solver picks an older _openmp_mutex

[isuruf]

Checklist

• I added a descriptive title
• I searched open reports and couldn't find a duplicate

What happened?

Following command

CONDA_SUBDIR=linux-64 CONDA_OVERRIDE_GLIBC=2.29 conda create -n a libgcc --solver=rattler

gives

Channels:
 - https://prefix.dev/conda-forge
Platform: linux-64
Collecting package metadata (repodata.json): done
Solving environment: done

## Package Plan ##

  environment location: /home/isuruf/.conda/envs/a

  added / updated specs:
    - libgcc


The following NEW packages will be INSTALLED:

  _libgcc_mutex      conda-forge/linux-64::_libgcc_mutex-0.1-conda_forge 
  _openmp_mutex      conda-forge/linux-64::_openmp_mutex-4.5-2_gnu 
  libgcc             conda-forge/linux-64::libgcc-15.2.0-he0feb66_17 
  libgomp            conda-forge/linux-64::libgomp-15.2.0-he0feb66_17

whereas

CONDA_SUBDIR=linux-64 CONDA_OVERRIDE_GLIBC=2.29 conda create -n a libgcc "_openmp_mutex=4.5=3_gnu" --solver=rattler

gives

Channels:
 - https://prefix.dev/conda-forge
Platform: linux-64
Collecting package metadata (repodata.json): done
Solving environment: done

## Package Plan ##

  environment location: /home/isuruf/.conda/envs/a

  added / updated specs:
    - _openmp_mutex==4.5=3_gnu
    - libgcc


The following NEW packages will be INSTALLED:

  _openmp_mutex      conda-forge/linux-64::_openmp_mutex-4.5-3_gnu 
  libgcc             conda-forge/linux-64::libgcc-15.2.0-he0feb66_17 
  libgomp            conda-forge/linux-64::libgomp-15.2.0-he0feb66_17 

Note that this is not specific to rattler solver. I can reproduce with libmamba and classic.

Additional Context

$ conda info

     active environment : base
    active env location : /home/isuruf/miniforge3
            shell level : 1
       user config file : /home/isuruf/.condarc
 populated config files : /etc/conda/condarc
                          /home/isuruf/.condarc
          conda version : 26.1.0
    conda-build version : 25.9.0
         python version : 3.10.16.final.0
                 solver : libmamba (default)
       virtual packages : __archspec=1=zen2
                          __conda=26.1.0=0
                          __cuda=13.1=0
                          __glibc=2.39=0
                          __linux=6.8.0=0
                          __unix=0=0
       base environment : /home/isuruf/miniforge3  (writable)
      conda av data dir : /home/isuruf/miniforge3/etc/conda
  conda av metadata url : None
           channel URLs : https://prefix.dev/conda-forge/linux-64
                          https://prefix.dev/conda-forge/noarch
          package cache : /home/isuruf/.conda/pkgs
       envs directories : /home/isuruf/.conda/envs
                          /home/isuruf/miniforge3/envs
    temporary directory : /tmp
               platform : linux-64
             user-agent : conda/26.1.0 requests/2.32.3 CPython/3.10.16 Linux/6.8.0-71-generic ubuntu/24.04.3 glibc/2.39 solver/libmamba conda-libmamba-solver/25.11.0 libmambapy/2.3.1
                UID:GID : 1079:1084
             netrc file : None
           offline mode : False

[jaimergp]

Pure py-rattler reproducer:

# /// script
# dependencies = ["py-rattler"]
# ///
import asyncio
import os
import rattler

def solve(specs: list[str], channels: list[str], subdirs: list[str]):

    async def _solve(specs: list[str], channels: list[str], subdirs: list[str]):
        return await rattler.solve(
                # Channels to use for solving
                sources=channels,
                platforms=subdirs,
                # The specs to solve for
                specs=[rattler.MatchSpec(spec, strict=False) for spec in specs],
                # Virtual packages define the specifications of the environment
                virtual_packages=rattler.VirtualPackage.detect(overrides=rattler.VirtualPackageOverrides.from_env()),
            )
    
    solved_records = asyncio.run(_solve(specs, channels, subdirs))
    print("solving for", specs)
    print()
    print(*sorted(solved_records, key=lambda r: r.name.normalized), sep="\n")
    print("****")

os.environ["CONDA_OVERRIDE_LINUX"] = "6.8.0"
os.environ["CONDA_OVERRIDE_GLIBC"] = "2.39"
os.environ["CONDA_OVERRIDE_UNIX"] = "1"
solve(["libgcc"], channels=["conda-forge"], subdirs=["linux-64", "noarch"])
solve(["libgcc", "_openmp_mutex=4.5=3_gnu"], channels=["conda-forge"], subdirs=["linux-64", "noarch"])

[baszalmstra]

I see the confusion, but browsing to https://prefix.dev/channels/conda-forge/packages/_openmp_mutex and filtering by architecture and version shows the culprit.

linux-64/_openmp_mutex-4.5-2_gnu.tar.bz2 has build number 16
linux-64/_openmp_mutex-4.5-3_gnu.conda has build number 2

Since 16 > 2, the package with build string 2_gnu is preferred over 3_gnu.

[wolfv]

Maybe we can expose the version sorting through the Python API?

[baszalmstra]

Closing as this is not a solver issue.