Player attrs: permits to remove an attribute by setting value to nil (#5716)

* Player attrs: permits to remove an attribute by setting value to nil

When doing player:set_attribute("attr", nil) remove attribute

Also remove a useless check on C++ API part (already done by checkplayer)

Fix #5709

doc/lua_api.txt

@@ -3028,7 +3028,9 @@ This is basically a reference to a C++ `ServerActiveObject`
         * `0`: player is drowning,
         * `1`-`10`: remaining number of bubbles
         * `11`: bubbles bar is not shown
-* `set_attribute(attribute, value)`: sets an extra attribute with value on player
+* `set_attribute(attribute, value)`:
+    * Sets an extra attribute with value on player.
+    * If value is nil, remove attribute from player.
 * `get_attribute(attribute)`: returns value for extra attribute. Returns nil if no attribute found.
 * `set_inventory_formspec(formspec)`
     * Redefine player's inventory form

src/content_sao.h

@@ -277,6 +277,16 @@ public:
 		return true;
 	}
 
+	inline void removeExtendedAttribute(const std::string &attr)
+	{
+		PlayerAttributes::iterator it = m_extra_attributes.find(attr);
+		if (it == m_extra_attributes.end())
+			return;
+
+		m_extra_attributes.erase(it);
+		m_extended_attributes_modified = true;
+	}
+
 	inline const PlayerAttributes &getExtendedAttributes()
 	{
 		return m_extra_attributes;

src/script/lua_api/l_object.cpp

@@ -1202,9 +1202,10 @@ int ObjectRef::l_set_attribute(lua_State *L)
 	}
 
 	std::string attr = luaL_checkstring(L, 2);
-	std::string value = luaL_checkstring(L, 3);
+	if (lua_isnil(L, 3)) {
-
+		co->removeExtendedAttribute(attr);
-	if (co->getType() == ACTIVEOBJECT_TYPE_PLAYER) {
+	} else {
+		std::string value = luaL_checkstring(L, 3);
 		co->setExtendedAttribute(attr, value);
 	}
 	return 1;