Add a test.

tests/multithreading/task_result_03.cc

@@ -0,0 +1,69 @@
+// ------------------------------------------------------------------------
+//
+// SPDX-License-Identifier: LGPL-2.1-or-later
+// Copyright (C) 2009 - 2024 by the deal.II authors
+//
+// This file is part of the deal.II library.
+//
+// Part of the source code is dual licensed under Apache-2.0 WITH
+// LLVM-exception OR LGPL-2.1-or-later. Detailed license information
+// governing the source code and code contributions can be found in
+// LICENSE.md and CONTRIBUTING.md at the top level directory of deal.II.
+//
+// ------------------------------------------------------------------------
+
+
+// Test TaskResult::try_emplace_task()
+
+#include <deal.II/base/task_result.h>
+
+#include "../tests.h"
+
+
+
+int
+main()
+{
+  initlog();
+
+  // Create a default-constructed TaskResult object not associated with a task:
+  Threads::TaskResult<int> task_result;
+
+  // Next run 10 threads in parallel that all try to set a task
+  // object. Calling TaskResult::operator=() will not work because
+  // some thread will try to set a task while there is already a
+  // running task, and that is considered an error.
+  //
+  // On the other hand, exactly one of the threads will succeed in
+  // emplacing a task with try_emplace_task().
+  //
+  // To make things a bit more interesting, and make sure that there
+  // really is contention, we let all the threads spin until a
+  // condition variable changes state. In other words, they will all
+  // try *at more or less the same time* to call try_emplace_task().
+  std::atomic<bool> condition(false);
+
+  const unsigned int     n_threads = 10;
+  std::list<std::thread> threads;
+  for (unsigned int t = 0; t < n_threads; ++t)
+    threads.emplace_back(std::thread([&]() {
+      while (condition.load() == false)
+        /* keep checking */;
+
+      task_result.try_emplace_task([]() { return 42; });
+    }));
+
+  // Now let all of these threads go to work:
+  condition = true;
+
+  // Wait for all of these threads to finish trying to emplace a task:
+  for (auto &t : threads)
+    t.join();
+
+  // Wait for that background task to finish, then get its
+  // value:
+  deallog << task_result.value() << std::endl;
+
+  // Ensure that we can continue to ask for the return value:
+  deallog << task_result.value() << std::endl;
+}

tests/multithreading/task_result_03.output

@@ -0,0 +1,3 @@
+
+DEAL::42
+DEAL::42