Add reusedSocket property on client request

README.md

@@ -165,6 +165,30 @@ setTimeout(() => {
 }, 2000);
 ```
 
+### Support `req.reusedSocket`
+
+This agent implements the `req.reusedSocket` to determine whether a request is send through a reused socket.
+
+When server closes connection at unfortunate time ([keep-alive race](https://code-examples.net/en/q/28a8069)), the http client will throw a `ECONNRESET` error. Under this circumstance, `req.reusedSocket` is useful when we want to retry the request automatically.
+
+```js
+const http = require('http');
+const Agent = require('agentkeepalive');
+const agent = new Agent();
+
+const req = http
+  .get('http://localhost:3000', { agent }, (res) => {
+    // ...
+  })
+  .on('error', (err) => {
+    if (req.reusedSocket && err.code === 'ECONNRESET') {
+      // retry the request or anything else...
+    }
+  })
+```
+
+This behavior is consistent with Node.js core. But through `agentkeepalive`, you can use this feature in older Node.js version.
+
 ## [Benchmark](https://github.com/node-modules/agentkeepalive/tree/master/benchmark)
 
 run the benchmark:

lib/agent.js

@@ -140,6 +140,8 @@ class Agent extends OriginalAgent {
     // reuseSocket(socket, req)
     super.reuseSocket(...args);
     const socket = args[0];
+    const req = args[1];
+    req.reusedSocket = true;
     const agentTimeout = this.options.timeout;
     if (getSocketTimeout(socket) !== agentTimeout) {
       // reset timeout before use

test/agent.test.js

@@ -1495,6 +1495,43 @@ describe('test/agent.test.js', () => {
     assert(Object.keys(agentkeepalive.sockets).length === 1);
   });
 
+  it('should set req.reusedSocket to true when reuse socket', done => {
+    const agent = new Agent({
+      keepAlive: true,
+    });
+
+    // First request
+    const req1 = http.get({
+      port,
+      path: '/',
+      agent,
+    }, res => {
+      assert(res.statusCode === 200);
+      res.on('data', () => {});
+      res.on('end', () => {
+        setTimeout(() => {
+          // Second request
+          const req2 = http.get({
+            port,
+            path: '/',
+            agent,
+          }, res => {
+            assert(res.statusCode === 200);
+            res.on('data', () => {});
+            res.on('end', () => {
+              done();
+            });
+          });
+          // Second request reuses the socket
+          assert(req2.reusedSocket);
+        }, 10);
+      });
+    });
+
+    // First request doesn't reuse the socket
+    assert(!req1.reusedSocket);
+  });
+
   describe('request timeout > agent timeout', () => {
     it('should use request timeout', done => {
       const agent = new Agent({