added before-flushing-head event to _http_server.ServerResponse.writeHead

doc/api/http.markdown

@@ -640,6 +640,26 @@ passed as the second parameter to the `'request'` event.
 The response implements the [Writable Stream][] interface. This is an
 [`EventEmitter`][] with the following events:
 
+### Event: 'beforeFlushingHead'
+
+`function (messageHead) {}`
+
+Indicates that the server is about to flush the HTTP headers to the underlying 
+socket. This event can be used for measuring time to first byte or customizing 
+the response headers and status code.
+
+The `messageHead` parameter allows overriding the http response code, message and headers. 
+It includes
+
+* `responseCode` {Number}
+* `responseMessage` {String}
+* `headers` {Object|Array}
+
+The headers member will be either an object of the form
+`{header-names: header value}`
+or an array of the form
+`[['header-name', 'header-value'] ...]`
+
 ### Event: 'close'
 
 `function () { }`

lib/_http_server.js

@@ -160,34 +160,38 @@ ServerResponse.prototype._implicitHeader = function() {
 ServerResponse.prototype.writeHead = function(statusCode, reason, obj) {
   var headers;
 
-  if (typeof reason === 'string') {
-    // writeHead(statusCode, reasonPhrase[, headers])
-    this.statusMessage = reason;
-  } else {
+  // writeHead(statusCode, reasonPhrase[, headers])
+  var args = {statusCode: statusCode, statusMessage: reason, headers: obj};
+
+  if (typeof args.statusMessage !== 'string') {
     // writeHead(statusCode[, headers])
-    this.statusMessage =
+    args.statusMessage =
         this.statusMessage || STATUS_CODES[statusCode] || 'unknown';
-    obj = reason;
+    args.headers = reason;
   }
-  this.statusCode = statusCode;
+
+  args.headers = args.headers || {};
 
   if (this._headers) {
     // Slow-case: when progressive API and header fields are passed.
-    if (obj) {
-      var keys = Object.keys(obj);
+    if (args.headers) {
+      var keys = Object.keys(args.headers);
       for (var i = 0; i < keys.length; i++) {
         var k = keys[i];
-        if (k) this.setHeader(k, obj[k]);
+        if (k) this.setHeader(k, args.headers[k]);
       }
     }
     // only progressive api is used
-    headers = this._renderHeaders();
-  } else {
-    // only writeHead() called
-    headers = obj;
+    args.headers = this._renderHeaders();
   }
 
-  var statusLine = 'HTTP/1.1 ' + statusCode.toString() + ' ' +
+  this.emit('beforeFlushingHead', args);
+
+  this.statusMessage = args.statusMessage;
+  this.statusCode = args.statusCode;
+  headers = args.headers;
+
+  var statusLine = 'HTTP/1.1 ' + this.statusCode.toString() + ' ' +
                    this.statusMessage + CRLF;
 
   if (statusCode === 204 || statusCode === 304 ||

test/parallel/test-http-before-flush-head-event.js

@@ -0,0 +1,49 @@
+'use strict';
+const common = require('../common'),
+  assert = require('assert'),
+  http = require('http');
+
+var testResBody = 'other stuff!\n';
+
+var server = http.createServer(function(req, res) {
+  res.on('beforeFlushingHead', function(args) {
+    args.statusCode = 201;
+    args.statusMessage = 'changed to show we can';
+    args.headers['Flush-Head'] = 'event-was-called';
+  });
+  res.writeHead(200, {
+    'Content-Type': 'text/plain'
+  });
+  res.end(testResBody);
+});
+server.listen(common.PORT);
+
+
+server.addListener('listening', function() {
+  var options = {
+    port: common.PORT,
+    path: '/',
+    method: 'GET'
+  };
+  var req = http.request(options, function(res) {
+    assert.ok(res.statusCode === 201,
+              'Response status code was not overridden from 200 to 201.');
+    assert.ok(res.statusMessage === 'changed to show we can',
+              'Response status message was not overridden.');
+    assert.ok('flush-head' in res.headers,
+              'Response headers didn\'t contain the flush-head header, ' +
+                'indicating the beforeFlushingHead event was not called or ' +
+                'did not allow adding headers.');
+    assert.ok(res.headers['flush-head'] === 'event-was-called',
+              'Response headers didn\'t contain the flush-head header ' +
+                'with value event-was-called, indicating the ' +
+                'beforeFlushingHead event was not called or did not allow ' +
+                'adding headers.');
+    res.addListener('end', function() {
+      server.close();
+      process.exit();
+    });
+    res.resume();
+  });
+  req.end();
+});