Use PARCEL_WORKERS environment variable (#589)

* Use PARCEL_WORKERS environment variable

* Update WorkerFarm.js

CONTRIBUTING.md

@@ -35,6 +35,12 @@ yarn test
 [node]: https://nodejs.org/
 [yarn]: https://yarnpkg.com/
 
+## Environment variables
+
+You can set `PARCEL_WORKERS` to the number of worker processes to spawn.
+
+`PARCEL_WORKERS=0` is handy for debugging, because that will cause all code to be run on the main thread. This allows you to place breakpoints in Asset code, for example.
+
 ## Financial contributions
 
 We also welcome financial contributions in full transparency on our [open collective](https://opencollective.com/parcel).

src/WorkerFarm.js

@@ -45,7 +45,9 @@ class WorkerFarm extends Farm {
     }
 
     await Promise.all(promises);
-    this.started = true;
+    if (this.options.maxConcurrentWorkers > 0) {
+      this.started = true;
+    }
   }
 
   receive(data) {
@@ -97,6 +99,10 @@ for (let key in EventEmitter.prototype) {
 }
 
 function getNumWorkers() {
+  if (process.env.PARCEL_WORKERS) {
+    return parseInt(process.env.PARCEL_WORKERS, 10);
+  }
+
   let cores;
   try {
     cores = require('physical-cpu-count');