Add attr path lookup helper

[zmitchell]

This adds a small executable id2attrpath with the following interface

$ id2attrpath <SOURCE> <ID>

that returns a space-delimited attribute path corresponding to the AttrSets.id passed in. Future PRs will extend this to Packages.id.

This program is only intended to make testing easier and shouldn't be used elsewhere.

[mkenigs]

Unclear to me what we want our convention to be for this var:

> rg "export DBPATH"
tests/pathological.bats
17:  export DBPATH="$BATS_FILE_TMPDIR/test-cli.sqlite";

tests/cli.bats
17:  export DBPATH="$BATS_FILE_TMPDIR/test-cli.sqlite";

tests/nixpkgs-flox.bats
43:  export DBPATH="$BATS_FILE_TMPDIR/test-nixpkgs-flox.sqlite";

tests/sqlite3.bats
17:  export DBPATH="$BATS_FILE_TMPDIR/test.sqlite";

tests/id2attrpath.bats
17:  export DBPATH="$BATS_FILE_TMPDIR/test-cli.sqlite";

[aakropotkin]

Let's just use the basement test.sqlite for all tests for consistency. It's not a real problem to use arbitrary names though.

[mkenigs]

I was wondering if it would be better to use an STL one liner instead of writing this algorithm and realized this will segfault if id2attrpath is called with ID = 0

[aakropotkin]

You can use attrPath.back() to handle the concern @mkenigs raised. Good catch.

[aakropotkin]

Handle the attrPath.back() or use nix::concatStringsSep and you'll be ready to roll.

size_t is nice too.

Changing the test db names is optional, I don't have strong opinions either way, but feel free to do it in this PR.

[aakropotkin]

Let's just use the basement test.sqlite for all tests for consistency. It's not a real problem to use arbitrary names though.

[aakropotkin]

What you wrote here works perfectly. There's a shorthand helper that the nix headers provide if you want something a bit less verbose :

#include <nix/util.hh>
//...
  std::cout << nix::concatStringsSep( " ", attrPath ) << std::endl;

( feel free to use either, I just wanted to share a tip )

[aakropotkin]

@zmitchell since you're out for the day I'm going to merge this ( especially the test cases ) into other pkgdb get * subcommand ticket.

Thanks for helping out on this!