libcamera: thread: Add a messaging passing API

Create a new Message class to model a message that can be passed to an
object living in another thread. Only an invalid message type is
currently defined, more messages will be added in the future.

The Thread class is extended with a messages queue, and the Object class
with thread affinity.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>

src/libcamera/object.cpp

@@ -9,6 +9,9 @@
 
 #include <libcamera/signal.h>
 
+#include "log.h"
+#include "thread.h"
+
 /**
  * \file object.h
  * \brief Base object to support automatic signal disconnection
@@ -24,13 +27,85 @@ namespace libcamera {
  * slots. By inheriting from Object, an object is automatically disconnected
  * from all connected signals when it gets destroyed.
  *
- * \sa Signal
+ * Object instances are bound to the thread in which they're created. When a
+ * message is posted to an object, its handler will run in the object's thread.
+ * This allows implementing easy message passing between threads by inheriting
+ * from the Object class.
+ *
+ * \sa Message, Signal, Thread
  */
 
+Object::Object()
+	: pendingMessages_(0)
+{
+	thread_ = Thread::current();
+}
+
 Object::~Object()
 {
 	for (SignalBase *signal : signals_)
 		signal->disconnect(this);
+
+	if (pendingMessages_)
+		thread()->removeMessages(this);
+}
+
+/**
+ * \brief Post a message to the object's thread
+ * \param[in] msg The message
+ *
+ * This method posts the message \a msg to the message queue of the object's
+ * thread, to be delivered to the object through the message() method in the
+ * context of its thread. Message ownership is passed to the thread, and the
+ * message will be deleted after being delivered.
+ *
+ * Messages are delivered through the thread's event loop. If the thread is not
+ * running its event loop the message will not be delivered until the event
+ * loop gets started.
+ */
+void Object::postMessage(std::unique_ptr<Message> msg)
+{
+	thread()->postMessage(std::move(msg), this);
+}
+
+/**
+ * \brief Message handler for the object
+ * \param[in] msg The message
+ *
+ * This virtual method receives messages for the object. It is called in the
+ * context of the object's thread, and can be overridden to process custom
+ * messages. The parent Object::message() method shall be called for any
+ * message not handled by the override method.
+ *
+ * The message \a msg is valid only for the duration of the call, no reference
+ * to it shall be kept after this method returns.
+ */
+void Object::message(Message *msg)
+{
+}
+
+/**
+ * \fn Object::thread()
+ * \brief Retrieve the thread the object is bound to
+ * \return The thread the object is bound to
+ */
+
+/**
+ * \brief Move the object to a different thread
+ * \param[in] thread The target thread
+ *
+ * This method moves the object from the current thread to the new \a thread.
+ * It shall be called from the thread in which the object currently lives,
+ * otherwise the behaviour is undefined.
+ */
+void Object::moveToThread(Thread *thread)
+{
+	ASSERT(Thread::current() == thread_);
+
+	if (thread_ == thread)
+		return;
+
+	thread->moveObject(this);
 }
 
 void Object::connect(SignalBase *signal)