http: improves expect header handling

Now returns a 417 error status or allows for an event listener on
the `checkExpectation` event. Before we were ignoring requests that
had misspelled `100-continue` values for expect headers.

This is a quick port of the work done here:
nodejs/node-v0.x-archive#7132 by alFReD-NSH
with surrounding discussion here:
nodejs/node-v0.x-archive#4651

Also updates all the instances of the deprecated
EventEmitter.listenerCount to the current self.listenerCount. Most
of these were in the new code ported over but there was another
legacy instance.

Refs: #2403
PR-URL: #4501
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/http.markdown

@@ -192,6 +192,17 @@ The request implements the [Writable Stream][] interface. This is an
 Emitted when the request has been aborted by the client. This event is only
 emitted on the first call to `abort()`.
 
+### Event: 'checkExpectation'
+
+`function (request, response) { }`
+
+Emitted each time a request with an http Expect header is received, where the
+value is not 100-continue. If this event isn't listened for, the server will
+automatically respond with a 417 Expectation Failed as appropriate.
+
+Note that when this event is emitted and handled, the `request` event will
+not be emitted.
+
 ### Event: 'connect'
 
 `function (response, socket, head) { }`

lib/_http_server.js

@@ -2,7 +2,6 @@
 
 const util = require('util');
 const net = require('net');
-const EventEmitter = require('events');
 const HTTPParser = process.binding('http_parser').HTTPParser;
 const assert = require('assert').ok;
 const common = require('_http_common');
@@ -391,7 +390,7 @@ function connectionListener(socket) {
       parser = null;
 
       var eventName = req.method === 'CONNECT' ? 'connect' : 'upgrade';
-      if (EventEmitter.listenerCount(self, eventName) > 0) {
+      if (self.listenerCount(eventName) > 0) {
         debug('SERVER have listener for %s', eventName);
         var bodyHead = d.slice(bytesParsed, d.length);
 
@@ -516,14 +515,23 @@ function connectionListener(socket) {
     }
 
     if (req.headers.expect !== undefined &&
-        (req.httpVersionMajor == 1 && req.httpVersionMinor == 1) &&
-        continueExpression.test(req.headers['expect'])) {
-      res._expect_continue = true;
-      if (EventEmitter.listenerCount(self, 'checkContinue') > 0) {
-        self.emit('checkContinue', req, res);
+        (req.httpVersionMajor == 1 && req.httpVersionMinor == 1)) {
+      if (continueExpression.test(req.headers.expect)) {
+        res._expect_continue = true;
+
+        if (self.listenerCount('checkContinue') > 0) {
+          self.emit('checkContinue', req, res);
+        } else {
+          res.writeContinue();
+          self.emit('request', req, res);
+        }
       } else {
-        res.writeContinue();
-        self.emit('request', req, res);
+        if (self.listenerCount('checkExpectation') > 0) {
+          self.emit('checkExpectation', req, res);
+        } else {
+          res.writeHead(417);
+          res.end();
+        }
       }
     } else {
       self.emit('request', req, res);

test/parallel/test-http-expect-handling.js

@@ -0,0 +1,55 @@
+// Spec documentation http://httpwg.github.io/specs/rfc7231.html#header.expect
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const http = require('http');
+
+const tests = [417, 417];
+
+let testsComplete = 0;
+let testIdx = 0;
+
+const s = http.createServer(function(req, res) {
+  throw new Error('this should never be executed');
+});
+
+s.listen(common.PORT, nextTest);
+
+function nextTest() {
+  const options = {
+    port: common.PORT,
+    headers: { 'Expect': 'meoww' }
+  };
+
+  if (testIdx === tests.length) {
+    return s.close();
+  }
+
+  const test = tests[testIdx];
+
+  if (testIdx > 0) {
+    s.on('checkExpectation', common.mustCall((req, res) => {
+      res.statusCode = 417;
+      res.end();
+    }));
+  }
+
+  http.get(options, function(response) {
+    console.log('client: expected status: ' + test);
+    console.log('client: statusCode: ' + response.statusCode);
+    assert.equal(response.statusCode, test);
+    assert.equal(response.statusMessage, 'Expectation Failed');
+
+    response.on('end', function() {
+      testsComplete++;
+      testIdx++;
+      nextTest();
+    });
+    response.resume();
+  });
+}
+
+
+process.on('exit', function() {
+  assert.equal(2, testsComplete);
+});