bug#34341: 26.1; url-retrieve-synchronously returns blank buffer

[Noam Postavsky]

Laurent Charignon <l.charignon@gmail.com> writes:

> #### Context
>
> The symptom of this issue first came up in the context of `ghub`, a package
> that offers interfacing with Github from emacs (issue:
> https://github.com/magit/ghub/issues/81).
>
> Multiple users of the `ghub` package started observing that
> synchronously retrieving content from the github API through ghub
> returned blank responses.

> (switch-to-buffer (url-retrieve-synchronously "
> https://api.github.com/users/charignon/repos";)
>                   (buffer-string))

> From bisecting, I know that the fix is in between 191a5568db (working) and
> 2a3bd6798e96708 (not working). It is hard to pinpoint further due to
> segfaults that were introduced then fixed in this range. My best guess is
> that "Fix unlikely races with GnuTLS, datagrams" (e87e6a24c4) resolved this.

I can confirm that cherrypicking this commit fixes the problem on my
Arch box which has gnutls 3.6.7 (doesn't occur at all on my Debian box
with gnutls 3.5.8).

Since the only recourse for users hitting is rebuilding --without-gnutls
or (advice-add 'gnutls-available-p :override #'ignore) which falls back
to the obsolete tls.el, I'd say this is important enough to backport to
emacs-26.  I'll do that in a couple of days if there are no objections.