Skip to content

LHAPATH Fix, master branch (2023.06.20.)

Attila Krasznahorkay requested to merge akraszna/atlasexternals:LhapathFix-master-20230620 into master

Taught FindLhapdf.cmake about the ATLAS specific data path. This is because to ensure that the order between the paths would be stable, all parts of $LHAPDF_DATA_PATH and $LHAPATH need to be defined in a single place, inside of the FindLhapdf.cmake module.

Longer version: @ewelina made me aware that the order of paths in $LHAPATH is different between Athena,master,latest and AthGeneration,master,latest. It took a bit of investigation to understand the exact reason for this, but I believe I now do...

Currently $LHAPATH is set up from 2 different places:

The way that environment variables are set up in AtlasLCG is that at the end of configuring a project, we ask the PACKAGES_FOUND global property of CMake for the full list of "packages" that were found by the project.

https://gitlab.cern.ch/atlas/atlasexternals/-/blob/master/Build/AtlasLCG/modules/LCGFunctions.cmake#L603

Then we iterate over these one-by-one, and collect the environment settings out of all of them.

As I now learned, the order of packages in this list is not "first seen, first in" as I thought previously, but rather "last seen, last in". I.e. if the project configuration comes across the previous two modules in the following order:

  • find_package(Lhapdf)
  • find_package(SuperchicEnvironment)
  • find_package(Lhapdf)

, then during the environment setup script generation these modules are listed in the SuperchicEnvironment, Lhapdf order. And I don't see a way of changing this.

As it happens, when configuring AthGeneration, it is find_package(SuperchicEnvironment) that is seen last by CMake before the environment generation is started. But when configuring Athena, it is instead find_package(Lhapdf). (Athena includes a few more packages that need LHAPDF, and those are processed after the generator packages.)

With all this in mind I still pondered for a bit about how to solve things, but in the end had to conclude that the only reliable setup is if the $LHAPATH environment variable's setup is not left up to two separate modules. Instead every element of $LHAPATH would have to be set up in a single place. (Yes, mucking about with PREPEND and APPEND could've fixed the issue for now, but I just didn't like the fragility of that setup.)

Very long story short, when this update is collected into atlas/athena, we will need to simplify https://gitlab.cern.ch/atlas/athena/-/blob/master/Generators/Superchic_i/SuperchicEnvironmentConfig.cmake at the same time.

Merge request reports