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/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