fix(nix): Resolve PATH mismatch for packages option in multi-user mode

[V-Subhankar-infy]

fix(nix): Resolve PATH mismatch and attribute-path error for packages in multi-user mode

Issues Fixed

Issue 1 — Packages installed to wrong profile in multi-user mode { Reported As Issue on Feb-16-2026 at #1573 }

Problem: In multi-user Nix installs, packages specified via the packages option were silently installed into the calling user's ~/.nix-profile (inside a su subshell) instead of /nix/var/nix/profiles/default — the global profile that is on PATH at runtime.

Reproduction:

# devcontainer-feature.json: multiUser: true, packages: "nixpkgs.vim"
$ type vim
# → vim: command not found

$ nix-env -p /nix/var/nix/profiles/default -q
# → vim is absent from the default profile

Root Cause: install.sh ran post-install-steps.sh inside su ${USERNAME} -c "...". Inside that subshell, nix-env defaulted to ~/.nix-profile. Since only /nix/var/nix/profiles/default/bin is on PATH at runtime, the installs were unreachable.

Fix:

• src/nix/install.sh — Removed the su subshell in the multi-user branch. nix-daemon.sh is sourced in the root context and NIX_FEATURE_INSTALL_PROFILE=/nix/var/nix/profiles/default is passed as a prefixed env var to post-install-steps.sh.
• src/nix/post-install-steps.sh — When NIX_FEATURE_INSTALL_PROFILE is set, constructs -p/--profile arg arrays and forwards them to every nix-env and nix profile install invocation. Single-user mode is unaffected (arrays remain empty).

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Issue 2 — Wrong attribute path for nodejs in packages scenario

Problem: The packages test scenario in scenarios.json used nodePackages.nodejs with useAttributePath: true. The correct nixpkgs attribute path is simply nodejs; the nodePackages. prefix resolves a different attribute set and caused the scenario to fail at build time.

Reproduction:

# scenarios.json: packages: "nodePackages.nodejs,...", useAttributePath: true
$ nix-env -iA nixpkgs.nodePackages.nodejs
# → error: attribute 'nodePackages.nodejs' in selection path not found

Fix:

• test/nix/scenarios.json — Corrected nodePackages.nodejs → nodejs in the packages string for the useAttributePath scenario.

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Tests

• Added check "vim_in_default_profile" to test/nix/packages.sh — runs nix-env -p /nix/var/nix/profiles/default -q | grep '^vim' to directly assert packages land in the global profile.
• Existing vim_installed / node_installed / yarn_installed checks confirm end-to-end PATH visibility at runtime.
• Corrected scenarios.json ensures the useAttributePath + packages scenario no longer fails on attribute resolution before the profile bug is even exercised.

Impact

• Multi-user containers with packages now correctly expose installed packages on PATH.
• The useAttributePath packages CI scenario is unblocked and runs cleanly.
• Single-user installs and the public feature interface are unchanged.

[Copilot]

Pull request overview

This PR updates the Nix feature’s multi-user installation flow so that configured packages (and optionally flakes) are installed into the default system profile that’s on users’ PATH, addressing PATH/profile mismatches in multi-user mode.

Changes:

• Install packages via nix-env and flakes via nix profile into a configurable target profile (used for multi-user default profile).
• In multi-user mode, run post-install steps against /nix/var/nix/profiles/default to align installs with the profile on PATH.
• Extend Nix feature tests to assert the default profile contains expected packages.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
test/nix/scenarios.json	Updates the attribute-path packages scenario to use nodejs (to be prefixed to nixpkgs.nodejs).
test/nix/packages.sh	Adds a test assertion that vim is present in the default Nix profile.
src/nix/post-install-steps.sh	Adds support for installing packages/flakes into a specified Nix profile (via -p/--profile).
src/nix/install.sh	Switches multi-user post-install execution to target /nix/var/nix/profiles/default directly.

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