Avoid creating multiple SIGHUP listeners for File Appender

[lamweili]

Fixes #556, Fixes #700, Fixes #785, Fixes #852, Fixes #886

Originally posted by @nomiddlename in #852 (comment)
I think the issue is that each file appender instance adds a SIGHUP handler, when they could all use the same handler. I'll see if I can work on a fix.

Implemented a fix based on @nomiddlename's comment, to create a single SIGHUP listener.

[lamweili]

This issue can be tested with the following codes in NodeJS.

const log4js = require('log4js');

log4js.configure({
  appenders: {
    a1: { type: 'file', filename: 'app1.log' },
    a2: { type: 'file', filename: 'app2.log' },
    a3: { type: 'file', filename: 'app3.log' },
    a4: { type: 'file', filename: 'app4.log' },
    a5: { type: 'file', filename: 'app5.log' },
    a6: { type: 'file', filename: 'app6.log' },
    a7: { type: 'file', filename: 'app7.log' },
    a8: { type: 'file', filename: 'app8.log' },
    a9: { type: 'file', filename: 'app9.log' },
    a10: { type: 'file', filename: 'app10.log' },
    a11: { type: 'file', filename: 'app11.log' },
    a12: { type: 'file', filename: 'app12.log' }
  },
  categories: {
    default: { 
      appenders: [ 'a1', 'a2', 'a3', 'a4', 'a5', 'a6', 'a7', 'a8', 'a9', 'a10', 'a11', 'a12' ], 
      level: 'debug'
    }
  }
});

> (node:18567) MaxListenersExceededWarning: Possible EventEmitter memory leak detected. 11 SIGHUP listeners added to [process]. Use emitter.setMaxListeners() to increase limit
(Use `node --trace-warnings ...` to show where the warning was created)

A check on the listener count:

process.listenerCount('SIGHUP');

12

[lamweili]

After the patch, no more MaxListenersExceededWarning.

A check on the listener count:

process.listenerCount('SIGHUP');

1

[nomiddlename]

Awesome - thanks for this. Could you add an automated test that covers it, please?

[lamweili]

Hi @nomiddlename, I added the automated test as requested.