net/http: Client cert auth with TLSClientConfig Certificates is not working while using GetCertificates works

[stlaz]

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

$ go version
go version go1.13 linux/amd64

Does this issue reproduce with the latest release?

it does reproduce with the latest Go version in Arch which usually corresponds to the latest release

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

go env Output

$ uname -ar
Linux some-hostname 5.2.13-arch1-1-ARCH #1 SMP PREEMPT Fri Sep 6 17:52:33 UTC 2019 x86_64 GNU/Linux
$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN="/home/__redacted__/go/bin"
GOCACHE="/home/__redacted__/.cache/go-build"
GOENV="/home/__redacted__/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/__redacted__/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/lib/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/lib/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
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-build789481132=/tmp/go-build -gno-record-gcc-switches"

What did you do?

I was writing a simple client with client cert authentication. My server allows for this kind of auth along with other possible authentication options. I followed the docs by specifying the Certificates field for the transport's TLSClientConfig as so:

https://play.golang.org/p/il0hsOJ-3UG

In this case, however, the cert and key were ignored for the authentication and since the server handles unauthenticated requests by redirecting somewhere else, this redirect indeed happened.

When I changed setting the Certificates field to instead use the GetClientCertificates, stuff started working and my server was correctly reacting to my client cert authentication attempts. The new, working code:

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

What did you expect to see?

I expected the Certificates field to be still working for client cert auth

What did you see instead?

cert/key pair were ignored, no client auth was attempted

[odeke-em]

Thank you for the report @stlaz and welcome to the Go project!

Kindly pinging a crypto expert @FiloSottile to take a look since this involves an interactions
between net/http and crypto/tls.