[feature] Introduce the requireProtocolSelection option

To comply with the WHATWG specification, the connection must fail if the
server does not return the `Sec-WebSocket-Protocol` header when a client
requests specific subprotocols. However, some servers omit this header,
as such behavior is permitted by RFC 6455.

This new option adds the ability to skip that validation, enabling the
client to work with servers that are not WHATWG-compliant.

Refs: #1862 (comment)

Author: lpinca

doc/ws.md

@@ -328,6 +328,9 @@ This class represents a WebSocket. It extends the `EventEmitter`.
     depending on the `protocolVersion`.
   - `perMessageDeflate` {Boolean|Object} Enable/disable permessage-deflate.
   - `protocolVersion` {Number} Value of the `Sec-WebSocket-Version` header.
+  - `requireProtocolSelection` {Boolean} Specifies whether to treat a missing
+    `Sec-WebSocket-Protocol` header in the server response as an error when
+    subprotocols are requested. Defaults to `true`.
   - `skipUTF8Validation` {Boolean} Specifies whether or not to skip UTF-8
     validation for text and close messages. Defaults to `false`. Set to `true`
     only if the server is trusted.

lib/websocket.js

@@ -650,6 +650,9 @@ module.exports = WebSocket;
  *     permessage-deflate
  * @param {Number} [options.protocolVersion=13] Value of the
  *     `Sec-WebSocket-Version` header
+ * @param {Boolean} [options.requireProtocolSelection=false] Specifies whether
+ *     to treat a missing `Sec-WebSocket-Protocol` header in the server response
+ *     as an error when subprotocols are requested.
  * @param {Boolean} [options.skipUTF8Validation=false] Specifies whether or
  *     not to skip UTF-8 validation for text and close messages
  * @private
@@ -663,6 +666,7 @@ function initAsClient(websocket, address, protocols, options) {
     maxPayload: 100 * 1024 * 1024,
     skipUTF8Validation: false,
     perMessageDeflate: true,
+    requireProtocolSelection: true,
     followRedirects: false,
     maxRedirects: 10,
     ...options,
@@ -959,7 +963,7 @@ function initAsClient(websocket, address, protocols, options) {
       } else if (!protocolSet.has(serverProt)) {
         protError = 'Server sent an invalid subprotocol';
       }
-    } else if (protocolSet.size) {
+    } else if (protocolSet.size && opts.requireProtocolSelection) {
       protError = 'Server sent no subprotocol';
     }
 

test/websocket.test.js

@@ -1402,6 +1402,29 @@ describe('WebSocket', () => {
       });
     });
 
+    it('honors the `requireProtocolSelection` option', (done) => {
+      const wss = new WebSocket.Server({
+        handleProtocols() {},
+        server
+      });
+
+      wss.on('connection', (ws) => {
+        assert.strictEqual(ws.protocol, '');
+        ws.on('close', () => wss.close(done));
+      });
+
+      const ws = new WebSocket(
+        `ws://localhost:${server.address().port}`,
+        'foo',
+        { requireProtocolSelection: false }
+      );
+
+      ws.on('open', () => {
+        assert.strictEqual(ws.protocol, '');
+        ws.close();
+      });
+    });
+
     it('honors the `createConnection` option', (done) => {
       const wss = new WebSocket.Server({ noServer: true, path: '/foo' });