RFC: Store garbage collection

[andreabedini]

This is a draft design for store garbage collection that I put together during ZuriHac. I admit I haven't explored existing designs.

The recipe is simple and similar to what nix does.

• We keep updated a list of unit-ids that the project plan references just like we do with plan.json
• At the same time we create a backlink from storedir/gc-root to the project dist directory.

This should allow garbage collection later on:

• we check the gc-root directory for old roots and removes them (by checking whether the symlink resolves).
• we traverse the dependency graph and do the proper garbage collection.

• Patches conform to the coding conventions.
• Any changes that could be relevant to users have been recorded in the changelog.
• The documentation has been updated, if necessary.
• Manual QA notes have been included.
• Tests have been added. (Ask for help if you don’t know how to write them! Ask for an exemption if tests are too complex for too little coverage!)

[andreabedini]

Related:

• [nix-local-build] Garbage collecting the store #3333
• https://github.com/phadej/cabal-extras/tree/master/cabal-store-gc

[fgaz]

I think we also need roots for installed executables