mf::WlShmBuffer::on_buffer_destroyed(wl_listener *listener, void *) is racy

[AlanGriffiths]

        std::lock_guard <std::mutex> lock{*shim->mutex};
        if (auto mir_buffer = shim->associated_buffer.lock()) {
            mir_buffer->buffer = nullptr;
        }

This can lead to a deadlock. If at the end of the "if" scope mir_buffer is the last remaining owner of the buffer, then the destructor will be called. The destructor also tries to lock the mutex: crunch!