Revert "net/http/httputil: allow ReverseProxy to call ModifyResponse on failed requests"

This reverts commit https://golang.org/cl/54030

Reason for revert: to not paint ourselves into a corner.
See https://github.com/golang/go/issues/23009

Fixes #23009
Updates #21255

Change-Id: I68caab078839b9d2bf645a7bbed8405a5a30cd22
Reviewed-on: https://go-review.googlesource.com/86435Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/net/http/httputil/reverseproxy.go

@@ -191,11 +191,10 @@ func (p *ReverseProxy) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
 	}
 
 	res, err := transport.RoundTrip(outreq)
-	if res == nil {
-		res = &http.Response{
-			StatusCode: http.StatusBadGateway,
-			Body:       http.NoBody,
-		}
+	if err != nil {
+		p.logf("http: proxy error: %v", err)
+		rw.WriteHeader(http.StatusBadGateway)
+		return
 	}
 
 	removeConnectionHeaders(res.Header)
@@ -205,16 +204,12 @@ func (p *ReverseProxy) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
 	}
 
 	if p.ModifyResponse != nil {
-		if err != nil {
+		if err := p.ModifyResponse(res); err != nil {
 			p.logf("http: proxy error: %v", err)
+			rw.WriteHeader(http.StatusBadGateway)
+			res.Body.Close()
+			return
 		}
-		err = p.ModifyResponse(res)
-	}
-	if err != nil {
-		p.logf("http: proxy error: %v", err)
-		rw.WriteHeader(http.StatusBadGateway)
-		res.Body.Close()
-		return
 	}
 
 	copyHeader(rw.Header(), res.Header)

src/net/http/httputil/reverseproxy_test.go

@@ -631,35 +631,6 @@ func TestReverseProxyModifyResponse(t *testing.T) {
 	}
 }
 
-// Issue 21255. Test ModifyResponse when an error from transport.RoundTrip
-// occurs, and that the proxy returns StatusOK.
-func TestReverseProxyModifyResponse_OnError(t *testing.T) {
-	// Always returns an error
-	errBackend := httptest.NewUnstartedServer(nil)
-	errBackend.Config.ErrorLog = log.New(ioutil.Discard, "", 0) // quiet for tests
-	defer errBackend.Close()
-
-	rpURL, _ := url.Parse(errBackend.URL)
-	rproxy := NewSingleHostReverseProxy(rpURL)
-	rproxy.ModifyResponse = func(resp *http.Response) error {
-		// Will be set for a non-nil error
-		resp.StatusCode = http.StatusOK
-		return nil
-	}
-
-	frontend := httptest.NewServer(rproxy)
-	defer frontend.Close()
-
-	resp, err := http.Get(frontend.URL)
-	if err != nil {
-		t.Fatalf("failed to reach proxy: %v", err)
-	}
-	if resp.StatusCode != http.StatusOK {
-		t.Errorf("err != nil: got res.StatusCode %d; expected %d", resp.StatusCode, http.StatusOK)
-	}
-	resp.Body.Close()
-}
-
 // Issue 16659: log errors from short read
 func TestReverseProxy_CopyBuffer(t *testing.T) {
 	backendServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {