bug#31603: 27.0.50; pinentry loopback disappeared

[adrian.lanz@wsl.ch]

With 'allow-loopback-pinentry' in gpg-agent.conf and (setq
epa-pinentry-mode 'loopback) in .emacs, I enjoy reliable pass-phrase
prompting and entering in the Emacs minibuffer for quite some time (at
least since September 2016).

Now unexpectedly, pinentry-curses started to pop-up for pass-phrase
prompting. I suspect commit 302e500087fd4cc1c5f37ec87c98e828b22aaa05
being the cause. I did not find a way to get pinentry-loopback back and
working.


Remark 1: The default graphical mode pinentry (X11 or similar) is
available as expected.

Remark 2: Text mode pinentry-curses is very unreliable (under my
configuration). Several attempts are often needed to get the pass-phrase
accepted. When pinentry-curses is automatically called to find-file a
*.gpg file, mangled parts of the pass-phrase are inserted as clear text
on top of the Emacs buffer into which the file is loaded. Mangled
pass-phrase parts may also otherwise passed to Emacs (seems as key
strokes with unpredictable effects).

[This bug report is, however, not intended towards pinentry-curses, but
pinentry-loopback. On the other hand, pinentry-curses seems the
fall-back of pinentry-loopback (and graphical mode pinentry).]

Use case: I access the system (at work) also on a daily basis from
outside - through VPN/SSH and in terminal/text mode, so that a
functioning text mode pinentry is a must (e.g. encrypted .authinfo.gpg
for Emacs/Gnus).

Workaround: I detected https://wiki.gnupg.org/AgentForwarding as an
intermediate solution - graphical pinentry, and external access with a
linux laptop, GnuPG configured.



Ubuntu 16.04.4 LTS (Stable)

GNU Emacs 27.0.50 (Snapshot)
[http://ppa.launchpad.net/ubuntu-elisp/ppa/ubuntu xenial main]