async_hooks,http: fix socket reuse with Agent

Under very specific circumstances the `http` implementation
could be brought to crash, because the Agent did not re-assign
the async id field properly after setting up a socket for reuse.

Fixes: nodejs#13325

lib/_http_agent.js

@@ -168,6 +168,7 @@ Agent.prototype.addRequest = function addRequest(req, options, port/*legacy*/,
     var socket = this.freeSockets[name].shift();
     // Assign the handle a new asyncId and run any init() hooks.
     socket._handle.asyncReset();
+    socket[async_id_symbol] = socket._handle.getAsyncId();
     debug('have free socket');
 
     // don't leak

test/parallel/test-async-hooks-http-agent.js

@@ -0,0 +1,79 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const async_id_symbol = process.binding('async_wrap').async_id_symbol;
+const http = require('http');
+
+// Regression test for https://github.com/nodejs/node/issues/13325
+// Checks that an http.Agent properly asyncReset()s a reused socket handle, and
+// re-assigns the fresh async id to the reused `net.Socket` instance.
+
+// Make sure a single socket is transpartently reused for 2 requests.
+const agent = new http.Agent({
+  keepAlive: true,
+  keepAliveMsecs: Infinity,
+  maxSockets: 1
+});
+
+const server = http.createServer(common.mustCall((req, res) => {
+  req.once('data', common.mustCallAtLeast(() => {
+    res.writeHead(200, {'Content-Type': 'text/plain'});
+    res.write('foo');
+  }));
+  req.on('end', common.mustCall(() => {
+    res.end('bar');
+  }));
+}, 2)).listen(0, common.mustCall(() => {
+  const port = server.address().port;
+  const payload = 'hello world';
+
+  // First request. This is useless except for adding a socket to the
+  // agent’s pool for reuse.
+  const r1 = http.request({
+    agent, port, method: 'POST'
+  }, common.mustCall((res) => {
+    // Remember which socket we used.
+    const socket = res.socket;
+    const asyncIdAtFirstRequest = socket[async_id_symbol];
+    assert.ok(asyncIdAtFirstRequest > 0, `${asyncIdAtFirstRequest} > 0`);
+    // Check that request and response share their socket.
+    assert.strictEqual(r1.socket, socket);
+
+    res.on('data', common.mustCallAtLeast(() => {}));
+    res.on('end', common.mustCall(() => {
+      // setImmediate() to give the agent time to register the freed socket.
+      setImmediate(common.mustCall(() => {
+        // The socket is free for reuse now.
+        assert.strictEqual(socket[async_id_symbol], -1);
+
+        // Second request. To re-create the exact conditions from the
+        // referenced issue, we use a POST request without chunked encoding
+        // (hence the Content-Length header) and call .end() after the
+        // response header has already been received.
+        const r2 = http.request({
+          agent, port, method: 'POST', headers: {
+            'Content-Length': payload.length
+          }
+        }, common.mustCall((res) => {
+          const asyncId = res.socket[async_id_symbol];
+          assert.ok(asyncId > 0, `${asyncId} > 0`);
+          assert.strictEqual(r2.socket, socket);
+          // Empty payload, to hit the “right” code path.
+          r2.end('');
+
+          res.on('data', common.mustCallAtLeast(() => {}));
+          res.on('end', common.mustCall(() => {
+            // Clean up to let the event loop stop.
+            server.close();
+            agent.destroy();
+          }));
+        }));
+
+        // Schedule a payload to be written immediately, but do not end the
+        // request just yet.
+        r2.write(payload);
+      }));
+    }));
+  }));
+  r1.end(payload);
+}));