Signing emails with mu4e (was Re: Hello, new committer here!)

[Pierre Langlois <pierre.langlois@gmx.com>]

Pierre Langlois writes:

> Hello Guix,
>
> So, as indicated in our contributing process on the manual, I'm happy to
> announce maintainers have agreed to give me commit access, thank you
> everybody!
>
> I believe I need somebody to add my GPG keys to the Guix keyring before
> I can push my first commit. Which I think is going to be
> https://issues.guix.gnu.org/43138 (although, if this one is urgent,
> don't hesitate to push it for me!).
>
> If I'm not mistaken, since I'm using a subkey as a signing key, it
> should be this way (that's what I've had to do sign a personal channel):
>
>     (;; primary: "41CA 12EA DE0C F33F 6885  A58F 5719 6E37 E00B 77FD"
>      "72D5 3D81 8CB6 F4A1 7258  374C A8FC 9E44 7F4F 7D54"
>      (name "planglois")

Huh, it looks like the email signature got messed up. I'm using mu4e but
it wasn't finding my key as it used to. I rolled-back to emacs 26 and
that seemed to have worked, except the email sent wasn't signed
properly.

So, with this email, if I do `M-x mml-secure-message-sign-pgpmime' and
try and send, I get a backtrace:

```
Debugger entered--Lisp error: (error "Couldn’t find any signer names; try setting `mml-s...")
  signal(error ("Couldn’t find any signer names; try setting `mml-s..."))
  error("Couldn't find any signer names%s" "; try setting `mml-secure-smime-sign-with-sender'.")
  mml-secure-epg-sign(OpenPGP t)
  mml2015-epg-sign((part (sign . "pgpmime") (tag-location . 477) (contents . "Pierre Langlois writes:\n\n> Hello Guix,\n>\n> So, as ...")))
  mml2015-sign((part (sign . "pgpmime") (tag-location . 477) (contents . "Pierre Langlois writes:\n\n> Hello Guix,\n>\n> So, as ...")))
  mml-pgpmime-sign-buffer((part (sign . "pgpmime") (tag-location . 477) (contents . "Pierre Langlois writes:\n\n> Hello Guix,\n>\n> So, as ...")))
  mml-generate-mime-1((part (sign . "pgpmime") (tag-location . 477) (contents . "Pierre Langlois writes:\n\n> Hello Guix,\n>\n> So, as ...")))
  mml-generate-mime(nil nil)
  message-encode-message-body()
  message-send-mail(nil)
  message-send-via-mail(nil)
  message-send(nil)
  message-send-and-exit(nil)
```

I've tried setting `mml-secure-smime-sign-with-sender' to `t' that's not
helping.  I'm not sure what's going on, are there any mu4e users here
that can sign emails? I wonder if it's do to with the recent gnupg
update instead of emacs.

Thanks,
Pierre