Upcoming migration for stdlib("c")

Almost since the inception of conda-forge, the baseline version of our standard library ("stdlib") for C has not changed. This library comes with extra complications because it is an essential part of the operating system, and one of the few things that conda/mamba/etc. cannot ship safely.

As the ecosystem has moved on and many packages are starting to require newer baseline versions, we need to follow suit at some point. However, to avoid breaking users on older systems, we need to have infrastructure in place that allows our packages to have sufficiently accurate metadata, such that conda can avoid the installation of a package requiring a newer stdlib on an old system.

After many discussions across conda-forge stakeholders, the solution we arrived at is the introduction of a new Jinja2-function {{ stdlib("c") }}, which reflects that a given recipe requires a C stdlib. Making this relationship explicit will make it easy to correctly reflect the requirement for newer stdlib versions per feedstock, as well as in our global pinning.

Up until now, the stdlib was handled implicitly as part of the compiler stack. In order to allow this transition to happen, we need to introduce this function to essentially all compiled recipes. This will be done in stages, first for a single migration, and then attached to all ongoing migrations in conda-forge.

The logic of the piggyback migrator tries to correctly handle most scenarios, but it is impossible to cover all corner cases. As for some general rules that all feedstock maintainers are free to apply independently:

• if a feedstock uses a - {{ compiler(...) }} jinja in the build section, add a line with - {{ stdlib("c") }} to the build environment.
• if a feedstock uses - sysroot_linux-64 2.17 # [linux64] (or a variation), remove this line and add the following to your conda_build_config.yaml:

  c_stdlib_version:              # [linux]
    - 2.17                       # [linux]
• if a feedstock sets MACOSX_DEPLOYMENT_TARGET in conda_build_config.yaml, for example to 10.13 for x86_64, replace that section with the following (note, this does not apply to MACOSX_SDK_VERSION!):

  c_stdlib_version:              # [osx and x86_64]
    - 10.13                      # [osx and x86_64]
• In meta.yaml, you can then remove any variations of - __glibc >=2.17 or - __osx >={{ MACOSX_DEPLOYMENT_TARGET }} # [osx and x86_64], as this will henceforth be handled through - {{ stdlib("c") }}.

Following the application of any of the above changes, the feedstock should be rerendered.

As these mechanisms begin rolling out, we will also update the maintainer documentation in the conda-forge knowledge base. For more details, see this issue.