Automatically determine if polling should be enabled to reduce CPU usage

lib/build-response.js

@@ -5,6 +5,7 @@ var prepare = require('prepare-response');
 var uglify = require('uglify-js');
 var watchify = require('watchify');
 var buildBundle = require('./build-bundle');
+var shouldPoll = require('./should-poll.js');
 
 module.exports = function send(path, options) {
   var bundle = buildBundle(path, options);
@@ -18,27 +19,38 @@ module.exports = function send(path, options) {
   } else if (options.cache === 'dynamic') {
     var response, resolve;
     var updatingTimeout;
-    bundle = watchify(bundle, {poll: true, delay: 0});
-    bundle.on('update', function () {
-      if (resolve) {
-        clearTimeout(updatingTimeout);
-      } else {
-        response = new Promise(function (_resolve) {
-          resolve = _resolve;
-        });
-      }
-      updatingTimeout = setTimeout(function rebuild() {
-        resolve(getResponse(bundle, options));
-        resolve = undefined;
-      }, 600);
+
+    var watchifyBundle = shouldPoll()
+    .then(function(poll) {
+      bundle = watchify(bundle, {poll: poll, delay: 0});
+      bundle.on('update', function () {
+        if (resolve) {
+          clearTimeout(updatingTimeout);
+        } else {
+          response = new Promise(function (_resolve) {
+            resolve = _resolve;
+          });
+        }
+        updatingTimeout = setTimeout(function rebuild() {
+          resolve(getResponse(bundle, options));
+          resolve = undefined;
+        }, 600);
+      });
+      return bundle;
     });
-    response = Promise.resolve(getResponse(bundle, options));
+
+    response = watchifyBundle.then(function(bundle) {
+      return getResponse(bundle, options);
+    });
+
     return {
       send: function (req, res, next) {
         response.done(function (response) { response.send(req, res, next); }, next);
       },
       dispose: function () {
-        bundle.close();
+        watchifyBundle.then(function(bundle) {
+          bundle.close();
+        });
       }
     };
   } else {

lib/should-poll.js

@@ -0,0 +1,50 @@
+'use strict';
+
+var Promise = require('promise');
+var buildBundle = require('./build-bundle');
+var fs = require('fs');
+var tmp = require('tmp');
+var watchify = require('watchify');
+
+function shouldPoll() {
+
+  return new Promise(function(resolve, reject) {
+    var file = tmp.fileSync();
+
+    var bundle = buildBundle(file.name, {});
+    watchify(bundle, { poll: false, delay: 0 });
+
+    var resolved = false;
+
+    function decide(pollEnabled) {
+      bundle.close();
+      file.removeCallback();
+      if (!resolved) {
+        resolved = true;
+        resolve(pollEnabled);
+      }
+    }
+
+    bundle.once('bundle', function(b) {
+      b.once('error', reject);
+      // end event is never fired without having a data
+      b.once('data', function() {});
+      b.once('end', function() {
+
+        bundle.once('update', function() {
+          decide(true);
+        });
+
+        setTimeout(decide, 1000, false);
+
+        fs.appendFileSync(file.name, 'var a = 1;');
+      });
+    });
+
+    bundle.once('error', reject);
+    bundle.bundle();
+  });
+
+}
+
+module.exports = shouldPoll;

package.json

@@ -34,7 +34,8 @@
     "ms": "^0.7.1",
     "prepare-response": "^1.1.2",
     "promise": "^7.0.3",
+    "tmp": "0.0.28",
     "uglify-js": "^2.4.23",
     "watchify": "^3.3.0"
   }
-}
+}

test/should-poll-test.js

@@ -0,0 +1,16 @@
+var shouldPoll = require('../lib/should-poll.js');
+var assert = require('assert');
+
+describe('should-poll', function() {
+  it('should be a function', function() {
+    assert.equal(typeof shouldPoll, 'function');
+  });
+  it('should resolve to true or false', function(done) {
+    shouldPoll()
+    .then(function(pollingEnabled) {
+      assert(typeof pollingEnabled === 'boolean');
+    })
+    .then(done)
+    .catch(done);
+  });
+});