[v12.x backport] http: added scheduling option to http agent

doc/api/http.md

@@ -116,6 +116,10 @@ changes:
   - version: v12.19.0
     pr-url: /~https://github.com/nodejs/node/pull/33617
     description: Add `maxTotalSockets` option to agent constructor.
+  - version: REPLACEME
+    pr-url: /~https://github.com/nodejs/node/pull/33278
+    description: Add `scheduling` option to specify the free socket
+                 scheduling strategy.
 -->
 
 * `options` {Object} Set of configurable options to set on the agent.
@@ -142,6 +146,18 @@ changes:
   * `maxFreeSockets` {number} Maximum number of sockets to leave open
     in a free state. Only relevant if `keepAlive` is set to `true`.
     **Default:** `256`.
+  * `scheduling` {string} Scheduling strategy to apply when picking
+    the next free socket to use. It can be `'fifo'` or `'lifo'`.
+    The main difference between the two scheduling strategies is that `'lifo'`
+    selects the most recently used socket, while `'fifo'` selects
+    the least recently used socket.
+    In case of a low rate of request per second, the `'lifo'` scheduling
+    will lower the risk of picking a socket that might have been closed
+    by the server due to inactivity.
+    In case of a high rate of request per second,
+    the `'fifo'` scheduling will maximize the number of open sockets,
+    while the `'lifo'` scheduling will keep it as low as possible.
+    **Default:** `'fifo'`.
   * `timeout` {number} Socket timeout in milliseconds.
     This will set the timeout when the socket is created.
 

lib/_http_agent.js

@@ -38,6 +38,7 @@ const { async_id_symbol } = require('internal/async_hooks').symbols;
 const {
   codes: {
     ERR_OUT_OF_RANGE,
+    ERR_INVALID_OPT_VALUE,
   },
 } = require('internal/errors');
 const { validateNumber } = require('internal/validators');
@@ -102,6 +103,12 @@ function Agent(options) {
     this.maxTotalSockets = Infinity;
   }
 
+  this.scheduling = this.options.scheduling || 'fifo';
+
+  if (this.scheduling !== 'fifo' && this.scheduling !== 'lifo') {
+    throw new ERR_INVALID_OPT_VALUE('scheduling', this.scheduling);
+  }
+
   this.on('free', (socket, options) => {
     const name = this.getName(options);
     debug('agent.on(free)', name);
@@ -238,7 +245,9 @@ Agent.prototype.addRequest = function addRequest(req, options, port/* legacy */,
     while (freeSockets.length && freeSockets[0].destroyed) {
       freeSockets.shift();
     }
-    socket = freeSockets.shift();
+    socket = this.scheduling === 'fifo' ?
+      freeSockets.shift() :
+      freeSockets.pop();
     if (!freeSockets.length)
       delete this.freeSockets[name];
   }

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

@@ -0,0 +1,148 @@
+'use strict';
+
+const common = require('../common');
+const assert = require('assert');
+const http = require('http');
+
+function createServer(count) {
+  return http.createServer(common.mustCallAtLeast((req, res) => {
+    // Return the remote port number used for this connection.
+    res.end(req.socket.remotePort.toString(10));
+  }), count);
+}
+
+function makeRequest(url, agent, callback) {
+  http
+    .request(url, { agent }, (res) => {
+      let data = '';
+      res.setEncoding('ascii');
+      res.on('data', (c) => {
+        data += c;
+      });
+      res.on('end', () => {
+        process.nextTick(callback, data);
+      });
+    })
+    .end();
+}
+
+function bulkRequest(url, agent, done) {
+  const ports = [];
+  let count = agent.maxSockets;
+
+  for (let i = 0; i < agent.maxSockets; i++) {
+    makeRequest(url, agent, callback);
+  }
+
+  function callback(port) {
+    count -= 1;
+    ports.push(port);
+    if (count === 0) {
+      done(ports);
+    }
+  }
+}
+
+function defaultTest() {
+  const server = createServer(8);
+  server.listen(0, onListen);
+
+  function onListen() {
+    const url = `http://localhost:${server.address().port}`;
+    const agent = new http.Agent({
+      keepAlive: true,
+      maxSockets: 5
+    });
+
+    bulkRequest(url, agent, (ports) => {
+      makeRequest(url, agent, (port) => {
+        assert.strictEqual(ports[0], port);
+        makeRequest(url, agent, (port) => {
+          assert.strictEqual(ports[1], port);
+          makeRequest(url, agent, (port) => {
+            assert.strictEqual(ports[2], port);
+            server.close();
+            agent.destroy();
+          });
+        });
+      });
+    });
+  }
+}
+
+function fifoTest() {
+  const server = createServer(8);
+  server.listen(0, onListen);
+
+  function onListen() {
+    const url = `http://localhost:${server.address().port}`;
+    const agent = new http.Agent({
+      keepAlive: true,
+      maxSockets: 5,
+      scheduling: 'fifo'
+    });
+
+    bulkRequest(url, agent, (ports) => {
+      makeRequest(url, agent, (port) => {
+        assert.strictEqual(ports[0], port);
+        makeRequest(url, agent, (port) => {
+          assert.strictEqual(ports[1], port);
+          makeRequest(url, agent, (port) => {
+            assert.strictEqual(ports[2], port);
+            server.close();
+            agent.destroy();
+          });
+        });
+      });
+    });
+  }
+}
+
+function lifoTest() {
+  const server = createServer(8);
+  server.listen(0, onListen);
+
+  function onListen() {
+    const url = `http://localhost:${server.address().port}`;
+    const agent = new http.Agent({
+      keepAlive: true,
+      maxSockets: 5,
+      scheduling: 'lifo'
+    });
+
+    bulkRequest(url, agent, (ports) => {
+      makeRequest(url, agent, (port) => {
+        assert.strictEqual(ports[ports.length - 1], port);
+        makeRequest(url, agent, (port) => {
+          assert.strictEqual(ports[ports.length - 1], port);
+          makeRequest(url, agent, (port) => {
+            assert.strictEqual(ports[ports.length - 1], port);
+            server.close();
+            agent.destroy();
+          });
+        });
+      });
+    });
+  }
+}
+
+function badSchedulingOptionTest() {
+  try {
+    new http.Agent({
+      keepAlive: true,
+      maxSockets: 5,
+      scheduling: 'filo'
+    });
+  } catch (err) {
+    assert.strictEqual(err.code, 'ERR_INVALID_OPT_VALUE');
+    assert.strictEqual(
+      err.message,
+      'The value "filo" is invalid for option "scheduling"'
+    );
+  }
+}
+
+defaultTest();
+fifoTest();
+lifoTest();
+badSchedulingOptionTest();