[feature] Add option to support late addition of headers

This supports the use-case where headers need to be added that depend on
the socket connection (e.g. for TLS channel binding).

doc/ws.md

@@ -293,6 +293,8 @@ This class represents a WebSocket. It extends the `EventEmitter`.
 - `address` {String|url.URL} The URL to which to connect.
 - `protocols` {String|Array} The list of subprotocols.
 - `options` {Object}
+  - `finishRequest` {Function} A function which can be used to customize the
+    http request before it is sent. See description below.
   - `followRedirects` {Boolean} Whether or not to follow redirects. Defaults to
     `false`.
   - `generateMask` {Function} The function used to generate the masking key. It
@@ -316,12 +318,24 @@ This class represents a WebSocket. It extends the `EventEmitter`.
     Options given do not have any effect if parsed from the URL given with the
     `address` parameter.
 
+Create a new WebSocket instance.
+
 `perMessageDeflate` default value is `true`. When using an object, parameters
 are the same of the server. The only difference is the direction of requests.
 For example, `serverNoContextTakeover` can be used to ask the server to disable
 context takeover.
 
-Create a new WebSocket instance.
+`finishRequest` is called with arguments
+
+- `request` {http.ClientRequest}
+- `websocket` {WebSocket}
+
+for each HTTP GET request (the initial one and any caused by redirects) when it
+is ready to be sent, to allow for last minute customization of the headers.  If
+`finishRequest` is set then it has the responsibility to call `request.end()`
+once it is done setting request headers. This is intended for niche use-cases
+where some headers can't be provided in advance e.g. because they depend on the
+underlying socket.
 
 #### IPC connections
 

lib/websocket.js

@@ -989,7 +989,11 @@ function initAsClient(websocket, address, protocols, options) {
     });
   });
 
-  req.end();
+  if (typeof opts.finishRequest === 'function') {
+    opts.finishRequest(req, websocket);
+  } else {
+    req.end();
+  }
 }
 
 /**

test/websocket.test.js

@@ -3857,6 +3857,31 @@ describe('WebSocket', () => {
         agent
       });
     });
+
+    it('supports late headers using finishRequest', (done) => {
+      const wss = new WebSocket.Server({ port: 0 }, () => {
+        const ws = new WebSocket(`ws://localhost:${wss.address().port}`, {
+          finishRequest(req, ws_arg) {
+            setTimeout( () => {
+              assert.strictEqual(ws_arg, ws);
+              assert.strictEqual(req, ws._req);
+              req.setHeader('Cookie', 'foo=bar');
+              req.end();
+            }, 150);
+          }
+        });
+
+        ws.on('close', (code) => {
+          assert.strictEqual(code, 1005);
+          wss.close(done);
+        });
+      });
+
+      wss.on('connection', (ws, req) => {
+        assert.strictEqual(req.headers.cookie, 'foo=bar');
+        ws.close();
+      });
+    });
   });
 
   describe('permessage-deflate', () => {