Don't remove furnace if something is inside it.

src/content_nodemeta.cpp

@@ -195,6 +195,24 @@ std::string FurnaceNodeMetadata::infoText()
 		return s;
 	}
 }
+bool FurnaceNodeMetadata::nodeRemovalDisabled()
+{
+	/*
+		Disable removal if furnace is not empty
+	*/
+	InventoryList *list[3] = {m_inventory->getList("src"),
+	m_inventory->getList("dst"), m_inventory->getList("fuel")};
+	
+	for(int i = 0; i < 3; i++) {
+		if(list[i] == NULL)
+			continue;
+		if(list[i]->getUsedSlots() == 0)
+			continue;
+		return true;
+	}
+	return false;
+	
+}
 void FurnaceNodeMetadata::inventoryModified()
 {
 	dstream<<"Furnace inventory modification callback"<<std::endl;

src/content_nodemeta.h

@@ -76,6 +76,7 @@ class FurnaceNodeMetadata : public NodeMetadata
 	virtual Inventory* getInventory() {return m_inventory;}
 	virtual void inventoryModified();
 	virtual bool step(float dtime);
+	virtual bool nodeRemovalDisabled();
 	virtual std::string getInventoryDrawSpecString();
 
 private: