bug#72319: ELPA archive download fails with signature error during rebuild

[Daniel Mendler]

The ELPA archive download fails during package archive rebuilding. The
rebuilding process on the ELPA server happens twice every day (e.g., in
the morning at 09:00 UTC or 11:00 local time in my time zone). As a
package maintainer I receive mails that new package releases have been
created at that time.

When I execute M-x package-upgrade-all shortly afterwards, approximately
in the time frame from 11:00 to 11:10, I observe the following failures:

Failed to download ‘gnu’ archive.
Failed to download ‘nongnu’ archive.

Failed to verify signature archive-contents.sig:
Bad signature from 645357D2883A0966 GNU ELPA Signing Agent (2023) 
<elpasign@elpa.gnu.org>
Command output:
gpg: Signature made Fri 26 Jul 2024 11:10:06 AM CEST
gpg:                using EDDSA key 0327BE68D64D9A1A66859F15645357D2883A0966
gpg: BAD signature from "GNU ELPA Signing Agent (2023) <elpasign@elpa.gnu.org>" 
[unknown]

I've already discussed with Stefan about this problem. The issue seems
to be that the archive-contents file is updated during the archive building
process, while the signature is only updated in the end. Ideally the
update of the archive-contents file should happen a little more
atomically: First build a new archive-contents file, sign it, and then
replace the old files as the final step.