Summary:

Would it be possible to create the 'gen' subdirectory in '/var/cache/dnf/updates-<hex-string>/repodata' up front as soon as that directory is created, and to avoid removing it while that cache is still in use?

Details:

The Mozilla Firefox build scripts have some code to "bootstrap" your local machine to install build prerequisites for devs. This code should be run regularly to ensure dependencies are up to date or else weird build failures can occur. To avoid being annoying, the script tries to avoid prompting the user for their password to `sudo dnf install` where possible by checking whether the prerequisites are all present:

https://searchfox.org/mozilla-central/rev/eea0fac2cb8cae1e5a2e38959eeaa0b7f6d23456/python/mozboot/mozboot/base.py#466,473,485-486

It does that using `dnf --cacheonly group list --installed' (without using sudo, obviously, or that would defeat the purpose of the check!)

Unfortunately, this can sometimes fail:

```
$ dnf --cacheonly group list --installed
Last metadata expiration check: 0:42:44 ago on Wed 08 Dec 2021 09:00:58 GMT.
[Errno 13] Permission denied: '/var/cache/dnf/updates-7eea87b22825bc0d/repodata/gen'
$ sudo ls -al /var/cache/dnf/updates-7eea87b22825bc0d/repodata/
total 15200
drwxr-xr-x. 1 root root      868 Dec  8 09:00 .
drwxr-xr-x. 1 root root       56 Dec  8 09:00 ..
-rw-r--r--. 1 root root    16908 Dec  8 09:00 <a-bunch-of-these>.xml.zck
-rw-r--r--. 1 root root     8306 Dec  8 09:00 repomd.xml
```

Retrying with sudo, it succeeds and the 'gen' subdirectory is created:

```
$ sudo dnf --cacheonly group list --installed
Last metadata expiration check: 0:43:02 ago on Wed 08 Dec 2021 09:00:58 GMT.
Installed Groups:
   <a-bunch-of-groups-listed>
$ sudo ls -al /var/cache/dnf/updates-7eea87b22825bc0d/repodata/
total 15200
drwxr-xr-x. 1 root root      874 Dec  8 09:44 .
drwxr-xr-x. 1 root root       56 Dec  8 09:00 ..
-rw-r--r--. 1 root root    16908 Dec  8 09:00 <a-bunch-of-these>.xml.zck
drwxr-xr-x. 1 root root        0 Dec  8 09:44 gen
-rw-r--r--. 1 root root     8306 Dec  8 09:00 repomd.xml
```

Now if I rerun `dnf --cacheonly group list --installed` (without sudo) it too succeeds.

So it looks like the 'gen' subdirectory being missing breaks `dnf --cacheonly` (when run without sudo) because the permissions on the parent directory don't allow regular users to create it. If it already exists, everything works.