net/http: populate Uncompressed Content-Length

[FireFart]

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

$ go version
go version go1.14.2 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
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/firefart/.cache/go-build"
GOENV="/home/firefart/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/firefart/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="gcc"
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"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build918042854=/tmp/go-build -gno-record-gcc-switches"

What did you do?

Currently when the net/http package receives a compressed response, the ContentLength is set to -1 and removed from the Header array:
https://golang.org/pkg/net/http/#Response (See field Uncompressed)

Often we are not interested in the body but only the Content-Length. If that's the case we often do io.Copy(ioutil.Discard, resp.Body) to consume the body.

Without checking the body length or the return from ioutil.Discard it's not possible to get the uncompressed Content-Length from the Response.

What did you expect to see?

I would suggest to introduce a new Property in the Response like UncompressedContentLength to make the uncompressed size available to the caller before the body is read. At this stage the size should already be known to go, as the Uncompressed property has already been processed.

What did you see instead?

[seankhliao]

Knowing that the body is compressed is different from knowing the size of the body.
The server may continue to stream the body, so you'd have no way of knowing the size of the body without reading it all.