net/http/httputil: copy header map if necessary in ReverseProxy

We were already making a copy of the map before removing
hop-by-hop headers. This commit does the same for proxied
headers mentioned in the "Connection" header.

A test is added to ensure request headers are not modified.

Updates #16875

Change-Id: I85329d212787958d5ad818915eb0538580a4653a
Reviewed-on: https://go-review.googlesource.com/28493Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/httputil/reverseproxy.go

@@ -152,11 +152,20 @@ func (p *ReverseProxy) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
 	p.Director(outreq)
 	outreq.Close = false
 
+	// We are modifying the same underlying map from req (shallow
+	// copied above) so we only copy it if necessary.
+	copiedHeaders := false
+
 	// Remove headers with the same name as the connection-tokens.
 	// See RFC 2616, section 14.10.
 	if c := outreq.Header.Get("Connection"); c != "" {
 		for _, f := range strings.Split(c, ",") {
 			if f = strings.TrimSpace(f); f != "" {
+				if !copiedHeaders {
+					outreq.Header = make(http.Header)
+					copyHeader(outreq.Header, req.Header)
+					copiedHeaders = true
+				}
 				outreq.Header.Del(f)
 			}
 		}
@@ -164,10 +173,7 @@ func (p *ReverseProxy) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
 
 	// Remove hop-by-hop headers to the backend. Especially
 	// important is "Connection" because we want a persistent
-	// connection, regardless of what the client sent to us. This
-	// is modifying the same underlying map from req (shallow
-	// copied above) so we only copy it if necessary.
-	copiedHeaders := false
+	// connection, regardless of what the client sent to us.
 	for _, h := range hopHeaders {
 		if outreq.Header.Get(h) != "" {
 			if !copiedHeaders {

src/net/http/httputil/reverseproxy_test.go

@@ -156,12 +156,17 @@ func TestReverseProxyStripHeadersPresentInConnection(t *testing.T) {
 		t.Fatal(err)
 	}
 	proxyHandler := NewSingleHostReverseProxy(backendURL)
-	frontend := httptest.NewServer(proxyHandler)
+	frontend := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		proxyHandler.ServeHTTP(w, r)
+		if c := r.Header.Get("Upgrade"); c != "original value" {
+			t.Errorf("handler modified header %q = %q; want %q", "Upgrade", c, "original value")
+		}
+	}))
 	defer frontend.Close()
 
 	getReq, _ := http.NewRequest("GET", frontend.URL, nil)
 	getReq.Header.Set("Connection", "Upgrade, "+fakeConnectionToken)
-	getReq.Header.Set("Upgrade", "foo")
+	getReq.Header.Set("Upgrade", "original value")
 	getReq.Header.Set(fakeConnectionToken, "should be deleted")
 	res, err := http.DefaultClient.Do(getReq)
 	if err != nil {