls.Config) (net.Conn, error) {
            return net.Dial(network, addr)
        },
    },
}
 
resp, _ := client.Get(url)
fmt.Printf("Client Proto: %d\n", resp.ProtoMajor)

Although this all looks a little wonky it actually works really well and performs nicely in production environments.

This project here contains complete working examples evoke types of servers and client. http://github.com/thrawn01/h2c-golang-example