domain: Properly exit() on domain disposal

This addresses nodejs#4034.  There are two problems happening:

1. The domain is not exited automatically when calling dispose() on it.
Then, since the domain is disposed, attempting to exit it again will do
nothing.

2. The active domain is stored on process.domain.  Since thrown errors
call `process.emit('uncaughtException', er)`, and the process is an
event emitter with a `.domain` member, it re-enters the domain a second
time before calling the error handler, pushing it onto the stack again.

Thus, if the handler calls `domain.dispose()`, then the domain is now on
the stack twice, and cannot be exited properly.  Since the domain is
disposed, any subsequent IO will be no-op'ed, since we've declared that
this context is done and best forgotten.

The solution here is twofold:

1. In EventEmitter.emit, do not enter the domain if `this===process`.
2. Automatically exit the domain when calling `domain.dispose()`.

lib/domain.js

@@ -59,7 +59,7 @@ function uncaughtHandler(er) {
       domain_thrown: true
     });
     exports.active.emit('error', er);
-    exports.active.exit();
+    if (exports.active) exports.active.exit();
   } else if (process.listeners('uncaughtException').length === 1) {
     // if there are other handlers, then they'll take care of it.
     // but if not, then we need to crash now.
@@ -207,8 +207,13 @@ Domain.prototype.bind = function(cb, interceptError) {
 };
 
 Domain.prototype.dispose = function() {
+  console.error('dispose', this, exports.active, process.domain)
+
   if (this._disposed) return;
 
+  // if we're the active domain, then get out now.
+  this.exit();
+
   this.emit('dispose');
 
   // remove error handlers.

lib/events.js

@@ -45,6 +45,8 @@ EventEmitter.prototype.setMaxListeners = function(n) {
   this._maxListeners = n;
 };
 
+// non-global reference, for speed.
+var PROCESS;
 
 EventEmitter.prototype.emit = function() {
   var type = arguments[0];
@@ -77,7 +79,10 @@ EventEmitter.prototype.emit = function() {
 
   if (typeof handler == 'function') {
     if (this.domain) {
-      this.domain.enter();
+      PROCESS = PROCESS || process;
+      if (this !== PROCESS) {
+        this.domain.enter();
+      }
     }
     switch (arguments.length) {
       // fast cases
@@ -97,14 +102,17 @@ EventEmitter.prototype.emit = function() {
         for (var i = 1; i < l; i++) args[i - 1] = arguments[i];
         handler.apply(this, args);
     }
-    if (this.domain) {
+    if (this.domain && this !== PROCESS) {
       this.domain.exit();
     }
     return true;
 
   } else if (isArray(handler)) {
     if (this.domain) {
-      this.domain.enter();
+      PROCESS = PROCESS || process;
+      if (this !== PROCESS) {
+        this.domain.enter();
+      }
     }
     var l = arguments.length;
     var args = new Array(l - 1);
@@ -114,7 +122,7 @@ EventEmitter.prototype.emit = function() {
     for (var i = 0, l = listeners.length; i < l; i++) {
       listeners[i].apply(this, args);
     }
-    if (this.domain) {
+    if (this.domain && this !== PROCESS) {
       this.domain.exit();
     }
     return true;

test/simple/test-domain-exit-dispose.js

@@ -0,0 +1,67 @@
+// Copyright Joyent, Inc. and other Node contributors.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a
+// copy of this software and associated documentation files (the
+// "Software"), to deal in the Software without restriction, including
+// without limitation the rights to use, copy, modify, merge, publish,
+// distribute, sublicense, and/or sell copies of the Software, and to permit
+// persons to whom the Software is furnished to do so, subject to the
+// following conditions:
+//
+// The above copyright notice and this permission notice shall be included
+// in all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
+// OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+// MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN
+// NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM,
+// DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
+// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE
+// USE OR OTHER DEALINGS IN THE SOFTWARE.
+
+var common = require('../common.js');
+var assert = require('assert');
+var domain = require('domain');
+var disposalFailed = false;
+
+// no matter what happens, we should increment a 10 times.
+var a = 0;
+log();
+function log(){
+  console.log(a++, process.domain);
+  if (a < 10) setTimeout(log, 20);
+}
+
+// in 50ms we'll throw an error.
+setTimeout(err, 50);
+function err(){
+  var d = domain.create();
+  d.on('error', handle);
+  d.run(err2);
+
+  function err2() {
+    // this timeout should never be called, since the domain gets
+    // disposed when the error happens.
+    setTimeout(function() {
+      console.error('This should not happen.');
+      disposalFailed = true;
+      process.exit(1);
+    });
+
+    // this function doesn't exist, and throws an error as a result.
+    err3();
+  }
+
+  function handle(e) {
+    // this should clean up everything properly.
+    d.dispose();
+    console.error(e);
+    console.error('in handler', process.domain, process.domain === d);
+  }
+}
+
+process.on('exit', function() {
+  assert.equal(a, 10);
+  assert.equal(disposalFailed, false);
+  console.log('ok');
+});