http: fix "Cannot call method 'emit' of null"

Fix the following exception:

  http.js:974
    this._httpMessage.emit('close');
                      ^
  TypeError: Cannot call method 'emit' of null
      at Socket.onServerResponseClose (http.js:974:21)
      at Socket.EventEmitter.emit (events.js:124:20)
      at net.js:421:10
      at process._tickCallback (node.js:386:13)
      at process._makeCallback (node.js:304:15)

Fixes #4586.

lib/http.js

@@ -967,7 +967,24 @@ exports.ServerResponse = ServerResponse;
 ServerResponse.prototype.statusCode = 200;
 
 function onServerResponseClose() {
-  this._httpMessage.emit('close');
+  // EventEmitter.emit makes a copy of the 'close' listeners array before
+  // calling the listeners. detachSocket() unregisters onServerResponseClose
+  // but if detachSocket() is called, directly or indirectly, by a 'close'
+  // listener, onServerResponseClose is still in that copy of the listeners
+  // array. That is, in the example below, b still gets called even though
+  // it's been removed by a:
+  //
+  //   var obj = new events.EventEmitter;
+  //   obj.on('event', a);
+  //   obj.on('event', b);
+  //   function a() { obj.removeListener('event', b) }
+  //   function b() { throw "BAM!" }
+  //   obj.emit('event');  // throws
+  //
+  // Ergo, we need to deal with stale 'close' events and handle the case
+  // where the ServerResponse object has already been deconstructed.
+  // Fortunately, that requires only a single if check. :-)
+  if (this._httpMessage) this._httpMessage.emit('close');
 }
 
 ServerResponse.prototype.assignSocket = function(socket) {

test/simple/test-http-server-stale-close.js

@@ -0,0 +1,51 @@
+// 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');
+var assert = require('assert');
+var http = require('http');
+var fork = require('child_process').fork;
+
+if (process.env.NODE_TEST_FORK) {
+  var req = http.request({
+    headers: {'Content-Length': '42'},
+    method: 'POST',
+    host: '127.0.0.1',
+    port: common.PORT,
+  }, process.exit);
+  req.write('BAM');
+  req.end();
+}
+else {
+  var server = http.createServer(function(req, res) {
+    res.writeHead(200, {'Content-Length': '42'});
+    req.pipe(res);
+    req.on('close', function() {
+      server.close();
+      res.end();
+    });
+  });
+  server.listen(common.PORT, function() {
+    fork(__filename, {
+      env: {NODE_TEST_FORK: '1'}
+    });
+  });
+}