Derek Zhou writes:

> When this thing happens, the tls handshakes are done properly. However,
> emacs did not write anything into gnutls before starting to read and
> obviously cannot get anything out at all. It is not really a hang, but
> write never happen and the display buffer stays empty.
>
> Derek

Took my nearly the whole day to debug, but this one-line patch fixed my
problem.
My server finishes tls handshake within the gnutls_boot itself, and if the
sentinel is not called right after, it will never be called so write
will not happen. Someone should review this carefully.