examples: add a new test case for starpu_tag_get_task()

As mentioned in r16755, calling starpu_tag_get_task() after
starpu_task_submit() is wrong because the task might have been already
destroyed. To not let StarPU automatically destroys a task, the flag
starpu_task::destroy must be set to 0.

examples/tag_example/tag_example.c

@@ -217,6 +217,34 @@ static void express_deps(unsigned i, unsigned j, unsigned piter)
 	}
 }
 
+static int test_starpu_tag_get_task()
+{
+	struct starpu_task *task;
+	int ret;
+
+	/* create a new task with a tag */
+	task = starpu_task_create();
+	task->callback_func = callback_cpu;
+	task->cl = &cl;
+	task->cl_arg = NULL;
+	task->destroy = 0; /* tell StarPU to not destroy the task */
+	task->use_tag = 1;
+	task->tag_id = TAG(99, 99, 99); /* arbitrary numbers */
+
+	/* execute the task */
+	ret = starpu_task_submit(task);
+	if (ret == -ENODEV) return 77;
+		STARPU_CHECK_RETURN_VALUE(ret, "starpu_task_submit");
+	starpu_task_wait_for_all();
+
+	/* check that starpu_tag_get_task() returns the correct task */
+	assert(starpu_tag_get_task(task->tag_id) == task);
+
+	starpu_task_destroy(task);
+
+	return 0;
+}
+
 int main(int argc STARPU_ATTRIBUTE_UNUSED , char **argv STARPU_ATTRIBUTE_UNUSED)
 {
 	int ret;
@@ -242,6 +270,10 @@ int main(int argc STARPU_ATTRIBUTE_UNUSED , char **argv STARPU_ATTRIBUTE_UNUSED)
 	if (ret == 0)
 	     starpu_task_wait_for_all();
 
+	ret = test_starpu_tag_get_task();
+	if (ret)
+		return ret;
+
 	tag_cleanup_grid(nk-2);
 	tag_cleanup_grid(nk-1);