libcamera: thread: Ensure deferred deletion of all objects before stopping

Objects can be scheduled for deletion with Object::deleteLater(), which
queues a deferred deletion to the thread's event loop. As the
deleteLater() function is meant to be called from a different thread,
this may race with thread termination, and deferred deletions queued
just before calling Thread::exit() may not be processed by the event
loop. Make sure they get processed when finishing the thread, before
stopping.

This eliminates the race condition that occurs when calling
Object::deleteLater() followed by Thread::exit() from the same thread.
Calling deleteLater() from neither the thread the object is bound to or
the thread calling Thread::exit() is still inherently racy.

The change fixes a failure in the object-delete unit test.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Milan Zamazal <mzamazal@redhat.com>

src/libcamera/base/object.cpp

@@ -116,8 +116,9 @@ Object::~Object()
  * event loop that the object belongs to. This ensures the object is destroyed
  * from the right context, as required by the libcamera threading model.
  *
- * If this function is called before the thread's event loop is started, the
+ * If this function is called before the thread's event loop is started or after
- * object will be deleted when the event loop starts.
+ * it has stopped, the object will be deleted when the event loop (re)starts. If
+ * this never occurs, the object will be leaked.
  *
  * Deferred deletion can be used to control the destruction context with shared
  * pointers. An object managed with shared pointers is deleted when the last

src/libcamera/base/thread.cpp

@@ -371,6 +371,12 @@ void Thread::run()
 
 void Thread::finishThread()
 {
+	/*
+	 * Objects may have been scheduled for deletion right before the thread
+	 * exited. Ensure they get deleted now, before the thread stops.
+	 */
+	dispatchMessages(Message::Type::DeferredDelete);
+
 	data_->mutex_.lock();
 	data_->running_ = false;
 	data_->mutex_.unlock();