Implement unloading of static_save=false objects according to existing docs (#10485)

src/server/luaentity_sao.h

@@ -42,6 +42,7 @@ public:
 	void step(float dtime, bool send_recommended);
 	std::string getClientInitializationData(u16 protocol_version);
 	bool isStaticAllowed() const { return m_prop.static_save; }
+	bool shouldUnload() const { return true; }
 	void getStaticData(std::string *result) const;
 	u16 punch(v3f dir, const ToolCapabilities *toolcap = nullptr,
 			ServerActiveObject *puncher = nullptr,

src/server/player_sao.h

@@ -83,6 +83,7 @@ public:
 	void addedToEnvironment(u32 dtime_s);
 	void removingFromEnvironment();
 	bool isStaticAllowed() const { return false; }
+	bool shouldUnload() const { return false; }
 	std::string getClientInitializationData(u16 protocol_version);
 	void getStaticData(std::string *result) const;
 	void step(float dtime, bool send_recommended);

src/server/serveractiveobject.h

@@ -125,6 +125,7 @@ public:
 		assert(isStaticAllowed());
 		*result = "";
 	}
+
 	/*
 		Return false in here to never save and instead remove object
 		on unload. getStaticData() will not be called in that case.
@@ -132,6 +133,14 @@ public:
 	virtual bool isStaticAllowed() const
 	{return true;}
 
+	/*
+		Return false here to never unload the object.
+		isStaticAllowed && shouldUnload -> unload when out of active block range
+		!isStaticAllowed && shouldUnload -> unload when block is unloaded
+	*/
+	virtual bool shouldUnload() const
+	{ return true; }
+
 	// Returns tool wear
 	virtual u16 punch(v3f dir,
 			const ToolCapabilities *toolcap = nullptr,

src/serverenvironment.cpp

@@ -1972,8 +1972,8 @@ void ServerEnvironment::deactivateFarObjects(bool _force_delete)
 		// force_delete might be overriden per object
 		bool force_delete = _force_delete;
 
-		// Do not deactivate if static data creation not allowed
+		// Do not deactivate if disallowed
-		if (!force_delete && !obj->isStaticAllowed())
+		if (!force_delete && !obj->shouldUnload())
 			return false;
 
 		// removeRemovedObjects() is responsible for these
@@ -2002,7 +2002,10 @@ void ServerEnvironment::deactivateFarObjects(bool _force_delete)
 		}
 
 		// If block is still active, don't remove
-		if (!force_delete && m_active_blocks.contains(blockpos_o))
+		bool still_active = obj->isStaticAllowed() ?
+			m_active_blocks.contains(blockpos_o) :
+			getMap().getBlockNoCreateNoEx(blockpos_o) != nullptr;
+		if (!force_delete && still_active)
 			return false;
 
 		verbosestream << "ServerEnvironment::deactivateFarObjects(): "