bug#28780: 25.3; Bad signature from GNU ELPA whe coding-system-for-read is utf-8

[Eli Zaretskii <eliz@gnu.org>]

> From: Iakov Davydov <dav02.davs@myths.ru>
> Date: Tue, 10 Oct 2017 16:09:57 +0000
> 
> When installing a package (e.g. auctex), an error message appears:
> Failed to verify signature auctex-11.91.0.tar.sig:
> Bad signature from 474F05837FBDEF9B GNU ELPA Signing Agent <elpasign@elpa.gnu.org>
> Command output:
> gpg: [don't know]: indeterminate length for invalid packet type 14
> gpg: Signature made Tue 25 Jul 2017 11:10:02 PM CEST using DSA key ID 7FBDEF9B
> gpg: BAD signature from "GNU ELPA Signing Agent <elpasign@elpa.gnu.org>" [unknown]
> 
> This only happens if the following line in my .emacs:
> (setq-default coding-system-for-read 'utf-8)
> 
> Steps to reproduce:
> 
> 1. Run emacs -Q
> 2. Execute `(setq-default coding-system-for-read 'utf-8)` (e.g. go to
> scratch, type this command, type C-x C-e).
> 3. M-x package-install auctex
> 4. Error message appears.
> 
> If step #2 is skipped, installation happens without an error.

Why do you need to do step 2?  You shouldn't do that, especially when
using commands that read binary files.  coding-system-for-read is for
temporarily forcing specific decoding of specific text, and should
otherwise be left alone.  And you definitely shouldn't change its
default value.

IOW, this sounds like a clear cockpit error.  Am I missing something?

> In fact, this or similar problem seems to be quite common, but often
> cannot be reproduced.
> 1. emacs bug#22971
> 2. https://emacs.stackexchange.com/questions/10046
> 3. https://github.com/jacksonrayhamilton/tern-context-coloring/issues/4
> 4. https://github.com/ananthakumaran/tide/issues/51
> 5. https://github.com/syl20bnr/spacemacs/issues/3669
> 6. https://www.reddit.com/r/emacs/comments/6t21lb/
> 7. https://www.linux.org.ru/forum/security/13718006 (In Russian)

I see no clear evidence that these reports are due to setting
coding-system-for-read.