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__ */

src/libcamera/message.cpp

@@ -68,4 +68,26 @@ Message::~Message()
  * \return The message receiver
  */
 
+/**
+ * \class SignalMessage
+ * \brief A message carrying a Signal across threads
+ */
+
+/**
+ * \fn SignalMessage::SignalMessage()
+ * \brief Construct a SignalMessage
+ * \param[in] slot The slot that the signal targets
+ * \param[in] pack The signal arguments
+ */
+
+/**
+ * \var SignalMessage::slot_
+ * \brief The slot that the signal targets
+ */
+
+/**
+ * \var SignalMessage::pack_
+ * \brief The signal arguments
+ */
+
 }; /* namespace libcamera */

src/libcamera/object.cpp

@@ -10,6 +10,7 @@
 #include <libcamera/signal.h>
 
 #include "log.h"
+#include "message.h"
 #include "thread.h"
 
 /**
@@ -32,6 +33,10 @@ namespace libcamera {
  * This allows implementing easy message passing between threads by inheriting
  * from the Object class.
  *
+ * 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
+ * in another thread.
+ *
  * \sa Message, Signal, Thread
  */
 
@@ -82,6 +87,16 @@ void Object::postMessage(std::unique_ptr<Message> msg)
  */
 void Object::message(Message *msg)
 {
+	switch (msg->type()) {
+	case Message::SignalMessage: {
+		SignalMessage *smsg = static_cast<SignalMessage *>(msg);
+		smsg->slot_->invokePack(smsg->pack_);
+		break;
+	}
+
+	default:
+		break;
+	}
 }
 
 /**

src/libcamera/signal.cpp

@@ -7,6 +7,10 @@
 
 #include <libcamera/signal.h>
 
+#include "message.h"
+#include "thread.h"
+#include "utils.h"
+
 /**
  * \file signal.h
  * \brief Signal & slot implementation
@@ -42,8 +46,30 @@ namespace libcamera {
  * to the same slot. Duplicate connections between a signal and a slot are
  * allowed and result in the slot being called multiple times for the same
  * signal emission.
+ *
+ * When a slot belongs to an instance of the Object class, the slot is called
+ * in the context of the thread that the object is bound to. If the signal is
+ * emitted from the same thread, the slot will be called synchronously, before
+ * Signal::emit() returns. If the signal is emitted from a different thread,
+ * the slot will be called asynchronously from the object's thread's event
+ * loop, after the Signal::emit() method returns, with a copy of the signal's
+ * arguments. The emitter shall thus ensure that any pointer or reference
+ * passed through the signal will remain valid after the signal is emitted.
  */
 
+void SlotBase::activatePack(void *pack)
+{
+	Object *obj = static_cast<Object *>(obj_);
+
+	if (Thread::current() == obj->thread()) {
+		invokePack(pack);
+	} else {
+		std::unique_ptr<Message> msg =
+			utils::make_unique<SignalMessage>(this, pack);
+		obj->postMessage(std::move(msg));
+	}
+}
+
 /**
  * \fn Signal::connect(T *object, void(T::*func)(Args...))
  * \brief Connect the signal to a member function slot