From 5ea2360b6643ec53018b1c080f4824b8238bff3a Mon Sep 17 00:00:00 2001
From: Akhil Indurti <aindurti@gmail.com>
Date: Tue, 14 Nov 2017 11:40:09 -0500
Subject: [PATCH] net/http/httputil: allow ReverseProxy to call ModifyResponse
 on failed requests

Previously when RoundTrip returned a non-nil error, the proxy returned a
StatusBadGateway error, instead of first calling ModifyResponse. This
commit first calls ModifyResponse, whether or not the error returned
from RoundTrip is nil.

Also closes response body when ModifyResponse returns an error. See #22658.

Fixes #21255

Change-Id: I5b5bf23a69ae5608f87d4ece756a1b4985ccaa9c
Reviewed-on: https://go-review.googlesource.com/54030
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
---
 src/net/http/httputil/reverseproxy.go      | 21 ++++++++++------
 src/net/http/httputil/reverseproxy_test.go | 29 ++++++++++++++++++++++
 2 files changed, 42 insertions(+), 8 deletions(-)

diff --git a/src/net/http/httputil/reverseproxy.go b/src/net/http/httputil/reverseproxy.go
index b96bb21019..aa22d5a2fd 100644
--- a/src/net/http/httputil/reverseproxy.go
+++ b/src/net/http/httputil/reverseproxy.go
@@ -191,10 +191,11 @@ func (p *ReverseProxy) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
 	}
 
 	res, err := transport.RoundTrip(outreq)
-	if err != nil {
-		p.logf("http: proxy error: %v", err)
-		rw.WriteHeader(http.StatusBadGateway)
-		return
+	if res == nil {
+		res = &http.Response{
+			StatusCode: http.StatusBadGateway,
+			Body:       http.NoBody,
+		}
 	}
 
 	removeConnectionHeaders(res.Header)
@@ -204,12 +205,16 @@ func (p *ReverseProxy) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
 	}
 
 	if p.ModifyResponse != nil {
-		if err := p.ModifyResponse(res); err != nil {
+		if 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)
diff --git a/src/net/http/httputil/reverseproxy_test.go b/src/net/http/httputil/reverseproxy_test.go
index 2232042d3e..822828e5c0 100644
--- a/src/net/http/httputil/reverseproxy_test.go
+++ b/src/net/http/httputil/reverseproxy_test.go
@@ -631,6 +631,35 @@ 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) {