http: add missing async_hooks destroy before asyncReset

Emit a destroy before calling asyncReset because otherwise the old
async_id never gets destroyed.

Refs: nodejs#19859

Author: basti1302

lib/_http_agent.js

@@ -25,7 +25,12 @@ const net = require('net');
 const util = require('util');
 const EventEmitter = require('events');
 const debug = util.debuglog('http');
-const { async_id_symbol } = require('internal/async_hooks').symbols;
+const {
+  destroyHooksExist,
+  emitDestroy,
+  symbols
+} = require('internal/async_hooks');
+const async_id_symbol = symbols.async_id_symbol;
 
 // New Agent code.
 
@@ -167,6 +172,9 @@ Agent.prototype.addRequest = function addRequest(req, options, port/* legacy */,
     var socket = this.freeSockets[name].shift();
     // Guard against an uninitialized or user supplied Socket.
     if (socket._handle && typeof socket._handle.asyncReset === 'function') {
+      if (destroyHooksExist() && socket._handle.getAsyncId()) {
+        emitDestroy(socket._handle.getAsyncId());
+      }
       // Assign the handle a new asyncId and run any init() hooks.
       socket._handle.asyncReset();
       socket[async_id_symbol] = socket._handle.getAsyncId();

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

@@ -0,0 +1,84 @@
+'use strict';
+// Flags: --expose-internals
+const common = require('../common');
+const assert = require('assert');
+const { async_id_symbol } = require('internal/async_hooks').symbols;
+const async_hooks = require('async_hooks');
+const http = require('http');
+
+// Regression test for https://github.com/nodejs/node/issues/19859
+// Checks that an http.Agent emits a destroy for the old asyncId before calling
+// asyncReset()s when reusing a socket handle. The setup is nearly identical to
+// parallel/test-async-hooks-http-agent (which focuses on the assertion that
+// a fresh asyncId is assigned to the net.Socket instance).
+
+const destroyedIds = new Set();
+async_hooks.createHook({
+  destroy: common.mustCallAtLeast((asyncId) => {
+    destroyedIds.add(asyncId);
+  }, 1)
+}).enable();
+
+// Make sure a single socket is transparently 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:
+        const r2 = http.request({
+          agent, port, method: 'POST'
+        }, common.mustCall((res) => {
+          assert.ok(destroyedIds.has(asyncIdAtFirstRequest));
+
+          // 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);
+}));