net/http/httptest: Server uses ipv4 instead of using both Ipv4 and Ipv6

[bdollma-te]

Go version

go version go1.21.0 linux/amd64

Output of go env in your module/workspace:

GO111MODULE=''
GOARCH='amd64'
GOBIN='/auto/srg-sce-swinfra-usr/emb/users/bdollma/sdflow/scripts/../bin'
GOCACHE='/auto/srg-sce-swinfra-usr/emb/users/bdollma/go/cache'
GOENV='/users/bdollma/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMODCACHE='/auto/srg-sce-swinfra-usr/emb/users/bdollma/sdflow/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/auto/srg-sce-swinfra-usr/emb/users/bdollma/sdflow/scripts/..'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/auto/srg-sce-swinfra-usr/emb/users/bdollma/go1.21.0'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/auto/srg-sce-swinfra-usr/emb/users/bdollma/go1.21.0/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.21.0'
GCCGO='gccgo'
GOAMD64='v1'
AR='ar'
CC='gcc'
CXX='g++'
CGO_ENABLED='1'
GOMOD='/dev/null'
GOWORK='/auto/srg-sce-swinfra-usr/emb/users/bdollma/sdflow/go.work'
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build1206231536=/tmp/go-build -gno-record-gcc-switches'

What did you do?

Create a httptest.Server using the NewServer.

func TestIssue(t *testing.T) {
	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		w.WriteHeader(http.StatusOK)
	}))
	defer srv.Close()

	u, err := url.Parse(srv.URL)
	require.NoError(t, err)
	port, err := strconv.Atoi(u.Port())
	require.NoError(t, err)
	url := fmt.Sprintf("http://localhost:%d", port)

	client := http.Client{}
	req, err := http.NewRequest(http.MethodPost, url, http.NoBody)
	require.NoError(t, err)
	req.Close = true
	resp, err := client.Do(req)
	require.NoError(t, err)
	require.Equal(t, http.StatusOK, resp.StatusCode)
}

Run the test:

go test -v . -run TestIssue -count 10000 -failfast
    http_server_test.go:280: 
                Error Trace:    ...
                Error:          Received unexpected error:
                                Post "http://localhost:44269": EOF
                Test:           TestIssue

What did you see happen?

After drilling down, the listener created for httptest.Server is IPv4 only:

go/src/net/http/httptest/server.go

Line 68 in 39f2032

l, err := net.Listen("tcp", "127.0.0.1:0")

and uses 127.0.0.1:0.

The test fails because at some point the client starts using IPv6. The server simply doesn't respond.

What did you expect to see?

At least a better error. Simply returning EOF doesn't remotely help pinpoint the error.
You can argue that I should have used srv.URL instead of parsing the port and using localhost:port. However, in real world scenarios the code expects a port and runs a DNS lookup.

Even better, why not listen at both ipv4 and ipv6 by default?

l, err := net.Listen("tcp", ":0")

[ianlancetaylor]

CC @neild

[gabyhelp]

Related Issues

• net/http/httptest: failed to listen on a port: listen tcp6 [::1]:0: socket #61452 (closed)
• redhat 6.0 go test failure while centos 6.0 tested fine #3854 (closed)
• net/http: No way to bind a server to a tcp4 address. #5197 (closed)
• net/http: error dropped when header read times out #56390
• net/http: test failed on freebsd-12.2-STABLE #43984 (closed)
• net/http: server will drop tcp connections after a certain number of concurrent connections are made #61924 (closed)
• net/http: TestTransportMaxConnsPerHost still flaky #31982 (closed)
• proposal: net/http/httptest: listen on IPv6/IPv4-agnostic loopback address #56998
• net/http/httptest: NewTLSServer doesn't support HTTP/2 requests #34939 (closed)
• proposal: net: UDPAddr/TCPAddr AddrPort should map 4in6 addresses to IPv4 netip.Addrs #53607

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[seankhliao]

Duplicate of #56998