[BUG] V6.1.0 and V6.1.1 don't seem to work with V6.0.0 caches

[thenonameguy]

Describe the bug

Our GHA jobs started failing today, after the new minor and patch releases.
It seems internally the Nix JSON output gets broken with the old caches used:

 error: [json.exception.parse_error.101] parse error at line 1, column 1: syntax error while parsing value - invalid literal; last read: 'D'
✖ Building shell in 4.94s
Error:   × Command `/nix/store/b3xyrhiwbdch46092bsvsnhy67119vsy-nix-2.24-devenv/
  │ bin/nix --show-trace --extra-experimental-features nix-command --extra-
  │ experimental-features flakes --option warn-dirty false --keep-going --max-
  │ jobs 2 --option eval-cache false --option extra-substituters https://
  │ devenv.cachix.org --option extra-trusted-public-keys devenv.cachix.org-
  │ 1:w1cLUi8dv3hnoSPGAuibQv+f9TZLr6cv/Hm9XgU50cw= print-dev-env --profile /
  │ home/runner/work/$REDACTED/.devenv/gc/shell -vv --log-format internal-
  │ json` failed with with exit code 1

Disabling the caching action or downgrading it with an explicit v6.0.0 version solved this problem.
It seems there might be a log message starting with D, breaking the JSON parsing of devenv.

To Reproduce

Sorry, I only have a private repo.

Expected behavior

Minor and patch version bumps don't break caches restored from former versions in any way.

Additional context

[deemp]

@thenonameguy

I'm getting this error with caches created by the latest versions.

https://github.com/nix-community/nix-vscode-extensions/actions/runs/13786299760/job/38555690114#step:6:10

The job passes if it doesn't restore.

https://github.com/nix-community/nix-vscode-extensions/actions/runs/13786047382/job/38554685144

If you have a chance, please try debugging with action-tmate.

[deemp]

@thenonameguy it'd be also nice if you could reproduce the problem with more verbosity.

[deemp]

I can reliably reproduce the issue by running the failing job again.
https://github.com/nix-community/nix-vscode-extensions/actions/runs/13792030765/job/38588057479

Everything is OK if I run nix store gc before nix develop template/#.

[deemp]

In my case, the problem is with codium --list-extensions. The devShell without that command runs OK.

UPD: not always

[deemp]

Everything is OK if I run nix store gc before nix develop template/#.

When building template#devShells.x86_64-linux.default, I get a derivation as an output

/nix/store/plxips6qsndkhis09jfklzy77f2n0zb1-nix-shell.drv

After GC and building, I get a normal path.

/nix/store/w198z2pigg9p26g74c2p8bbdygkrrhfz-nix-shell

Something very wrong happens during saving/restoring a cache.

[deemp]

@thenonameguy

v6.1.0 introduced changes to the SQL script that merges databases of two stores (existing and restored).

If there's a problem, it should be in that script https://github.com/nix-community/cache-nix-action/blob/aee88ae5efbbeb38ac5d9862ecbebdb404a19e69/src/templates/merge.sql

The script can be used with local stores created via nix build --store tmp/store1 nixpkgs#hello.

[deemp]

@thenonameguy should be fixed in v6.1.2.