The interface for pollable output streams.
The default implementation of can_poll always returns TRUE.
The default implementation of write_nonblocking calls g_pollable_output_stream_is_writable(), and then calls OutputStream.write if it returns TRUE. This means you only need to override it if it is possible that your is_writable implementation may return TRUE when the stream is not actually writable.
The default implementation of writev_nonblocking calls g_pollable_output_stream_write_nonblocking() for each vector, and converts its return value and error (if set) to a GPollableReturn You should override this where possible to avoid having to allocate a GError to return G_IO_ERROR_WOULD_BLOCK.
The parent interface.