go: net/http: transport caches permanently broken persistent connections if write error happens during h2 handshake

The fix for https://github.com/golang/go/issues/34978 is not correct, the issue is still present.

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

$ go version
go version go1.13.12 linux/amd64

Does this issue reproduce with the latest release?

Yes

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

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/cfriesen/.cache/go-build"
GOENV="/home/cfriesen/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/cfriesen/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"
GOMOD="/home/cfriesen/devel/h2bug/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 -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build843802376=/tmp/go-build -gno-record-gcc-switches"

What did you do?

Test-case is here: https://github.com/mikedanese/h2bug

What did you expect to see?

The persistent connection is not cached by http.Transport.

What did you see instead?

The persistent connection is cached by http.Transport.

More

Commit https://go.googlesource.com/go/+/88186e5e232625f9c91d639e0cb90a88c6cf1172 was submitted in order to fix https://github.com/golang/go/issues/34978, but it appears to be not quite correct due to how http2/transport.go gets included into http via h2_bundle.go with an “http2” prefix added.

If I run the above testcase using “dlv test”, then add a breakpoint in src/net/http/transport.go right after the assignment to isH2DialError, and then run “print pconn.alt”, it gives me

(dlv) print pconn.alt
net/http.RoundTripper(golang.org/x/net/http2.erringRoundTripper) {
        err: error(*errors.errorString) *{
                s: "broken pipe",},}

At the same time, “isH2DialError” is “false” when it clearly should be “true” for the bugfix to be correct.

Trying to explicitly assert the types gave the following interesting results:

(dlv) print pconn.alt.(http2.erringRoundTripper)
golang.org/x/net/http2.erringRoundTripper {
        err: error(*errors.errorString) *{
                s: "broken pipe",},}
(dlv) print pconn.alt.(http.http2erringRoundTripper)
Command failed: interface conversion: net/http.RoundTripper is golang.org/x/net/http2.erringRoundTripper, not net/http.http2erringRoundTripper