test: ensure Stop() thread helps drain the queue

Exercise the case where tasks remain pending and verify that the
thread calling Stop() participates in draining the queue

src/test/threadpool_tests.cpp

@@ -39,6 +39,7 @@ struct ThreadPoolFixture {
 // 10) Ensure Interrupt() prevents further submissions.
 // 11) Start() must not cause a deadlock when called during Stop().
 // 12) Ensure queued tasks complete after Interrupt().
+// 13) Ensure the Stop() calling thread helps drain the queue.
 BOOST_FIXTURE_TEST_SUITE(threadpool_tests, ThreadPoolFixture)
 
 #define WAIT_FOR(futures)                                                         \
@@ -380,4 +381,40 @@ BOOST_AUTO_TEST_CASE(queued_tasks_complete_after_interrupt)
     WAIT_FOR(blocking_tasks);
 }
 
+// Test 13, ensure the Stop() calling thread helps drain the queue
+BOOST_AUTO_TEST_CASE(stop_active_wait_drains_queue)
+{
+    ThreadPool threadPool(POOL_NAME);
+    threadPool.Start(NUM_WORKERS_DEFAULT);
+
+    std::counting_semaphore<> blocker(0);
+    const auto blocking_tasks = BlockWorkers(threadPool, blocker, NUM_WORKERS_DEFAULT);
+
+    auto main_thread_id = std::this_thread::get_id();
+    std::atomic<int> main_thread_tasks{0};
+    const size_t num_tasks = 20;
+    for (size_t i = 0; i < num_tasks; i++) {
+        (void)Submit(threadPool, [&main_thread_tasks, main_thread_id]() {
+            if (std::this_thread::get_id() == main_thread_id)
+                main_thread_tasks.fetch_add(1, std::memory_order_relaxed);
+        });
+    }
+    BOOST_CHECK_EQUAL(threadPool.WorkQueueSize(), num_tasks);
+
+    // Delay release so Stop() drain all tasks from the calling thread
+    std::thread unblocker([&blocker, &threadPool]() {
+        while (threadPool.WorkQueueSize() > 0) {
+            std::this_thread::yield();
+        }
+        blocker.release(NUM_WORKERS_DEFAULT);
+    });
+
+    threadPool.Stop();
+    unblocker.join();
+
+    // Check the main thread processed all tasks
+    BOOST_CHECK_EQUAL(main_thread_tasks.load(), num_tasks);
+    WAIT_FOR(blocking_tasks);
+}
+
 BOOST_AUTO_TEST_SUITE_END()