test: for derivation output selection with .

[dottharun]

Motivation

added test and fixes #7236

As discussed in #7236, after #6589 the command nix build nixpkgs#libxml2.dev correctly builds the dev output

• and this pr adds test for this behavior

Also as pointed out by @Ericson2314 Here

• Respect the outputSpecified attribute #6589 uses outputSpecified attribute in derivations (Nixpkgs convention) to enable this behavior

Context

• The test uses nix build command with --json and --no-link flags
• which are then passed to jq with match, keys filters for checking the output names and derivations
• outputSpecified attribute in Nixpkgs Here
• meta.outputsToInstall attribute is used in the test because originally with nix: Respect meta.outputsToInstall, and use all outputs by default #6426 outputsToInstall is given more priority that command-line argument, thats why we are testing that behaviour too
• this pr marks my first contribution to the nix code-base except documentation capturing all my learnings

Priorities and Process

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[roberth]

IIUC this is also reflected in the output symlinks.

[edolstra]

flakes.sh is getting very big again, could you put this test in a separate file?

[Ericson2314]

Yeah our existing functional tests are rather unsystematic, we need to start doing something about that so we have a better idea of test coverage.

[roberth]

flakes.sh is getting very big again, could you put this test in a separate file?

Figuring out a better categorization for test cases seems out of scope for this PR.

[Ericson2314]

tests/functional/build.sh looks better even among existing files though

[dottharun]

tests/functional/build.sh looks better even among existing files though

this is a flake only behavior though

• Can I do it here tests/functional/flakes/build-paths.sh?

[Ericson2314]

Thanks @dottharun!