Allow managing object observers

-----

Co-authored-by: sfan5 <sfan5@live.de>
Co-authored-by: SmallJoker <SmallJoker@users.noreply.github.com>

src/server/serveractiveobject.h

@@ -21,6 +21,7 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 
 #include <cassert>
 #include <unordered_set>
+#include <optional>
 #include "irrlichttypes_bloated.h"
 #include "activeobject.h"
 #include "itemgroup.h"
@@ -236,7 +237,25 @@ public:
 	*/
 	v3s16 m_static_block = v3s16(1337,1337,1337);
 
+	// Names of players to whom the object is to be sent, not considering parents.
+	using Observers = std::optional<std::unordered_set<std::string>>;
+	Observers m_observers;
+
+	/// Invalidate final observer cache. This needs to be done whenever
+	/// the observers of this object or any of its ancestors may have changed.
+	void invalidateEffectiveObservers();
+	/// Cache `m_effective_observers` with the names of all observers,
+	/// also indirect observers (object attachment chain).
+	const Observers &getEffectiveObservers();
+	/// Force a recalculation of final observers (including all parents).
+	const Observers &recalculateEffectiveObservers();
+	/// Whether the object is sent to `player_name`
+	bool isEffectivelyObservedBy(const std::string &player_name);
+
 protected:
+	// Cached intersection of m_observers of this object and all its parents.
+	std::optional<Observers> m_effective_observers;
+
 	virtual void onMarkedForDeactivation() {}
 	virtual void onMarkedForRemoval() {}