net/http: allow Client.CheckRedirect to use most recent response

Fixes #10069

Change-Id: I3819ff597d5a0c8e785403bf9d65a054f50655a6
Reviewed-on: https://go-review.googlesource.com/23207
Reviewed-by: Russ Cox <rsc@golang.org>

src/net/http/client.go

@@ -47,6 +47,9 @@ type Client struct {
 	// method returns both the previous Response (with its Body
 	// closed) and CheckRedirect's error (wrapped in a url.Error)
 	// instead of issuing the Request req.
+	// As a special case, if CheckRedirect returns ErrUseLastResponse,
+	// then the most recent response is returned with its body
+	// unclosed, along with a nil error.
 	//
 	// If CheckRedirect is nil, the Client uses its default policy,
 	// which is to stop after 10 consecutive requests.
@@ -417,6 +420,12 @@ func (c *Client) Get(url string) (resp *Response, err error) {
 
 func alwaysFalse() bool { return false }
 
+// ErrUseLastResponse can be returned by Client.CheckRedirect hooks to
+// control how redirects are processed. If returned, the next request
+// is not sent and the most recent response is returned with its body
+// unclosed.
+var ErrUseLastResponse = errors.New("net/http: use last response")
+
 // checkRedirect calls either the user's configured CheckRedirect
 // function, or the default.
 func (c *Client) checkRedirect(req *Request, via []*Request) error {
@@ -468,6 +477,7 @@ func (c *Client) doFollowingRedirects(req *Request, shouldRedirect func(int) boo
 			ireq := reqs[0]
 			req = &Request{
 				Method:   ireq.Method,
+				Response: resp,
 				URL:      u,
 				Header:   make(Header),
 				Cancel:   ireq.Cancel,
@@ -481,7 +491,27 @@ func (c *Client) doFollowingRedirects(req *Request, shouldRedirect func(int) boo
 			if ref := refererForURL(reqs[len(reqs)-1].URL, req.URL); ref != "" {
 				req.Header.Set("Referer", ref)
 			}
-			if err := c.checkRedirect(req, reqs); err != nil {
+			err = c.checkRedirect(req, reqs)
+
+			// Sentinel error to let users select the
+			// previous response, without closing its
+			// body. See Issue 10069.
+			if err == ErrUseLastResponse {
+				return resp, nil
+			}
+
+			// Close the previous response's body. But
+			// read at least some of the body so if it's
+			// small the underlying TCP connection will be
+			// re-used. No need to check for errors: if it
+			// fails, the Transport won't reuse it anyway.
+			const maxBodySlurpSize = 2 << 10
+			if resp.ContentLength == -1 || resp.ContentLength <= maxBodySlurpSize {
+				io.CopyN(ioutil.Discard, resp.Body, maxBodySlurpSize)
+			}
+			resp.Body.Close()
+
+			if err != nil {
 				// Special case for Go 1 compatibility: return both the response
 				// and an error if the CheckRedirect function failed.
 				// See https://golang.org/issue/3795
@@ -508,14 +538,6 @@ func (c *Client) doFollowingRedirects(req *Request, shouldRedirect func(int) boo
 		if !shouldRedirect(resp.StatusCode) {
 			return resp, nil
 		}
-
-		// Read the body if small so underlying TCP connection will be re-used.
-		// No need to check for errors: if it fails, Transport won't reuse it anyway.
-		const maxBodySlurpSize = 2 << 10
-		if resp.ContentLength == -1 || resp.ContentLength <= maxBodySlurpSize {
-			io.CopyN(ioutil.Discard, resp.Body, maxBodySlurpSize)
-		}
-		resp.Body.Close()
 	}
 }
 

src/net/http/client_test.go

@@ -366,6 +366,44 @@ func TestPostRedirects(t *testing.T) {
 	}
 }
 
+func TestClientRedirectUseResponse(t *testing.T) {
+	defer afterTest(t)
+	const body = "Hello, world."
+	var ts *httptest.Server
+	ts = httptest.NewServer(HandlerFunc(func(w ResponseWriter, r *Request) {
+		if strings.Contains(r.URL.Path, "/other") {
+			io.WriteString(w, "wrong body")
+		} else {
+			w.Header().Set("Location", ts.URL+"/other")
+			w.WriteHeader(StatusFound)
+			io.WriteString(w, body)
+		}
+	}))
+	defer ts.Close()
+
+	c := &Client{CheckRedirect: func(req *Request, via []*Request) error {
+		if req.Response == nil {
+			t.Error("expected non-nil Request.Response")
+		}
+		return ErrUseLastResponse
+	}}
+	res, err := c.Get(ts.URL)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if res.StatusCode != StatusFound {
+		t.Errorf("status = %d; want %d", res.StatusCode, StatusFound)
+	}
+	defer res.Body.Close()
+	slurp, err := ioutil.ReadAll(res.Body)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(slurp) != body {
+		t.Errorf("body = %q; want %q", slurp, body)
+	}
+}
+
 var expectedCookies = []*Cookie{
 	{Name: "ChocolateChip", Value: "tasty"},
 	{Name: "First", Value: "Hit"},

src/net/http/request.go

@@ -255,6 +255,11 @@ type Request struct {
 	// set, it is undefined whether Cancel is respected.
 	Cancel <-chan struct{}
 
+	// Response is the redirect response which caused this request
+	// to be created. This field is only populated during client
+	// redirects.
+	Response *Response
+
 	// ctx is either the client or server context. It should only
 	// be modified via copying the whole Request using WithContext.
 	// It is unexported to prevent people from using Context wrong

src/net/http/response.go

@@ -96,7 +96,7 @@ type Response struct {
 	// any trailer values sent by the server.
 	Trailer Header
 
-	// The Request that was sent to obtain this Response.
+	// Request is the request that was sent to obtain this Response.
 	// Request's Body is nil (having already been consumed).
 	// This is only populated for Client requests.
 	Request *Request