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>

include/libcamera/signal.h

@@ -8,6 +8,8 @@
 #define __LIBCAMERA_SIGNAL_H__
 
 #include <list>
+#include <tuple>
+#include <type_traits>
 #include <vector>
 
 #include <libcamera/object.h>
@@ -27,6 +29,9 @@ public:
 	void *obj() { return obj_; }
 	bool isObject() const { return isObject_; }
 
+	void activatePack(void *pack);
+	virtual void invokePack(void *pack) = 0;
+
 protected:
 	void *obj_;
 	bool isObject_;
@@ -35,24 +40,70 @@ protected:
 template<typename... Args>
 class SlotArgs : public SlotBase
 {
+private:
+#ifndef __DOXYGEN__
+	/*
+	 * This is a cheap partial implementation of std::integer_sequence<>
+	 * from C++14.
+	 */
+	template<int...>
+	struct sequence {
+	};
+
+	template<int N, int... S>
+	struct generator : generator<N-1, N-1, S...> {
+	};
+
+	template<int... S>
+	struct generator<0, S...> {
+		typedef sequence<S...> type;
+	};
+#endif
+
+	using PackType = std::tuple<typename std::remove_reference<Args>::type...>;
+
+	template<int... S>
+	void invokePack(void *pack, sequence<S...>)
+	{
+		PackType *args = static_cast<PackType *>(pack);
+		invoke(std::get<S>(*args)...);
+		delete args;
+	}
+
 public:
 	SlotArgs(void *obj, bool isObject)
 		: SlotBase(obj, isObject) {}
 
-	virtual void invoke(Args... args) = 0;
+	void invokePack(void *pack) override
+	{
+		invokePack(pack, typename generator<sizeof...(Args)>::type());
+	}
 
-protected:
+	virtual void activate(Args... args) = 0;
-	friend class Signal<Args...>;
+	virtual void invoke(Args... args) = 0;
 };
 
 template<typename T, typename... Args>
 class SlotMember : public SlotArgs<Args...>
 {
 public:
+	using PackType = std::tuple<typename std::remove_reference<Args>::type...>;
+
 	SlotMember(T *obj, bool isObject, void (T::*func)(Args...))
 		: SlotArgs<Args...>(obj, isObject), func_(func) {}
 
-	void invoke(Args... args) { (static_cast<T *>(this->obj_)->*func_)(args...); }
+	void activate(Args... args)
+	{
+		if (this->isObject_)
+			SlotBase::activatePack(new PackType{ args... });
+		else
+			(static_cast<T *>(this->obj_)->*func_)(args...);
+	}
+
+	void invoke(Args... args)
+	{
+		(static_cast<T *>(this->obj_)->*func_)(args...);
+	}
 
 private:
 	friend class Signal<Args...>;
@@ -66,7 +117,8 @@ public:
 	SlotStatic(void (*func)(Args...))
 		: SlotArgs<Args...>(nullptr, false), func_(func) {}
 
-	void invoke(Args... args) { (*func_)(args...); }
+	void activate(Args... args) { (*func_)(args...); }
+	void invoke(Args... args) {}
 
 private:
 	friend class Signal<Args...>;
@@ -186,9 +238,8 @@ public:
 		 * disconnect operation, invalidating the iterator.
 		 */
 		std::vector<SlotBase *> slots{ slots_.begin(), slots_.end() };
-		for (SlotBase *slot : slots) {
+		for (SlotBase *slot : slots)
-			static_cast<SlotArgs<Args...> *>(slot)->invoke(args...);
+			static_cast<SlotArgs<Args...> *>(slot)->activate(args...);
-		}
 	}
 };
 

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