contrib/net/http: copy request in RoundTrip

contrib/net/http/roundtripper.go

@@ -49,13 +49,19 @@ func (rt *roundTripper) RoundTrip(req *http.Request) (res *http.Response, err er
 	if rt.cfg.before != nil {
 		rt.cfg.before(req, span)
 	}
-	// inject the span context into the http request
-	err = tracer.Inject(span.Context(), tracer.HTTPHeadersCarrier(req.Header))
+	r2 := req.WithContext(ctx)
+	// deep copy of the Header
+	r2.Header = make(http.Header, len(req.Header))
+	for k, s := range req.Header {
+		r2.Header[k] = append([]string(nil), s...)
+	}
+	// inject the span context into the http request copy
+	err = tracer.Inject(span.Context(), tracer.HTTPHeadersCarrier(r2.Header))
 	if err != nil {
 		// this should never happen
 		fmt.Fprintf(os.Stderr, "contrib/net/http.Roundtrip: failed to inject http headers: %v\n", err)
 	}
-	res, err = rt.base.RoundTrip(req.WithContext(ctx))
+	res, err = rt.base.RoundTrip(r2)
 	if err != nil {
 		span.SetTag("http.errors", err.Error())
 		span.SetTag(ext.Error, err)

contrib/net/http/roundtripper_test.go

@@ -229,6 +229,31 @@ func TestRoundTripperAnalyticsSettings(t *testing.T) {
 	})
 }
 
+// TestRoundTripperCopy is a regression test ensuring that RoundTrip
+// does not modify the request per the RoundTripper contract. See:
+// https://cs.opensource.google/go/go/+/refs/tags/go1.18.1:src/net/http/client.go;l=129-133
+func TestRoundTripperCopy(t *testing.T) {
+	mt := mocktracer.Start()
+	defer mt.Stop()
+
+	s := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		_, err := tracer.Extract(tracer.HTTPHeadersCarrier(r.Header))
+		assert.NoError(t, err)
+		w.Write([]byte("Hello World"))
+	}))
+	defer s.Close()
+
+	initialReq, err := http.NewRequest("GET", s.URL+"/hello/world", nil)
+	assert.NoError(t, err)
+	req, err := http.NewRequest("GET", s.URL+"/hello/world", nil)
+	assert.NoError(t, err)
+	rt := WrapRoundTripper(http.DefaultTransport).(*roundTripper)
+	_, err = rt.RoundTrip(req)
+	assert.NoError(t, err)
+	assert.Len(t, req.Header, 0)
+	assert.Equal(t, initialReq, req)
+}
+
 func TestServiceName(t *testing.T) {
 	s := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		w.Write([]byte("Hello World"))