test: Add test http agent keepalive LIFO behavior

Add a test that ensures the HTTP agent reuses sockets
in a LIFO fashion rather than FIFO.

test/sequential/test-http-keepalive-lifo-sockets.js

@@ -0,0 +1,95 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const http = require('http');
+
+// To test the LIFO behavior of the agent:
+// Send a request and store the local TCP port
+// number used to send a request to the server.
+
+// Store a list of port numbers used to process requests...
+let port_list = [];
+
+const server = http.createServer(common.mustCallAtLeast((req, res) => {
+  // Return the remote port number used for this connection.
+  res.end(req.socket.remotePort.toString(10));
+}), 6);
+
+server.listen(0, function() {
+  const agent = http.Agent({
+    keepAlive: true,
+    maxSockets: 4
+  });
+
+  // Fill the agent with connections
+  const fill_request_count = agent.maxSockets;
+
+  makeReqs(fill_request_count, function(er) {
+    assert.ifError(er);
+    // Assert that different ports were used.
+    assert.strictEqual(port_list.length, fill_request_count);
+
+    // Now the agent is fully populated with connections to the server
+    // just waiting to be reused.
+
+    // For LIFO behavior the last used port should always be used
+    // to send a serial string of requests.
+
+    // If there was FIFO behavior for each serial request a
+    // the oldest used port should be used to send the request.
+
+    const fill_ports = port_list;
+    port_list = [];
+    makeReq(common.mustCall(() => {
+      // Since the agent's free socket list isn't updated
+      // until after the response is processed. Send the next
+      // request completely after the first one is finished processing.
+      process.nextTick(common.mustCall(() => {
+        makeReq(common.mustCall(() => {
+          // LIFO should always use the last used port to send the next
+          // request, so verify that behavior.
+          const lastPort = fill_ports[fill_ports.length - 1];
+          assert.deepStrictEqual(port_list, [lastPort, lastPort]);
+          agent.destroy();
+          server.close();
+        }));
+      }));
+    }));
+  });
+
+  process.on('exit', function() {
+    console.log('ok');
+  });
+
+  function makeReqs(n, cb) {
+    for (let i = 0; i < n; i++) makeReq(then);
+
+    function then(er) {
+      if (er) return cb(er);
+      else if (--n === 0) process.nextTick(cb);
+    }
+  }
+
+  function makeReq(cb) {
+    http
+      .request(
+        {
+          port: server.address().port,
+          path: '/',
+          agent: agent
+        },
+        function(res) {
+          let data = '';
+          res.setEncoding('ascii');
+          res.on('data', function(c) {
+            data += c;
+          });
+          res.on('end', function() {
+            port_list.push(data);
+            cb();
+          });
+        }
+      )
+      .end();
+  }
+});