Fix a couple on exit issues

- run handlers on a natural exit (ie no `process.exit` calls)
- preserve exit code when `process.exit` is called during `process.on('exit')`

Exit code was not previous preserved because we only intercepted explicit exits
(ie calls to `process.exit`) but did not intercept natural exits.

`process.exit` in node has two sets of semantics.

1. When called not while exiting, it begins exiting
2. When called while exiting (ie during a `process.on('exit')` handlers) it
    exits immediately, preventing other exit handlers from firing.

Fix #6

Author: hjdivad

index.js

@@ -4,6 +4,12 @@ var exit;
 var handlers = [];
 var lastTime;
 
+process.on('beforeExit', function () {
+  if (handlers.length === 0) { return; }
+
+  return module.exports._flush();
+});
+
 /*
  * To allow cooperative async exit handlers, we unfortunately must hijack
  * process.exit.
@@ -29,6 +35,25 @@ module.exports.captureExit = function() {
   exit = process.exit;
 
   process.exit = function(code) {
+    if (handlers.length === 0 && lastTime === undefined) {
+      // synchronously exit.
+      //
+      // We do this brecause either
+      //
+      //  1.  The process exited due to a call to `process.exit` but we have no
+      //      async work to do because no handlers had been attached.  It
+      //      doesn't really matter whether we take this branch or not in this
+      //      case.
+      //
+      //  2.  The process exited naturally.  We did our async work during
+      //      `beforeExit` and are in this function because someone else has
+      //      called `process.exit` during an `on('exit')` hook.  The only way
+      //      for us to preserve the exit code in this case is to exit
+      //      synchronously.
+      //
+      return exit.call(process, code);
+    }
+
     var own = lastTime = module.exports._flush(lastTime, code)
       .then(function() {
         // if another chain has started, let it exit

test-natural-exit-subprocess-error.js

@@ -0,0 +1,14 @@
+'use strict';
+
+var capture = require('./');
+capture.captureExit();
+
+capture.onExit(function () {
+  console.log('onExit');
+});
+
+process.on('exit', function () {
+  console.log('exit');
+  process.exit(1);
+});
+

test-natural-exit-subprocess.js

@@ -0,0 +1,12 @@
+'use strict';
+
+var capture = require('./');
+capture.captureExit();
+
+capture.onExit(function () {
+  console.log('onExit');
+})
+
+process.on('exit', function () {
+  console.log('exit');
+});

test.js

@@ -3,6 +3,7 @@ var RSVP = require('rsvp');
 
 var originalExit = process.exit; // keep this around for good measure.
 var exit = require('./');
+var childProcess = require('child_process');
 
 describe('capture-exit', function() {
   beforeEach(function() {
@@ -206,6 +207,29 @@ describe('capture-exit', function() {
   });
 });
 
+describe('natural exit', function() {
+  it('runs handlers on a natural exit', function() {
+    var output = childProcess.execSync('node test-natural-exit-subprocess.js');
+    expect(output+'').to.include('onExit');
+    expect(output+'').to.include('exit');
+  });
+
+  it('exits with error code if an on exit handler calls process.exit with code', function() {
+    var succeeded = false;
+    try {
+      var output = childProcess.execSync('node test-natural-exit-subprocess-error.js');
+      succeeded = true;
+    } catch(e) {
+      expect(e.output+'').to.include('onExit');
+      expect(e.output+'').to.include('exit');
+    }
+
+    if (succeeded) {
+      throw new Error('Unexpected zero exit status for process.exit(1)');
+    }
+  });
+});
+
 function delay(milliseconds) {
   return new RSVP.Promise(function(resolve) {
     setTimeout(resolve, milliseconds);