Apply configure options to git checkout when adding external projects

Usage
1

Hi,
in ITK we have some third-party modules that we checkout using CMake’s ExternalProject_Add() function’s git capabilities:
https://cmake.org/cmake/help/latest/module/ExternalProject.html

Specifically, we donwload DCMTK:
https://github.com/InsightSoftwareConsortium/ITK/blob/9baa34b6998cfb4034d64f0c4f37072e05db356e/Modules/ThirdParty/DCMTK/CMakeLists.txt#L304

using a given git commit hash:
https://github.com/InsightSoftwareConsortium/ITK/blob/9baa34b6998cfb4034d64f0c4f37072e05db356e/Modules/ThirdParty/DCMTK/DCMTKGitTag.cmake

which results in git telling us that we are in detached HEAD status:
CDash

In order to silence the warning, we would pass the -c advice.detachedHead=false flag to the git checkout command.

However, it is not evident to me how CMake allows us to do so: I found this post where a GIT_CONFIG field is specified with the exact flag that we would require. However, I’m wondering whether this effectively applies to the git checkout step.

Thanks

2

That seems to apply to the git clone only according to the docs. However, I see that if you’re using Git >= 1.7.7, ExternalProject should apply this config automatically as of CMake 3.20. See this MR.

3

Thanks for the answer @ben.boeckel.

What that MR brings seems like what we’re looking for. We should update our minimum required version in ITK to benefit from it (we’re now requiring 3.16.3). Thanks Again Ben.