Someone blogged that the following happens with gpg version 2.2.27-3ubuntu2.1:
$ gpgv </dev/null gpgv: can't allocate lock for '/home/XXX/.gnupg/trustedkeys.gpg' gpgv: verify signatures failed: Unknown system error
For details see https://www.osso.nl/blog/2023/gpgv-can-t-allocate-lock-for/
I haven't cited more text from the blog because it's copyrighted and not licensed under a permissive license.
The same happens with a very recent 2.4:
$ gpgv --version gpgv (GnuPG) 2.4.4-beta56 libgcrypt 1.11.0 Copyright (C) 2023 g10 Code GmbH License GNU GPL-3.0-or-later <https://gnu.org/licenses/gpl.html> This is free software: you are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by law. $ cp /home/XXX/.gnupg/pubring.kbx /home/XXX/.gnupg/trustedkeys.kbx $ gpgv </dev/null gpgv: can't allocate lock for '/home/XXX/.gnupg/trustedkeys.kbx' gpgv: verify signatures failed: Unknown system error
Problem seems to be that there is no ~/trustedkeys.gpg file and that the fallback to the kbx file does not anymore work. I can replicate that with 2.40 and 2.4.4-beta.
I found that the warning is emitted when it tries to call keybox_compress.
It should not be called when it's READONLY (which gpgv specifies).