If the contents of a named cached (append only cache) are dependent upon for another rule, is the only way to guarantee the cache is populated to have the rule set as

ProcessCacheScope.PER_SESSION

Rule A -> package install into named cache Rule B (depends on A) -> reads from named cache

I don't think even that is a guarantee. I can

rm -rf ~/.cache/pants/named_caches

while pantsd is running. What issue are you solving where regular caching isn't enough? Would immutable input digests be an option? If it has to be filled before rule-run it sounds more apt that way.

Seems pyenv is doing exactly this https://github.com/pantsbuild/pants/blob/main/src/python/pants/backend/python/providers/pyenv/rules.py#L272-L275 I believe “per session” is per invocation of the pants cli, which is “good enough” for me I have a problem very similar to the pyenv use case where I need to download dependencies for later use. I looked at using digests for this but unfortunately the tool is leaking full filesystem paths, so. I need the fully qualified path to be stable across sandboxes 😞

A potential hack around that is to use a run script which does something like

export MYPATH=$(realpath .my-cache)

etc. I do that for various reasons in several of my plugins.