stdenv: fix unbound NIX_LOG_FD in nix develop

[tie]

Description of changes

When running nix develop for a package, Nix records the stdenv environment with NIX_LOG_FD set. That is, when the actual development shell runs, it uses the functions that attempt to log to NIX_LOG_FD, but this variable is not actually set.

As a workaround, check whether NIX_LOG_FD is set at runtime.

Example (before this change):

$ nix develop --file . hello
$ echo "${NIX_LOG_FD-unset}"
unset
$ runPhase unpackPhase
bash: "$NIX_LOG_FD": Bad file descriptor
Running phase: unpackPhase
unpacking source archive /nix/store/pa10z4ngm0g83kx9mssrqzz30s84vq7k-hello-2.12.1.tar.gz
bash: "$NIX_LOG_FD": Bad file descriptor
source root is hello-2.12.1
bash: "$NIX_LOG_FD": Bad file descriptor
setting SOURCE_DATE_EPOCH to timestamp 1653865426 of file hello-2.12.1/ChangeLog

After this change:

$ nix develop --file . hello
$ runPhase unpackPhase
Running phase: unpackPhase
unpacking source archive /nix/store/pa10z4ngm0g83kx9mssrqzz30s84vq7k-hello-2.12.1.tar.gz
source root is hello-2.12.1
setting SOURCE_DATE_EPOCH to timestamp 1653865426 of file hello-2.12.1/ChangeLog

Notice that bash: "$NIX_LOG_FD": Bad file descriptor lines are gone.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 24.11 Release Notes (or backporting 23.11 and 24.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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[drupol]

Same issue here, but the origin seems to be different.

❯ rm -rf .venv && direnv reload
direnv: loading ~/Code/work/python-project/.envrc
direnv: using flake
direnv: nix-direnv: Renewed cache
Executing venvHook
Creating new venv environment in path: './.venv'
/home/pol/.config/direnv/lib/hm-nix-direnv.sh:1792: "$NIX_LOG_FD": Bad file descriptor
Installing dependencies from lock file

Package operations: 71 installs, 0 updates, 0 removals

  - Installing typing-extensions (4.12.2): Pending...
  - Installing typing-extensions (4.12.2)
  - Installing annotated-types (0.7.0)
  ...

[tie]

direnv: using flake

@drupol, IIRC nix-direnv (ab)uses nix develop, so probably the same underlying issue (but with more layers of abstractions on top)…

[philiptaron]

Yep.

[trofi]

Is it a side-effect of NixOS/nix#8987 by chance?

[tie]

Is it a side-effect of NixOS/nix#8987 by chance?

@trofi, yes, these functions were recorded in a separate derivation where NIX_LOG_FD was set. I’m OK with this Nix behavior, but we should probably document that stdenv’s setup.sh and hooks shouldn’t have significant side effects when sourced (and use phases instead e.g. when creating files).