Refactor ServerSession.generateResponse() to group all logic around setting each parameter -- both the internal setting and the response value -- together, and from here work around an issue in Firefox that means the client will fail if the server sends an unsolicited server_max_window_bits parameter.

lib/client_session.js

@@ -64,12 +64,12 @@ ClientSession.prototype.activate = function(params) {
 
   this._ownContextTakeover = !(this._acceptNoContextTakeover || params.client_no_context_takeover);
   this._ownWindowBits = Math.min(
-    this._acceptMaxWindowBits || common.DEFAULT_MAX_WINDOW_BITS,
-    params.client_max_window_bits || common.DEFAULT_MAX_WINDOW_BITS
+    this._acceptMaxWindowBits || common.MAX_WINDOW_BITS,
+    params.client_max_window_bits || common.MAX_WINDOW_BITS
   );
 
   this._peerContextTakeover = !params.server_no_context_takeover;
-  this._peerWindowBits = params.server_max_window_bits || common.DEFAULT_MAX_WINDOW_BITS;
+  this._peerWindowBits = params.server_max_window_bits || common.MAX_WINDOW_BITS;
 
   return true;
 };

lib/common.js

@@ -8,7 +8,7 @@ var common = {
     'client_max_window_bits'
   ],
 
-  DEFAULT_MAX_WINDOW_BITS: 15,
+  MAX_WINDOW_BITS:   15,
   VALID_WINDOW_BITS: [8, 9, 10, 11, 12, 13, 14, 15],
 
   concat: function(buffers, length) {

lib/server_session.js

@@ -21,42 +21,46 @@ ServerSession.validParams = function(params) {
 };
 
 ServerSession.prototype.generateResponse = function() {
-  var params = {};
+  var response = {};
 
   // https://tools.ietf.org/html/draft-ietf-hybi-permessage-compression#section-8.1.1.1
-  if (this._acceptNoContextTakeover || this._params.server_no_context_takeover)
-    params.server_no_context_takeover = true;
+
+  this._ownContextTakeover = !this._acceptNoContextTakeover &&
+                             !this._params.server_no_context_takeover;
+
+  if (!this._ownContextTakeover) response.server_no_context_takeover = true;
 
   // https://tools.ietf.org/html/draft-ietf-hybi-permessage-compression#section-8.1.1.2
-  if (this._requestNoContextTakeover || this._params.client_no_context_takeover)
-    params.client_no_context_takeover = true;
+
+  this._peerContextTakeover = !this._requestNoContextTakeover &&
+                              !this._params.client_no_context_takeover;
+
+  if (!this._peerContextTakeover) response.client_no_context_takeover = true;
 
   // https://tools.ietf.org/html/draft-ietf-hybi-permessage-compression#section-8.1.2.1
-  var acceptMax, serverMax;
-  if (this._acceptMaxWindowBits || this._params.server_max_window_bits) {
-    acceptMax = this._acceptMaxWindowBits || common.DEFAULT_MAX_WINDOW_BITS;
-    serverMax = this._params.server_max_window_bits || common.DEFAULT_MAX_WINDOW_BITS;
-    params.server_max_window_bits = Math.min(acceptMax, serverMax);
-  }
+
+  this._ownWindowBits = Math.min(this._acceptMaxWindowBits || common.MAX_WINDOW_BITS,
+                                 this._params.server_max_window_bits || common.MAX_WINDOW_BITS);
+
+  // In violation of the spec, Firefox closes the connection if it does not
+  // send server_max_window_bits but the server includes this in its response
+  if (this._ownWindowBits < common.MAX_WINDOW_BITS && this._params.server_max_window_bits)
+    response.server_max_window_bits = this._ownWindowBits;
 
   // https://tools.ietf.org/html/draft-ietf-hybi-permessage-compression#section-8.1.2.2
+
   var clientMax = this._params.client_max_window_bits, requestMax;
   if (clientMax) {
-    if (clientMax === true) {
-      if (this._requestMaxWindowBits) params.client_max_window_bits = this._requestMaxWindowBits;
-    } else {
-      requestMax = this._requestMaxWindowBits || common.DEFAULT_MAX_WINDOW_BITS;
-      params.client_max_window_bits = Math.min(requestMax, clientMax);
-    }
+    if (clientMax === true) clientMax = common.MAX_WINDOW_BITS;
+    this._peerWindowBits = Math.min(this._requestMaxWindowBits || common.MAX_WINDOW_BITS, clientMax);
+  } else {
+    this._peerWindowBits = common.MAX_WINDOW_BITS;
   }
 
-  this._ownContextTakeover = !params.server_no_context_takeover;
-  this._ownWindowBits = params.server_max_window_bits || common.DEFAULT_MAX_WINDOW_BITS;
+  if (this._peerWindowBits < common.MAX_WINDOW_BITS)
+    response.client_max_window_bits = this._peerWindowBits;
 
-  this._peerContextTakeover = !params.client_no_context_takeover;
-  this._peerWindowBits = params.client_max_window_bits || common.DEFAULT_MAX_WINDOW_BITS;
-
-  return params;
+  return response;
 };
 
 module.exports = ServerSession;

spec/server_session_spec.js

@@ -2,7 +2,7 @@ var PermessageDeflate = require('../lib/permessage_deflate'),
     zlib = require('zlib'),
     test = require('jstest').Test
 
-test.describe("ClientSession", function() { with(this) {
+test.describe("ServerSession", function() { with(this) {
   before(function() { with(this) {
     this.ext      = PermessageDeflate.configure(options)
     this.session  = ext.createServerSession([offer])
@@ -183,8 +183,8 @@ test.describe("ClientSession", function() { with(this) {
     define("options", {maxWindowBits: 12})
 
     describe("with an empty offer", function() { with(this) {
-      it("includes server_max_window_bits in the response", function() { with(this) {
-        assertEqual( {server_max_window_bits: 12}, response() )
+      it("does not include server_max_window_bits in the response", function() { with(this) {
+        assertEqual( {}, response() )
       }})
 
       it("uses context takeover and 12 window bits for deflating outgoing messages", function() { with(this) {