libcamera: object: Document and ensure Object deletion constraints

Object instances are meant to be deleted from the thread they are bound
to, and this requirement is documented. There are however exceptions to
the rule, as threads may be stopped and restarted, with objects bound to
them not being deleted and recreated for every stop/restart cycle. Bound
objects may therefore need to be deleted after the thread has stopped,
making it impossible to use Object::deleteLater().

Document the lifetime requirements more precisely, and enforce them with
an assertion.

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

src/libcamera/base/object.cpp

@@ -40,8 +40,9 @@ LOG_DEFINE_CATEGORY(Object)
  * Object class.
  *
  * Deleting an object from a thread other than the one the object is bound to is
- * unsafe, unless the caller ensures that the object isn't processing any
- * message concurrently.
+ * unsafe, unless the caller ensures that the object's thread is stopped and no
+ * parent or child of the object gets deleted concurrently. See
+ * Object::~Object() for more information.
  *
  * Object slots connected to signals will also run in the context of the
  * object's thread, regardless of whether the signal is emitted in the same or
@@ -84,9 +85,20 @@ Object::Object(Object *parent)
  * Object instances shall be destroyed from the thread they are bound to,
  * otherwise undefined behaviour may occur. If deletion of an Object needs to
  * be scheduled from a different thread, deleteLater() shall be used.
+ *
+ * As an exception to this rule, Object instances may be deleted from a
+ * different thread if the thread the instance is bound to is stopped through
+ * the whole duration of the object's destruction, *and* the parent and children
+ * of the object do not get deleted concurrently. The caller is responsible for
+ * fulfilling those requirements.
+ *
+ * In all cases Object instances shall be deleted before the Thread they are
+ * bound to.
  */
 Object::~Object()
 {
+	ASSERT(Thread::current() == thread_ || !thread_->isRunning());
+
 	/*
 	 * Move signals to a private list to avoid concurrent iteration and
 	 * deletion of items from Signal::disconnect().