Fixed race condition when triggering watch event

Author: Frank Schmid

lib/wpwatch.js

@@ -31,9 +31,10 @@ module.exports = {
     };
 
     // This starts the watch and waits for the immediate compile that follows to end or fail.
+    let lastHash = null;
     const startWatch = (callback) => {
       let firstRun = true;
-      compiler.watch(watchOptions, (err, stats) => {
+      const watcher = compiler.watch(watchOptions, (err, stats) => {
         if (err) {
           if (firstRun) {
             firstRun = false;
@@ -42,17 +43,39 @@ module.exports = {
           throw err;
         }
 
+        process.env.SLS_DEBUG && this.serverless.cli.log(`Webpack watch invoke: HASH NEW=${stats.hash} CUR=${lastHash}`);
+
+        // If the file hash did not change there were no effective code changes detected (comment only changed do not change the compile hash!)
+        // See here: https://webpack.js.org/api/node/#watching (note below watching)
+        if (stats && stats.hash === lastHash) {
+          if (firstRun) {
+            firstRun = false;
+            callback();
+          }
+          return;
+        }
+
         if (stats) {
+          lastHash = stats.hash;
           this.serverless.cli.consoleLog(stats.toString(consoleStats));
         }
 
-        this.serverless.cli.log('Watching for changes...');
-
         if (firstRun) {
           firstRun = false;
+          this.serverless.cli.log('Watching for changes...');
           callback();
-        }else{
-          this.serverless.pluginManager.spawn('webpack:compile:watch');
+        } else {
+          // We will close the watcher while the compile event is triggered and resume afterwards to prevent race conditions.
+          watcher.close(() => {
+            return this.serverless.pluginManager.spawn('webpack:compile:watch')
+            .then(() => {
+              // Resume watching after we triggered the compile:watch event
+              return BbPromise.fromCallback(cb => {
+                startWatch(cb);
+              })
+              .then(() => this.serverless.cli.log('Watching for changes...'));
+            });
+          });
         }
       });
     };