io: add qio_task_wait_thread to join with a background thread

Add the ability for a caller to wait for completion of the
background thread to synchronously dispatch its result, without
needing to wait for the main loop to run the idle callback.

This method needs very careful usage to avoid a dangerous
race condition with the free'ing of the task. The completion
callback is normally invoked from an idle callback registered
with the main loop context. The qio_task_wait_thread method
must only be called if the completion callback has not yet
run. The only safe way to achieve this is to run the
qio_task_wait_thread method from the thread that executes
the main loop.

It is generally a bad idea to use this method since it will
block execution of the main loop, however, the design of
the character devices and its usage from vhostuser already
requires blocking execution.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20190211182442.8542-3-berrange@redhat.com>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>

1 files changed, 28 insertions, 1 deletions

diff --git a/include/io/task.h b/include/io/task.h
index 9e09b95b2e..57d8ba835e 100644
--- a/include/io/task.h
+++ b/include/io/task.h
@@ -232,7 +232,8 @@ QIOTask *qio_task_new(Object *source,
  *
  * Run a task in a background thread. When @worker
  * returns it will call qio_task_complete() in
- * the event thread context that provided.
+ * the thread that is running the main loop associated
+ * with @context.
  */
 void qio_task_run_in_thread(QIOTask *task,
                             QIOTaskWorker worker,
@@ -240,6 +241,32 @@ void qio_task_run_in_thread(QIOTask *task,
                             GDestroyNotify destroy,
                             GMainContext *context);
 
+
+/**
+ * qio_task_wait_thread:
+ * @task: the task struct
+ *
+ * Wait for completion of a task that was previously
+ * invoked using qio_task_run_in_thread. This MUST
+ * ONLY be invoked if the task has not already
+ * completed, since after the completion callback
+ * is invoked, @task will have been freed.
+ *
+ * To avoid racing with execution of the completion
+ * callback provided with qio_task_new, this method
+ * MUST ONLY be invoked from the thread that is
+ * running the main loop associated with @context
+ * parameter to qio_task_run_in_thread.
+ *
+ * When the thread has completed, the completion
+ * callback provided to qio_task_new will be invoked.
+ * When that callback returns @task will be freed,
+ * so @task must not be referenced after this
+ * method completes.
+ */
+void qio_task_wait_thread(QIOTask *task);
+
+
 /**
  * qio_task_complete:
  * @task: the task struct