http: fast-forward teardown of unread messages

When a response finishes and the incoming message was fully received
but never read, _dump() no longer goes through resume(): read(0) at
EOF schedules the 'end' emission directly, skipping the resume_ tick,
the 'resume' emit and the flow() machinery.

Reduces the per-request nextTick count of a hello-world HTTP server
from 7 to 6 and CPU per request by ~4%.

Signed-off-by: Matteo Collina <hello@matteocollina.com>

Author: mcollina

lib/_http_incoming.js

@@ -512,7 +512,15 @@ IncomingMessage.prototype._dump = function _dump() {
     // If there is buffered data, it may trigger 'data' events.
     // Remove 'data' event listeners explicitly.
     this.removeAllListeners('data');
-    this.resume();
+    const state = this._readableState;
+    if (state.ended && !state.destroyed && state.length === 0) {
+      // The message was fully received and never read: there is nothing to
+      // pull off the wire. Go straight to the 'end' emission instead of
+      // paying for the resume() and flow() scheduling machinery.
+      this.read(0);
+    } else {
+      this.resume();
+    }
   }
 };