net/http: don't assume Response.Request is populated after redirect errors

Fixes #15577

Change-Id: I5f023790a393b17235db2e66c02c2483773ddc1a
Reviewed-on: https://go-review.googlesource.com/22857Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/http/client.go

@@ -442,7 +442,7 @@ func (c *Client) doFollowingRedirects(req *Request, shouldRedirect func(int) boo
 		req.closeBody()
 		method := valueOrDefault(reqs[0].Method, "GET")
 		var urlStr string
-		if resp != nil {
+		if resp != nil && resp.Request != nil {
 			urlStr = resp.Request.URL.String()
 		} else {
 			urlStr = req.URL.String()

src/net/http/client_test.go

@@ -1168,3 +1168,26 @@ func TestReferer(t *testing.T) {
 		}
 	}
 }
+
+// issue15577Tripper returns a Response with a redirect response
+// header and doesn't populate its Response.Request field.
+type issue15577Tripper struct{}
+
+func (issue15577Tripper) RoundTrip(*Request) (*Response, error) {
+	resp := &Response{
+		StatusCode: 303,
+		Header:     map[string][]string{"Location": {"http://www.example.com/"}},
+		Body:       ioutil.NopCloser(strings.NewReader("")),
+	}
+	return resp, nil
+}
+
+// Issue 15577: don't assume the roundtripper's response populates its Request field.
+func TestClientRedirectResponseWithoutRequest(t *testing.T) {
+	c := &Client{
+		CheckRedirect: func(*Request, []*Request) error { return fmt.Errorf("no redirects!") },
+		Transport:     issue15577Tripper{},
+	}
+	// Check that this doesn't crash:
+	c.Get("http://dummy.tld")
+}