Mocha: CI tests: MaxListenersExceededWarning: Possible EventEmitter memory leak detected.

[juergba]

In the log files of our CI tests, following warnings can be found:

MaxListenersExceededWarning: Possible EventEmitter memory leak detected. 11 uncaughtException listeners added to [process].

Please investigate, explain and evtl. fix those warnings.

[adam-edison]

Confirmed I've been seeing this bug for the last 3 weeks. Running on node 10, using mocha version 6.2.2. This outputs to the console while tests are running using either mocha or mocha --watch.

(node:13167) MaxListenersExceededWarning: 
Possible EventEmitter memory leak detected. 
11 uncaughtException listeners added to [process]. 
Use emitter.setMaxListeners() to increase limit

Suggested cause:

• listeners are not being removed after they have been used and no longer needed

Many other libraries have faced this issue. See winston:

• https://github.com/node-ts/logger/pull/13

[jehy]

I described this bug in details here: https://github.com/mochajs/mocha/issues/4171

[juergba]

fixed by #4147.

Our CI tests still log a few warnings, but they are test specific and will be improved later.