ObjectRef: Add add_velocity() (#3208)

Allow changing the velocity of objects relatively to their current velocity

doc/lua_api.txt

@@ -4029,6 +4029,10 @@ This is basically a reference to a C++ `ServerActiveObject`
 ##### LuaEntitySAO-only (no-op for other objects)
 * `set_velocity(vel)`
     * `vel` is a vector, e.g. `{x=0.0, y=2.3, z=1.0}`
+* `add_velocity(vel)`
+    * `vel` is a vector, e.g. `{x=0.0, y=2.3, z=1.0}`
+    * In comparison to using get_velocity, adding the velocity and then using
+      set_velocity, add_velocity is supposed to avoid synchronization problems.
 * `get_velocity()`: returns the velocity, a vector
 * `set_acceleration(acc)`
     * `acc` is a vector

src/content_sao.h

@@ -121,6 +121,10 @@ public:
 	s16 getHP() const;
 	/* LuaEntitySAO-specific */
 	void setVelocity(v3f velocity);
+	void addVelocity(v3f velocity)
+	{
+		m_velocity += velocity;
+	}
 	v3f getVelocity();
 	void setAcceleration(v3f acceleration);
 	v3f getAcceleration();

src/script/lua_api/l_object.cpp

@@ -850,6 +850,20 @@ int ObjectRef::l_set_velocity(lua_State *L)
 	return 0;
 }
 
+// add_velocity(self, {x=num, y=num, z=num})
+int ObjectRef::l_add_velocity(lua_State *L)
+{
+	NO_MAP_LOCK_REQUIRED;
+	ObjectRef *ref = checkobject(L, 1);
+	LuaEntitySAO *co = getluaobject(ref);
+	if (!co)
+		return 0;
+	v3f pos = checkFloatPos(L, 2);
+	// Do it
+	co->addVelocity(pos);
+	return 0;
+}
+
 // get_velocity(self)
 int ObjectRef::l_get_velocity(lua_State *L)
 {
@@ -1840,6 +1854,7 @@ const luaL_Reg ObjectRef::methods[] = {
 	luamethod(ObjectRef, get_nametag_attributes),
 	// LuaEntitySAO-only
 	luamethod_aliased(ObjectRef, set_velocity, setvelocity),
+	luamethod(ObjectRef, add_velocity),
 	luamethod_aliased(ObjectRef, get_velocity, getvelocity),
 	luamethod_aliased(ObjectRef, set_acceleration, setacceleration),
 	luamethod_aliased(ObjectRef, get_acceleration, getacceleration),

src/script/lua_api/l_object.h

@@ -161,6 +161,9 @@ private:
 	// set_velocity(self, {x=num, y=num, z=num})
 	static int l_set_velocity(lua_State *L);
 
+	// add_velocity(self, {x=num, y=num, z=num})
+	static int l_add_velocity(lua_State *L);
+
 	// get_velocity(self)
 	static int l_get_velocity(lua_State *L);