net/http: cannot set custom/special pseudo-headers when using HTTP/2

[jadr2ddude]

What version of Go are you using (go version)?

$ go version
go version go1.12.5 linux/amd64

Does this issue reproduce with the latest release?

Yes.

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/jadenw/.cache/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/jadenw/go"
GOPROXY=""
GORACE=""
GOROOT="/usr/lib/go"
GOTMPDIR=""
GOTOOLDIR="/usr/lib/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build648789117=/tmp/go-build -gno-record-gcc-switches"

What did you do?

I attempted to implement websockets over HTTP/2, AKA RFC 8441, with net/http.

What did you expect to see?

Some mechanism of setting :protocol as defined in section 5 of RFC 8441.

What did you see instead?

There is no way to specify extra pseudo-headers in a request with net/http, at least that I can find in the header encoding code.

I am really not sure what the best way to solve this would be, or whether it is fully worthwhile. However, I would really like to be able to use a user-provided *http.Client for this, and this is the only thing I have not yet found a workaround for.

Clarification: pseudo-headers do not satisfy httpguts.ValidHeaderFieldName and therefore cannot be passed in with the regular headers

[jadr2ddude]

Also, a link to what I have currently of the client side of the HTTP/2 handshake: https://github.com/jadr2ddude/exp/blob/1ed5641ad76ca5761af3a652406bf49b7cac2ce3/ws/handshake.go#L248

[andybons]

@bradfitz @mikioh

[mikedanese]

This is somewhat related to #27244