perf: skip ETag generation for no-store responses

lib/response.js

@@ -158,8 +158,11 @@ res.send = function send(body) {
   }
 
   // determine if ETag should be generated
+  // Skip ETag for no-store responses as the client will ignore it anyway
   var etagFn = app.get('etag fn')
-  var generateETag = !this.get('ETag') && typeof etagFn === 'function'
+  var cacheControl = this.get('Cache-Control')
+  var isNoStore = cacheControl && /(?:^|,)\s*no-store\s*(?:,|$)/i.test(cacheControl)
+  var generateETag = !isNoStore && !this.get('ETag') && typeof etagFn === 'function'
 
   // populate Content-Length
   var len

test/res.send.js

@@ -456,6 +456,42 @@ describe('res', function(){
         .expect(utils.shouldNotHaveHeader('ETag'))
         .expect(200, done);
       })
+
+      it('should not send ETag when Cache-Control: no-store is set', function (done) {
+        var app = express();
+
+        app.use(function (req, res) {
+          res.set('Cache-Control', 'no-store');
+          var str = Array(1000).join('-');
+          res.send(str);
+        });
+
+        app.enable('etag');
+
+        request(app)
+        .get('/')
+        .expect('Cache-Control', 'no-store')
+        .expect(utils.shouldNotHaveHeader('ETag'))
+        .expect(200, done);
+      })
+
+      it('should not send ETag when Cache-Control contains no-store', function (done) {
+        var app = express();
+
+        app.use(function (req, res) {
+          res.set('Cache-Control', 'private, no-store, max-age=0');
+          var str = Array(1000).join('-');
+          res.send(str);
+        });
+
+        app.enable('etag');
+
+        request(app)
+        .get('/')
+        .expect('Cache-Control', 'private, no-store, max-age=0')
+        .expect(utils.shouldNotHaveHeader('ETag'))
+        .expect(200, done);
+      })
     });
 
     describe('when disabled', function () {