libcamera: signal: Support cross-thread signals

Allow signals to cross thread boundaries by posting them to the
recipient through messages instead of calling the slot directly when the
recipient lives in a different thread.

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

src/libcamera/include/message.h

@@ -10,6 +10,7 @@
 namespace libcamera {
 
 class Object;
+class SlotBase;
 class Thread;
 
 class Message
@@ -17,6 +18,7 @@ class Message
 public:
 	enum Type {
 		None = 0,
+		SignalMessage = 1,
 	};
 
 	Message(Type type);
@@ -32,6 +34,18 @@ private:
 	Object *receiver_;
 };
 
+class SignalMessage : public Message
+{
+public:
+	SignalMessage(SlotBase *slot, void *pack)
+		: Message(Message::SignalMessage), slot_(slot), pack_(pack)
+	{
+	}
+
+	SlotBase *slot_;
+	void *pack_;
+};
+
 } /* namespace libcamera */
 
 #endif /* __LIBCAMERA_MESSAGE_H__ */