http: check for number of open connections during drain

If connections reach 0, we close earlier.

Author: kaiburjack

lib/_http_server.js

@@ -598,6 +598,7 @@ function Server(options, requestListener) {
 
   this.drainStarted = false;
   this.drainTimeout = options.drainTimeout || 0;
+  this.drainConnectionsPollInterval = options.drainConnectionsPollInterval || 500;
 
   if (requestListener) {
     // Do we want to gracefully drain existing keep-alive connections
@@ -662,7 +663,9 @@ Server.prototype.closeIdleConnections = function closeIdleConnections(callback)
   if (!this[kConnections]) {
     return;
   }
+
   const fn = () => {
+    // Destroy all still remaining idle connections.
     const connections = this[kConnections].idle();
     for (let i = 0, l = connections.length; i < l; i++) {
       if (connections[i].socket._httpMessage && !connections[i].socket._httpMessage.finished) {
@@ -673,10 +676,28 @@ Server.prototype.closeIdleConnections = function closeIdleConnections(callback)
     if (typeof callback === 'function') {
       callback();
     }
+  }
+
+  // For graceful draining, if drainTimeout > 0 (see below)
+  let startTime = Date.now()
+  let timer = null;
+  let graceIterations = 3;
+  const checkFn = () => {
+    const connections = this[kConnections].all();
+    if (Date.now() - startTime > this.drainTimeout || (connections.length === 0 && graceIterations-- <= 0)) {
+      // Either drain period has ended, or we have no open connections anymore.
+      // We can close now.
+      clearInterval(timer);
+      timer = null;
+      fn();
+    }
   };
-  this.drainStarted = true;
-  if (this.drainTimeout > 0) {
-    setTimeout(() => { fn() }, this.drainTimeout);
+
+  if (this.drainTimeout > 0 && !this.drainStarted) {
+    this.drainStarted = true;
+    // Start polling for idle connections.
+    // If all connections are gone or drain period ends, we call the callback.
+    timer = setInterval(checkFn, this.drainConnectionsPollInterval);
   } else {
     fn();
   }