test: add regression test for quic UpdateDataStats null-deref

Exercises the crash path in Session::SendDataStats() where on_exit fires
UpdateDataStats() after Destroy() has already reset impl_ to nullptr.

Refs: #62126

Author: zerone0x

test/parallel/test-quic-session-update-data-stats-after-close.mjs

@@ -0,0 +1,54 @@
+// Flags: --experimental-quic --no-warnings
+// Regression test for https://github.com/nodejs/node/pull/62126
+// UpdateDataStats() must not crash when called after the session's impl_ has
+// been reset to null (i.e. after the session is destroyed).
+//
+// The crash path is in Session::SendDatagram():
+//   auto on_exit = OnScopeLeave([&] { ...; UpdateDataStats(); });
+// If the send encounters an error it calls Close(SILENT) → Destroy() →
+// impl_.reset(). The on_exit lambda then fires with impl_ == nullptr.
+
+import { hasQuic, skip, mustCall } from '../common/index.mjs';
+import * as fixtures from '../common/fixtures.mjs';
+
+if (!hasQuic) {
+  skip('QUIC is not enabled');
+}
+
+const { listen, connect } = await import('node:quic');
+const { createPrivateKey } = await import('node:crypto');
+
+const keys = createPrivateKey(fixtures.readKey('agent1-key.pem'));
+const certs = fixtures.readKey('agent1-cert.pem');
+
+// Datagrams must be enabled on both sides for sendDatagram() to work.
+const kDatagramOptions = {
+  transportParams: {
+    maxDatagramFrameSize: 1200n,
+  },
+};
+
+const serverEndpoint = await listen(
+  mustCall((serverSession) => {
+    serverSession.opened.then(mustCall(async () => {
+      // Send a datagram then immediately close. This exercises the
+      // UpdateDataStats() call that fires via on_exit after SendDatagram —
+      // the close can race with or precede the stats update, leaving
+      // impl_ == nullptr. Before the fix this would crash.
+      serverSession.sendDatagram(Buffer.from('hello')).catch(() => {});
+      serverSession.close();
+    }));
+  }),
+  { keys, certs, ...kDatagramOptions },
+);
+
+const clientSession = await connect(serverEndpoint.address, kDatagramOptions);
+await clientSession.opened;
+
+// Mirror the race on the client side.
+clientSession.sendDatagram(Buffer.from('world')).catch(() => {});
+clientSession.close();
+
+await clientSession.closed;
+serverEndpoint.close();
+await serverEndpoint.closed;