http: don't Clean query string in relative redirects

R=adg, rsc, kevlar, r
CC=golang-dev
https://golang.org/cl/4476045

src/pkg/http/serve_test.go

@@ -693,3 +693,20 @@ func TestTimeoutHandler(t *testing.T) {
 		t.Errorf("expected Write error of %v; got %v", e, g)
 	}
 }
+
+// Verifies we don't path.Clean() on the wrong parts in redirects.
+func TestRedirectMunging(t *testing.T) {
+	req, _ := NewRequest("GET", "http://example.com/", nil)
+
+	resp := httptest.NewRecorder()
+	Redirect(resp, req, "/foo?next=http://bar.com/", 302)
+	if g, e := resp.Header().Get("Location"), "/foo?next=http://bar.com/"; g != e {
+		t.Errorf("Location header was %q; want %q", g, e)
+	}
+
+	resp = httptest.NewRecorder()
+	Redirect(resp, req, "http://localhost:8080/_ah/login?continue=http://localhost:8080/", 302)
+	if g, e := resp.Header().Get("Location"), "http://localhost:8080/_ah/login?continue=http://localhost:8080/"; g != e {
+		t.Errorf("Location header was %q; want %q", g, e)
+	}
+}

src/pkg/http/server.go

@@ -581,12 +581,18 @@ func Redirect(w ResponseWriter, r *Request, url string, code int) {
 				url = olddir + url
 			}
 
+			var query string
+			if i := strings.Index(url, "?"); i != -1 {
+				url, query = url[:i], url[i:]
+			}
+
 			// clean up but preserve trailing slash
 			trailing := url[len(url)-1] == '/'
 			url = path.Clean(url)
 			if trailing && url[len(url)-1] != '/' {
 				url += "/"
 			}
+			url += query
 		}
 	}