PKG_HASH erroring on undefined when rendering recipe

[beckermr]

When only rendering a recipe, the new undefined jinja2 checks error when standard variables that rattler-build is supposed to insert are not present.

I get that the PKG_HASH is not yet defined, but erroring on it being missing is not the right thing to do here. The reason to error on undefined variables is to alert the user of a mistake. However, this is not a mistake by thew user. Instead rattler-build should insert a sentinel value or do something else.

Here is a reproducer:

$ cat slepc.yaml 
schema_version: 1

context:
  version: 3.19.1
  sha256: 280737e9ef762d7f0079ad3ad29913215c799ebf124651c723c1972f71fbc0db
  build: 0

package:
  name: slepc
  version: ${{ version }}

source:
  url: http://slepc.upv.es/download/distrib/slepc-${{ version }}.tar.gz
  sha256: ${{ sha256 }}

build:
  number: ${{ build }}
  string: real_h${{ PKG_HASH }}_${{ build }}
  skip:
    - win

requirements:
  run:
    - petsc
    - suitesparse
  run_exports:
    - ${{ pin_subpackage('slepc', upper_bound='x.x') }}

about:
  summary: SLEPc Scalable Library for Eigenvalue Problem Computations
  license: BSD-2-Clause
  license_file: LICENSE.md
  homepage: http://slepc.upv.es/

extra:
  recipe-maintainers:
    - dalcinl
    - joseeroman
    - minrk
$ rattler-build build --render-only -r slepc.yaml
 Processing recipe at path: /Users/beckermr/Desktop/conda-forge/cf-scripts/tests/test_recipe_yaml/slepc.yaml
Error:   × Failed to render recipe with variants
  ╰─▶ Jinja template error: Failed to render build string template: undefined value (in <string>:1)
    ╭─[/Users/beckermr/Desktop/conda-forge/cf-scripts/tests/test_recipe_yaml/slepc.yaml:18:11]
 17 │   number: ${{ build }}
 18 │   string: real_h${{ PKG_HASH }}_${{ build }}
    ·           ────┬────
    ·               ╰── Failed to render build string template: undefined value (in <string>:1)
 19 │   skip:
    ╰────

cc @wolfv @Hofer-Julian

[beckermr]

This same logic should apply to any variable rattler-build is supposed to define. Those variables by definition are never missing.

[wolfv]

It should actually be ${{ hash }}.

[beckermr]

Huh?

PKG_HASH is documented here: https://rattler-build.prefix.dev/latest/build_script/#default-environment-variables-set-during-the-build-process

[beckermr]

Those default env vars or any default jinja2 vars should never error regardless.

[wolfv]

Yeah it's defined in the build script, but in the Jinja it's hash (${{ hash }}). We should maybe change / alias that but there is only extremely few recipes using this wrongly at this point.

[beckermr]

Is this in the spec? Conda build always populated both.

[wolfv]

I don't think that conda-build ever used ${{ hash }}.

[beckermr]

Sorry I was not clear! What I meant to say is that conda-build always populated any env vars in the build script into the jinja2.

[wolfv]

Yeah that is not the case anymore, especially outside the script key