bug#25059: gnutls: asynchronous spurious "fatal error"

[Andy Wingo <wingo@igalia.com>]

Using Emacs 25.1.1 and GnuTLS 3.5.4, consider this interaction:

    (url-retrieve "https://www.gnu.org/"
                  #'(lambda (status)
                      (message "success")))

If I evaluate this a few times in a row directly in my scratch buffer, I
get messages like this:

    Contacting host: www.gnu.org:443
    #<buffer  *http www.gnu.org:443*-960341>
    success
    Contacting host: www.gnu.org:443
    #<buffer  *http www.gnu.org:443*-228158>
    success
    Contacting host: www.gnu.org:443
    #<buffer  *http www.gnu.org:443*-76553>
    success
    Contacting host: www.gnu.org:443
    #<buffer  *http www.gnu.org:443*-944613>
    success
    Contacting host: www.gnu.org:443
    #<buffer  *http www.gnu.org:443*-171378>
    success

And then, after a couple seconds:

    gnutls.c: [0] (Emacs) fatal error: The TLS connection was non-properly terminated.

Indeed if I wait a couple seconds after a single fetch, I always get the
above message after some timeout of inactivity.  Here is a log with
gnutls-log-level set to 2:

    Contacting host: www.gnu.org:443
    gnutls.c: [1] (Emacs) allocating credentials
    gnutls.c: [2] (Emacs) allocating x509 credentials
    gnutls.c: [2] (Emacs) using default verification flags
    gnutls.c: [1] (Emacs) setting the trustfile:  /etc/ssl/certs/ca-certificates.crt
    gnutls.c: [1] (Emacs) setting the trustfile:  /etc/pki/tls/certs/ca-bundle.crt
    gnutls.c: [1] (Emacs) gnutls callbacks
    gnutls.c: [1] (Emacs) gnutls_init
    gnutls.c: [1] (Emacs) got non-default priority string: NORMAL
    gnutls.c: [1] (Emacs) setting the priority string
    gnutls.c: [audit] Note that the security level of the Diffie-Hellman key exchange has been lowered to 256 bits and this may allow decryption of the session data
    gnutls.c: [1] (Emacs) non-fatal error: Resource temporarily unavailable, try again. [3088 times]
    gnutls.c: [2] received curve SECP256R1
    gnutls.c: [1] (Emacs) non-fatal error: Resource temporarily unavailable, try again. [2816 times]
    #<buffer  *http www.gnu.org:443*-346477>
    gnutls.c: [1] (Emacs) non-fatal error: Resource temporarily unavailable, try again. [5 times]
    success

Then after a couple seconds:

    gnutls.c: [0] (Emacs) fatal error: The TLS connection was non-properly terminated.
    gnutls.c: [2] (Emacs) Deallocating x509 credentials

No idea what that non-fatal error appearing thousands of times is, I
suspect Emacs is polling on a non-blocking file descriptor or
something.  Anyway this error appears to not affect anything as it's
asynchronous and it is not handleable by anything, and just makes people
think they have problems :)  Can you make it go away?

Regards,

Andy