http: allow _httpMessage to be GC'ed

Set `socket._httpMessage` to `null` before emitting the `'connect'` or
`'upgrade'` event.

PR-URL: nodejs#18865
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>

lib/_http_agent.js

@@ -277,6 +277,7 @@ function installListeners(agent, s, options) {
     s.removeListener('close', onClose);
     s.removeListener('free', onFree);
     s.removeListener('agentRemove', onRemove);
+    s._httpMessage = null;
   }
   s.on('agentRemove', onRemove);
 }

test/parallel/test-http-connect.js

@@ -49,6 +49,10 @@ server.listen(0, common.mustCall(() => {
     path: 'google.com:443'
   }, common.mustNotCall());
 
+  req.on('socket', common.mustCall((socket) => {
+    assert.strictEqual(socket._httpMessage, req);
+  }));
+
   req.on('close', common.mustCall());
 
   req.on('connect', common.mustCall((res, socket, firstBodyChunk) => {
@@ -60,6 +64,7 @@ server.listen(0, common.mustCall(() => {
     // Make sure this socket has detached.
     assert(!socket.ondata);
     assert(!socket.onend);
+    assert.strictEqual(socket._httpMessage, null);
     assert.strictEqual(socket.listeners('connect').length, 0);
     assert.strictEqual(socket.listeners('data').length, 0);