http: add Agent.agentKeepAliveTimeoutBuffer option

PR-URL: https://github.com/nodejs/node/pull/59315
Reviewed-By: Jason Zhang <xzha4350@gmail.com>

test/parallel/test-http-agent-keep-alive-timeout-buffer.js

@@ -0,0 +1,44 @@
+'use strict';
+
+const common = require('../common');
+const assert = require('assert');
+const http = require('http');
+
+// Ensure agentKeepAliveTimeoutBuffer option sets the correct value or falls back to default.
+{
+  const agent1 = new http.Agent({ agentKeepAliveTimeoutBuffer: 1500, keepAlive: true });
+  assert.strictEqual(agent1.agentKeepAliveTimeoutBuffer, 1500);
+
+  const agent2 = new http.Agent({ agentKeepAliveTimeoutBuffer: -100, keepAlive: true });
+  assert.strictEqual(agent2.agentKeepAliveTimeoutBuffer, 1000);
+
+  const agent3 = new http.Agent({ agentKeepAliveTimeoutBuffer: Infinity, keepAlive: true });
+  assert.strictEqual(agent3.agentKeepAliveTimeoutBuffer, 1000);
+
+  const agent4 = new http.Agent({ keepAlive: true });
+  assert.strictEqual(agent4.agentKeepAliveTimeoutBuffer, 1000);
+}
+
+// Integration test with server sending Keep-Alive timeout header.
+{
+  const SERVER_TIMEOUT = 3;
+  const BUFFER = 1500;
+
+  const server = http.createServer((req, res) => {
+    res.setHeader('Keep-Alive', `timeout=${SERVER_TIMEOUT}`);
+    res.end('ok');
+  });
+
+  server.listen(0, common.mustCall(() => {
+    const agent = new http.Agent({ agentKeepAliveTimeoutBuffer: BUFFER, keepAlive: true });
+    assert.strictEqual(agent.agentKeepAliveTimeoutBuffer, BUFFER);
+
+    http.get({ port: server.address().port, agent }, (res) => {
+      res.resume();
+      res.on('end', () => {
+        agent.destroy();
+        server.close();
+      });
+    });
+  }));
+}