net/http/httputil: ReverseProxy request cancellation

If an inbound connection is closed, cancel the outbound http request.

This is particularly useful if the outbound request may consume resources
unnecessarily until it is cancelled.

Fixes #8406

Change-Id: I738c4489186ce342f7e21d0ea3f529722c5b443a
Signed-off-by: Peter Waller <p@pwaller.net>
Reviewed-on: https://go-review.googlesource.com/2320Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/httputil/reverseproxy.go

@@ -100,6 +100,24 @@ var hopHeaders = []string{
 	"Upgrade",
 }
 
+type requestCanceler interface {
+	CancelRequest(*http.Request)
+}
+
+type runOnFirstRead struct {
+	io.Reader
+
+	fn func() // Run before first Read, then set to nil
+}
+
+func (c *runOnFirstRead) Read(bs []byte) (int, error) {
+	if c.fn != nil {
+		c.fn()
+		c.fn = nil
+	}
+	return c.Reader.Read(bs)
+}
+
 func (p *ReverseProxy) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
 	transport := p.Transport
 	if transport == nil {
@@ -109,6 +127,34 @@ func (p *ReverseProxy) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
 	outreq := new(http.Request)
 	*outreq = *req // includes shallow copies of maps, but okay
 
+	if closeNotifier, ok := rw.(http.CloseNotifier); ok {
+		if requestCanceler, ok := transport.(requestCanceler); ok {
+			reqDone := make(chan struct{})
+			defer close(reqDone)
+
+			clientGone := closeNotifier.CloseNotify()
+
+			outreq.Body = struct {
+				io.Reader
+				io.Closer
+			}{
+				Reader: &runOnFirstRead{
+					Reader: outreq.Body,
+					fn: func() {
+						go func() {
+							select {
+							case <-clientGone:
+								requestCanceler.CancelRequest(outreq)
+							case <-reqDone:
+							}
+						}()
+					},
+				},
+				Closer: outreq.Body,
+			}
+		}
+	}
+
 	p.Director(outreq)
 	outreq.Proto = "HTTP/1.1"
 	outreq.ProtoMajor = 1

src/net/http/httputil/reverseproxy_test.go

@@ -8,6 +8,7 @@ package httputil
 
 import (
 	"io/ioutil"
+	"log"
 	"net/http"
 	"net/http/httptest"
 	"net/url"
@@ -211,3 +212,58 @@ func TestReverseProxyFlushInterval(t *testing.T) {
 		t.Error("maxLatencyWriter flushLoop() never exited")
 	}
 }
+
+func TestReverseProxyCancellation(t *testing.T) {
+	const backendResponse = "I am the backend"
+
+	reqInFlight := make(chan struct{})
+	backend := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		close(reqInFlight)
+
+		select {
+		case <-time.After(10 * time.Second):
+			// Note: this should only happen in broken implementations, and the
+			// closenotify case should be instantaneous.
+			t.Log("Failed to close backend connection")
+			t.Fail()
+		case <-w.(http.CloseNotifier).CloseNotify():
+		}
+
+		w.WriteHeader(http.StatusOK)
+		w.Write([]byte(backendResponse))
+	}))
+
+	defer backend.Close()
+
+	backend.Config.ErrorLog = log.New(ioutil.Discard, "", 0)
+
+	backendURL, err := url.Parse(backend.URL)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	proxyHandler := NewSingleHostReverseProxy(backendURL)
+
+	// Discards errors of the form:
+	// http: proxy error: read tcp 127.0.0.1:44643: use of closed network connection
+	proxyHandler.ErrorLog = log.New(ioutil.Discard, "", 0)
+
+	frontend := httptest.NewServer(proxyHandler)
+	defer frontend.Close()
+
+	getReq, _ := http.NewRequest("GET", frontend.URL, nil)
+	go func() {
+		<-reqInFlight
+		http.DefaultTransport.(*http.Transport).CancelRequest(getReq)
+	}()
+	res, err := http.DefaultClient.Do(getReq)
+	if res != nil {
+		t.Fatal("Non-nil response")
+	}
+	if err == nil {
+		// This should be an error like:
+		// Get http://127.0.0.1:58079: read tcp 127.0.0.1:58079:
+		//    use of closed network connection
+		t.Fatal("DefaultClient.Do() returned nil error")
+	}
+}