Cache in a `~/.pants` directory, instead of `~/.nce`

[huonw]

scie-pants currently caches its setup (e.g. downloaded interpreters, venvs, etc.) into ~/.nce, which is different to where all the usual pants caches are: ~/.cache/pants (by default). This is confusing for users.

It'd be nice if we could put it all in one place, e.g. return to the PANTS_SETUP_CACHE="${PANTS_SETUP_CACHE:-${XDG_CACHE_HOME:-$HOME/.cache}/pants/setup}" logic that the old ./pants script used.

This should now be possible in some form with a-scie/lift#41, although there's some questions about migration.

[huonw]

With #284, scie-pants uses appropriate dep versions to support this: science >= 0.2.0 (https://github.com/a-scie/lift/releases/tag/v0.2.0) and jump >= 0.12 (https://github.com/a-scie/jump/releases/tag/v0.12.0).

[kaos]

[..] there's some questions about migration.

Can you elaborate on this?

When you upgrade scie-pants, the cache key will change, effectively abandoning any previous cache entries, right? So I'm not sure what needs migrating here..

[huonw]

Ah hm, let me dig that out of my memory... maybe it was: People may have CI configuration that caches the old directory that needs updating or else their builds may get notably slower (eg suggestions in https://www.pantsbuild.org/2.19/docs/using-pants/using-pants-in-ci#directories-to-cache).

I guess there's also the smaller factor of the nce caches turning into wasted disk space, and thus maybe we could be nice and help people clean up.

I agree we don't need to actually move things on disk, though.

[kaos]

People may have CI configuration that caches the old directory that needs updating or else their builds may get notably slower (eg suggestions in https://www.pantsbuild.org/2.19/docs/using-pants/using-pants-in-ci#directories-to-cache).

Right. So we'd need to update the docs there.

I guess there's also the smaller factor of the nce caches turning into wasted disk space, and thus maybe we could be nice and help people clean up.

I see this as an unrelated issue.
Relates to: pantsbuild/pants#11167

I agree we don't need to actually move things on disk, though.

Ack.

So it comes down to clear documentation/communication about the change so users can adapt as appropriate when upgrading.

[huonw]

For CI, I think a "perfect state" for the change would do more. In particular, someone who has already set up CI and had it chugging along for a while won't be rereading the docs to notice. (Although of course we need to update the docs for new users.)

So, we might want to consider a switchover period (eg opt in by setting an environment variable), warnings and/or other active help so people are alerted.

However, we might decide to do less, but let's do so explicitly rather than accidentally.

[kaos]

yea, I imagined part of the "/communication" part to be messages (warnings) from scie-pants itself at runtime.