net/http/httputil: eliminate duplicate alloc/copy in ReverseProxy

This commit elimates the request allocation and shallow copy duplication
already done by req.WithContext.

name         old time/op    new time/op    delta
ServeHTTP-4     216µs ±36%     212µs ±15%     ~     (p=0.853 n=10+10)

name         old alloc/op   new alloc/op   delta
ServeHTTP-4     917kB ±36%    1137kB ± 0%     ~     (p=0.352 n=10+10)

name         old allocs/op  new allocs/op  delta
ServeHTTP-4      5.00 ± 0%      4.00 ± 0%  -20.00%  (p=0.000 n=10+10)

Change-Id: I514a59c30b037c7a65c355b06fd82c2d6ff17bb0
Reviewed-on: https://go-review.googlesource.com/35569Reviewed-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

@@ -149,12 +149,10 @@ func (p *ReverseProxy) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
 		}()
 	}
 
-	outreq := new(http.Request)
-	*outreq = *req // includes shallow copies of maps, but okay
+	outreq := req.WithContext(ctx) // includes shallow copies of maps, but okay
 	if req.ContentLength == 0 {
 		outreq.Body = nil // Issue 16036: nil Body for http.Transport retries
 	}
-	outreq = outreq.WithContext(ctx)
 
 	p.Director(outreq)
 	outreq.Close = false

src/net/http/httputil/reverseproxy_test.go

@@ -664,3 +664,30 @@ func TestReverseProxy_CopyBuffer(t *testing.T) {
 		}
 	}
 }
+
+type staticTransport struct {
+	res *http.Response
+}
+
+func (t *staticTransport) RoundTrip(r *http.Request) (*http.Response, error) {
+	return t.res, nil
+}
+
+func BenchmarkServeHTTP(b *testing.B) {
+	res := &http.Response{
+		StatusCode: 200,
+		Body:       ioutil.NopCloser(strings.NewReader("")),
+	}
+	proxy := &ReverseProxy{
+		Director:  func(*http.Request) {},
+		Transport: &staticTransport{res},
+	}
+
+	w := httptest.NewRecorder()
+	r := httptest.NewRequest("GET", "/", nil)
+
+	b.ReportAllocs()
+	for i := 0; i < b.N; i++ {
+		proxy.ServeHTTP(w, r)
+	}
+}