Trying to debug pants cache in docker. Was wondering if I can get some help? 🙏 We are trying to build our pants jar inside of a docker instance running on ubuntu. In order to speed up the build, I was hoping to persist the pants cache for future builds. It seems like the right way to do that is this:

ENV XDG_CACHE_HOME /root/.cache

(src)

RUN --mount=type=cache,target=/root/.cache/ ./pants binary some/jar/to/build:

But all subsequent docker runs dont use any cached data, and the build starts from scratch. Are there any examples of pants + docker + caching that are known to work?

Hey @cuddly-flag-68085, I'm wondering if playing with where the cache directory is might make a difference: https://www.pantsbuild.org/docs/troubleshooting#how-to-change-your-cache-directory

I tried that but no luck. It seems to be writing the cache data to the right place. I wonder is it possible the cache key is not deterministic in this environment. Do you happen to know where the pants cache key construction code is?

Hm, I'd be surprised if the environment is causing the cache key to not be stable - you're using the same Docker image each time I take it The cache keys are computed automatically through the Rules API - tl;dr, Pants splits up the steps of your builds into small parts, which are modeled by "rules". Rules are pure functions that take certain input - if the inputs are the same, the cache can be used. (The engine handles impure things like reading from the file system). See https://www.pantsbuild.org/docs/rules-api-tips#fyi-caching-semantics for more info The only thing I could imagine messing up the cache is if the OS is changing because some processes depend on the platform. But I doubt you're doing that One other thing to check is to compare the output of

env

in your terminal before and after, in case certain env vars are changing? (Altho Pants discards most env vars for hermiticity)

Which version of Pants are you on?

We're using 1.26.0.