This function finishes the surface and drops all references to external resources. For example, for the Xlib backend it means that cairo will no longer access the drawable, which can be freed. After calling Surface.finish the only valid operations on a surface are getting and setting user, referencing and destroying, and flushing and finishing it. Further drawing to the surface will not affect the surface but will instead trigger a CAIRO_STATUS_SURFACE_FINISHED error. When the last call to Surface.destroy decreases the reference count to zero, cairo will call Surface.finish if it hasn't been called already, before freeing the resources associated with the surface. Since 1.0

class Surface