notes/node_modules/signal-exit/index.js

// Note: since nyc uses this module to output coverage, any lines
// that are in the direct sync flow of nyc's outputCoverage are
// ignored, since we can never get coverage for them.
var assert = require('assert')
var signals = require('./signals.js')

var EE = require('events')
/* istanbul ignore if */
if (typeof EE !== 'function') {
  EE = EE.EventEmitter
}

var emitter
if (process.__signal_exit_emitter__) {
  emitter = process.__signal_exit_emitter__
} else {
  emitter = process.__signal_exit_emitter__ = new EE()
  emitter.count = 0
  emitter.emitted = {}
}

// Because this emitter is a global, we have to check to see if a
// previous version of this library failed to enable infinite listeners.
// I know what you're about to say.  But literally everything about
// signal-exit is a compromise with evil.  Get used to it.
if (!emitter.infinite) {
  emitter.setMaxListeners(Infinity)
  emitter.infinite = true
}

module.exports = function (cb, opts) {
  assert.equal(typeof cb, 'function', 'a callback must be provided for exit handler')

  if (loaded === false) {
    load()
  }

  var ev = 'exit'
  if (opts && opts.alwaysLast) {
    ev = 'afterexit'
  }

  var remove = function () {
    emitter.removeListener(ev, cb)
    if (emitter.listeners('exit').length === 0 &&
        emitter.listeners('afterexit').length === 0) {
      unload()
    }
  }
  emitter.on(ev, cb)

  return remove
}

module.exports.unload = unload
function unload () {
  if (!loaded) {
    return
  }
  loaded = false

  signals.forEach(function (sig) {
    try {
      process.removeListener(sig, sigListeners[sig])
    } catch (er) {}
  })
  process.emit = originalProcessEmit
  process.reallyExit = originalProcessReallyExit
  emitter.count -= 1
}

function emit (event, code, signal) {
  if (emitter.emitted[event]) {
    return
  }
  emitter.emitted[event] = true
  emitter.emit(event, code, signal)
}

// { <signal>: <listener fn>, ... }
var sigListeners = {}
signals.forEach(function (sig) {
  sigListeners[sig] = function listener () {
    // If there are no other listeners, an exit is coming!
    // Simplest way: remove us and then re-send the signal.
    // We know that this will kill the process, so we can
    // safely emit now.
    var listeners = process.listeners(sig)
    if (listeners.length === emitter.count) {
      unload()
      emit('exit', null, sig)
      /* istanbul ignore next */
      emit('afterexit', null, sig)
      /* istanbul ignore next */
      process.kill(process.pid, sig)
    }
  }
})

module.exports.signals = function () {
  return signals
}

module.exports.load = load

var loaded = false

function load () {
  if (loaded) {
    return
  }
  loaded = true

  // This is the number of onSignalExit's that are in play.
  // It's important so that we can count the correct number of
  // listeners on signals, and don't wait for the other one to
  // handle it instead of us.
  emitter.count += 1

  signals = signals.filter(function (sig) {
    try {
      process.on(sig, sigListeners[sig])
      return true
    } catch (er) {
      return false
    }
  })

  process.emit = processEmit
  process.reallyExit = processReallyExit
}

var originalProcessReallyExit = process.reallyExit
function processReallyExit (code) {
  process.exitCode = code || 0
  emit('exit', process.exitCode, null)
  /* istanbul ignore next */
  emit('afterexit', process.exitCode, null)
  /* istanbul ignore next */
  originalProcessReallyExit.call(process, process.exitCode)
}

var originalProcessEmit = process.emit
function processEmit (ev, arg) {
  if (ev === 'exit') {
    if (arg !== undefined) {
      process.exitCode = arg
    }
    var ret = originalProcessEmit.apply(this, arguments)
    emit('exit', process.exitCode, null)
    /* istanbul ignore next */
    emit('afterexit', process.exitCode, null)
    return ret
  } else {
    return originalProcessEmit.apply(this, arguments)
  }
}