Fix merging stores

[deemp]

We use single-user Nix installation provided by the nix-quick-install-action.

Scenario:

• There's a store with /nix/var/nix/db.
• Assume restore-prefixes-all-matches restores a store A, then store B.
  • These stores also have /nix/var/nix/db.
• Assume B is the last store to be restored.
• Then, the final store will have /nix/var/nix/db from B.

However, ideally, restore-prefixes-all-matches should really merge stores and databases, not just overwrite databases.

It's currently unclear how to merge stores.

Some links that may be helpful

• Rebuild sqlite db from scratch? NixOS/nix#3091
• https://discourse.nixos.org/t/super-colliding-nix-stores/28462
• https://discourse.nixos.org/t/help-nix-store-sqlite-db-documentation/37729

Reproduce a similar scenario

To get two stores, run

nix build --no-link nixpkgs#hello --store store1
nix build --no-link nixpkgs#gawk --store store2

Then, the databases are in

store1/nix/var/nix/db/db.sqlite
store2/nix/var/nix/db/db.sqlite

I can copy paths from one store to another via cp.

Questions

• How to merge the databases?
• Where to merge?
  • In the main near restoreCache
  • Determine whether a cache contains a /nix/var/nix/db/db.sqlite

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/merge-two-nix-stores/41256/1

[deemp]

Should be resolved now. Here's the sql script.

We skip all existing /nix/store paths and all stuff in /nix/var except /nix/var/nix/db/db.sqlite (link).