Meteor : findOne in Tracker.autorun on server crashes ... what to do?

I want to populate an in-memory list of restricted words form a list stored into the database, because is desired to be dynamic. Sound simple huh? Well no, it is not! Why ?

Here is a short code of it :

if (Meteor.isServer) {
  Meteor.startup(function() {

    var configCollection = new Mongo.Collection('config');

    // An wrapper object for easy referencing
    var words = {
      faulty: ['f_word_here']
    }; // Some default faulty words

    var updateFaultyWords = function() {
      var config = configCollection.findOne();
      if (config) {
        words.faulty = config.faultyWords;
      }
    };

    // ------- Problematic Code ------

    Tracker.autorun(function() {
      updateFaultyWords();
    });


    // -------------------------------

    // later somewhere in the code

    var allowWord = function(word) {
      return words.faulty.indexOf(word) === -1;
    };
  });
}

I am using Tracker here because like it says in the docs I want to update my list in a reactive mode.

Tracker.autorun allows you to run a function that depends on reactive data sources. Whenever those data sources are updated with new data, the function will be rerun.

However this method crashes big time with a stack trace that I cannot understand:

W20151209-17:36:55.802(1)? (STDERR)           
W20151209-17:36:55.802(1)? (STDERR) /Users/tiberiu/.meteor/packages/meteor-tool/.1.1.10.1b51q9m++os.osx.x86_64+web.browser+web.cordova/mt-os.osx.x86_64/dev_bundle/server-lib/node_modules/fibers/future.js:245
W20151209-17:36:55.802(1)? (STDERR)                         throw(ex);
W20151209-17:36:55.803(1)? (STDERR)                               ^
W20151209-17:36:55.865(1)? (STDERR) Error: Can't call yield in a noYieldsAllowed block!
W20151209-17:36:55.865(1)? (STDERR)     at Function.Fiber.yield (packages/meteor/fiber_helpers.js:8:1)
W20151209-17:36:55.865(1)? (STDERR)     at Function.wait (/Users/tiberiu/.meteor/packages/meteor-tool/.1.1.10.1b51q9m++os.osx.x86_64+web.browser+web.cordova/mt-os.osx.x86_64/dev_bundle/server-lib/node_modules/fibers/future.js:183:14)
W20151209-17:36:55.865(1)? (STDERR)     at Object.Future.wait (/Users/tiberiu/.meteor/packages/meteor-tool/.1.1.10.1b51q9m++os.osx.x86_64+web.browser+web.cordova/mt-os.osx.x86_64/dev_bundle/server-lib/node_modules/fibers/future.js:397:10)
W20151209-17:36:55.865(1)? (STDERR)     at [object Object]._.extend._nextObject (packages/mongo/mongo_driver.js:986:1)
W20151209-17:36:55.865(1)? (STDERR)     at [object Object]._.extend.forEach (packages/mongo/mongo_driver.js:1020:1)
W20151209-17:36:55.866(1)? (STDERR)     at [object Object]._.extend.map (packages/mongo/mongo_driver.js:1030:1)
W20151209-17:36:55.866(1)? (STDERR)     at [object Object]._.extend.fetch (packages/mongo/mongo_driver.js:1054:1)
W20151209-17:36:55.866(1)? (STDERR)     at [object Object].Cursor.(anonymous function) [as fetch] (packages/mongo/mongo_driver.js:869:1)
W20151209-17:36:55.866(1)? (STDERR)     at [object Object].MongoConnection.findOne (packages/mongo/mongo_driver.js:776:1)
W20151209-17:36:55.867(1)? (STDERR)     at [object Object]._.extend.findOne (packages/mongo/collection.js:305:1)

What am I doing wrong ? Should I report this as a bug ?

You cannot use autorun on the server; it is client-side only functionality.

Use cursor.observe or cursor.observeChanges on the cursor that is returned by "find":

http://docs.meteor.com/#/full/observe

http://docs.meteor.com/#/full/observe_changes

I would also avoid using findOne if you have more than item in your collection (not sure if you're just omitting the arguments here or are just using it as a test case).