http: do not rely on the 'agentRemove' event

Do not use the `'agentRemove'` event to null `socket._httpMessage` as
that event is public and can be used to not keep a request in the agent.

PR-URL: #20786
Fixes: #20690
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>

lib/_http_agent.js

@@ -276,7 +276,6 @@ function installListeners(agent, s, options) {
     s.removeListener('close', onClose);
     s.removeListener('free', onFree);
     s.removeListener('agentRemove', onRemove);
-    s._httpMessage = null;
   }
   s.on('agentRemove', onRemove);
 }

lib/_http_client.js

@@ -459,6 +459,7 @@ function socketOnData(d) {
       socket.removeListener('close', socketCloseListener);
       socket.removeListener('error', socketErrorListener);
 
+      socket._httpMessage = null;
       socket.readableFlowing = null;
 
       req.emit(eventName, res, socket, bodyHead);

test/parallel/test-http-agent-remove.js

@@ -0,0 +1,21 @@
+'use strict';
+const { mustCall } = require('../common');
+
+const http = require('http');
+const { strictEqual } = require('assert');
+
+const server = http.createServer(mustCall((req, res) => {
+  res.flushHeaders();
+}));
+
+server.listen(0, mustCall(() => {
+  const req = http.get({
+    port: server.address().port
+  }, mustCall(() => {
+    const { socket } = req;
+    socket.emit('agentRemove');
+    strictEqual(socket._httpMessage, req);
+    socket.destroy();
+    server.close();
+  }));
+}));