Pants: Add a goal to introspect and garbage collect caches.

[stuhood]

This might look like:

• Adding a goal that reports the disk usage in all cache locations (the LMDB store and named caches in particular)
• Exposing manually running GC for the cache locations (in alignment with the GC that pantsd does) and/or completely clearing them.
  
  
  
  • This could also expose a smart "explicit" GC as described in https://github.com/pantsbuild/pants/issues/11167#issuecomment-776143353
  
  

One possibility would be that this would be one goal with an optional preview mode: ./pants gc.

[stuhood]

Managing pantsd status is a bit abstract to me: maybe someone else has a clearer idea of what that means.

But the "how much disk space is pants using" point is a bit clearer. I imagine it looking like:

• Adding a goal that reports the disk usage in all cache locations (the LMDB store and named caches in particular)
• Expose manually running GC for the cache locations (in alignment with the GC that pantsd does) and/or completely clearing them.

[stuhood]

I've added some information about clearing caches to https://www.pantsbuild.org/v2.3/docs/troubleshooting#cache-or-pantsd-invalidation-issues.

[illicitonion]

Following on from https://pantsbuild.slack.com/archives/C0105PY6BM5/p1612892422002100

It could be handy to have some goal (or standalone utility, or flag, or something), to perform a garbage collection of the lmdb store, but rather than being time-based, using local action cache entries as GC roots, and only keeping digests referenced by these roots. Allows us to prune "we made a copy of every input file" while preserving "we can avoid doing expensive work". Ideally we could order the action cache entries by time, too, so that there's some LRU cut-off.

[stuhood]

Great idea.

I'm going to go ahead and drop the pantsd aspect from this ticket, because we haven't gotten any feedback elucidating what that might look like.