pkg/docker/config: correct defaultPerUIDPathFormat

[nalind]

If I'm reading containers/podman#4227 right, we should make defaultPerUIDPathFormat correspond to the value that we'd compute by combining the default value of ${XDG_RUNTIME_DIR} (typically /run/user/$(id -u)) and xdgRuntimeDirPath.

[mtrmac]

Thanks. The non-XDG_RUNTIME_DIR default was, IIRC, intentionally different, e.g. #424 says:

Looking at pam_systemd(8), it removes /run/user when “the last concurrent session of a user ends”; so, if we created the directory without systemd knowing about the session, it can delete the directory at an unpredictable time when some other session terminates.

so it may not be all quite as simple (but I haven’t reread all of the previous issues/PRs/bugs filed).

One clear problem with the current code is that it does not work at all for non-root users who are not allowed to create subdirectories in /run/containers (OTOH that’s also the case for /run/user/$UID), so I’m not at all saying that the current code is right, just that the intent of this code path was not to match $XDG_RUNTIME_DIR.

[nalind]

One clear problem with the current code is that it does not work at all for non-root users who are not allowed to create subdirectories in /run/containers (OTOH that’s also the case for /run/user/$UID), so I’m not at all saying that the current code is right, just that the intent of this code path was not to match $XDG_RUNTIME_DIR.

Yeah, the "not being able to create /run/containers or subdirectories under it" was what had me thinking this was an error. If it's intentional, though, should I just close this one?

[mtrmac]

See also the conversation in containers/podman#4232 , not sure how that will turn out.

[mtrmac]

The original motivation for this PR, containers/podman#4227 , was resolved by containers/podman#4337 , having Podman mostly defer to this code for the defaults.

That does not mean that these non-$XDG_RUNTIME_DIR defaults work all that well (they really don’t), OTOH I don’t think we have any good choices.

The longer-term plan is to have (system-wide and per-user) registries.conf set up a set of credential helpers, and to use such a helper instead of a flat file for the credentials. That should make the common/ideal case better.

OTOH that does nothing for users who choose to use files, or have to use files because nothing else is available, and do not have $XDG_RUNTIME_DIR; still, I don’t think we have good choices.