Ability to surgically remove some packages from Nix global store

[ezyang]

Right now, we don't have GC (#3333) so the only way to clean up the store is to just delete it wholesale.

Once, I tried deleting a specific package by unregistering it from the database. But I guess someone else was depending on it because cabal new-build choked to death and I couldn't recover without deleting the store.

So it would seem like it would be helpful and orthogonal to GC to allow users to delete specific packages from the store, recursively removing anything that depended on them. (Maybe not that useful? I don't know.)

[lspitzner]

It should be easy to add this functionality to pkgdbgc. I have put it on my to-do.

@ezyang shouldn't ghc-pkg have warned you when you unregistered the package?

[phadej]

https://nixos.org/nixos/nix-pills/garbage-collector.html

is ingenious approach. Then

• each dist-newstyle could contain indirect gc-roots
• installdir could act as indirect gc-roots repository

Then GC roots can be discovered easily. And there would be general GC root mechanism to add more if needed.