net/http/httputil: ReverseProxy does not pass unannounced trailers

[tcolgate]

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

go version devel +2d20ded584 Sat May 20 00:45:56 2017 +0000 linux/amd64

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

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/tristan/src/go"
GORACE=""
GOROOT="/home/tristan/src/go"
GOTOOLDIR="/home/tristan/src/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build356611585=/tmp/go-build -gno-record-gcc-switches"
CXX="g++"
CGO_ENABLED="1"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"

What did you do?

Attempt to use grpc-go via a httputil.ReverseProxy. grpc.Server does not pre-anounce its trailers. ReverseProxy only passes on pre-anounced trailers.

https://play.golang.org/p/Lt7QPq9vmX

What did you expect to see?

All trailers from upstream should be passed on

What did you see instead?

Only trailers set in the Trailer header are passed on

[gopherbot]

CL https://golang.org/cl/43712 mentions this issue.

[tcolgate]

With the above patch, both helloworld and routeguide grpc-go example will work via httputil.ReverseProxy

[odeke-em]

I think this might be a breaking change and that's the behavior for any handler/handlefunc
in Go and not only for httputil.ReverseProxy, unless I am mistaken.

I defer to @bradfitz for his view.

[tcolgate]

The runtime has explicit support for unannounced trailers (marking the headers with the http.TraolerPrefix prefix. It would seem odd if it were expected behaviour for tReversdProxy to drop them. Perhaps I'm missing your point?