net/http: Transport doesn't support specify Transfer-Encoding explicitly

[zema1]

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

$ go version
go version go1.12.6 darwin/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="/Users/zg/Library/Caches/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/zg/go"
GOPROXY=""
GORACE=""
GOROOT="/usr/local/go"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/zg/Development/gunkit/go.mod"
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 -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/lg/cht6v2795qn9z07kmcl6h3c40000gn/T/go-build863622379=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

I know http.Transport will use chunked Encoding automatically when Request has body and Request.Content-Length was set < 0. But this troubled me when I use net.http in my own penetration testing tool, which always need to build a custom chunked body. The code simply looks like the following snippet：

buf := &bytes.Buffer{}
ck := NewCustomChunkedWriter(buf)
ck.WriteString("test")
ck.Close()
req, _ = http.NewRequest("POST", "http://xx.com", buf)
req.Header.Set("Transfer-Encoding", "chunked")

Howover, it didn't work as expected. I tried to trace the roundtrip method and found that Transfer-Encoding is in a map named reqWriteExcludeHeader, which looks like a whitelist when writing request header.

And if I set req.Transfer-Encoding = []string{"chunked"}， the request body will be chunked twice! one from my customChunkedWriter, the other from the internal chunked writer.

Finally, I believe there is actually no way to make it. I think It's not a intended behavior. It should be great if it acts like Accept-Encoding.

If the Transport requests gzip on its own and gets a gzipped response, it's transparently decoded in the Response.Body. However, if the user explicitly requested gzip it is not automatically uncompressed.

Related issue: #28026

What did you expect to see?

If I send Transfer-Encoding explicitly, transport should keep the header and do nothing about the request body.

What did you see instead?

Described as above.

[bcmills]

Duplicate of #28026