httptest: empty response (EOF)

[andig]

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

$ go version
go version go1.16.5 darwin/arm64

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="arm64"
GOBIN=""
GOCACHE="/Users/andig/Library/Caches/go-build"
GOENV="/Users/andig/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="arm64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/andig/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/andig/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/opt/homebrew/Cellar/go/1.16.5/libexec"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/opt/homebrew/Cellar/go/1.16.5/libexec/pkg/tool/darwin_arm64"
GOVCS=""
GOVERSION="go1.16.5"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/andig/htdocs/evcc/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 -arch arm64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/sv/rs_453y57xj86xsbz3kw1mbc0000gn/T/go-build3556353379=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

Testing an http client doing JSON request using httptest my test always failed.

This is the test:

func TestHttp(t *testing.T) {
	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		_, _ = fmt.Println(w, `{"list":[{"useMeter":true}]}`)
	}))
	defer ts.Close()

	resp, err := http.Get(ts.URL)
	if err != nil {
		panic(err)
	}
	defer resp.Body.Close()

	b, err := io.ReadAll(resp.Body)
	if err != nil {
		panic(err)
	}

	t.Log(string(b))

	var res interface{}
	err = json.Unmarshal(b, &res)
	t.Log(err)
}

What did you expect to see?

Successful test, the JSON is valid.

What did you see instead?

unexpected end of JSON input

The error is from inside the json decoder. If I change the test to

var res interface{}
err = json.NewDecoder(resp.Body).Decode(&res)
t.Log(err)

the error changes to

EOF

Stepping through the original test I can see that the io.ReadAlls result is already empty which it probably shouldn't be?

[andig]

It does help to use Fprintln here...

[pgarneau]

So what did you end up doing? Getting a similar issue and causing one of my tests to be flaky.

[davecheney]

@pgarneau please open a new issue