FetchContent: Quickly hits MAXPATH limit on Windows

Problem

The name of a FetchContent project is used four times in the file-path that is generated by the populate step of FetchContent, which quickly exceeds the 260 characters limit for paths on Windows.

In general, that should not be a problem, because modern Windows allows to exceeds that limit (by setting a registry key). However, the used programs must support it. And sadly, ninja does not do it and bails out if a path exceeds the 260 character limit for file-paths.

Solution

Do not use the name of the FetchContent project multiple times in the generated file-path and possibly use CMAKE_OBJECT_PATH_MAX.

Example to demonstrate the problem

Create the following CMakeLists.txt file and configure it using the Ninja configurator in a Visual Studio Developer Command-Prompt:

...
cmake_minimum_required( VERSION 3.25 )
project( Maxpath_FetchContent_Problem )
include( FetchContent )
FetchContent_Declare( quite_long_name_for_fetchcontent_project
    GIT_REPOSITORY https://github.com/google/googletest.git
    GIT_TAG        703bd9caab50b139428cea1aaff9974ebee5742e # release-1.10.0
)
FetchContent_MakeAvailable( quite_long_name_for_fetchcontent_project )

This results in:

C:\Users\dbahadir\workspace\BUILD@Maxpath_FetchContent_Problem>cmake -G "Ninja Multi-Config" --toolchain ..\toolchains\x86_64-windows-msvc.clang.llvm-only.cmake -S ..\Maxpath_FetchContent_Problem -B .

-- The C compiler identification is Clang 15.0.1 with GNU-like command-line
-- The CXX compiler identification is Clang 15.0.1 with GNU-like command-line
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: C:/Program Files/Microsoft Visual Studio/2022/Professional/VC/Tools/Llvm/x64/bin/clang.exe - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: C:/Program Files/Microsoft Visual Studio/2022/Professional/VC/Tools/Llvm/x64/bin/clang++.exe - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
ninja: error: Stat(C:/Users/dbahadir/workspace/BUILD@Maxpath_FetchContent_Problem/_deps/quite_long_name_for_fetchcontent_project-subbuild/quite_long_name_for_fetchcontent_project-populate-prefix/src/quite_long_name_for_fetchcontent_project-populate-stamp/Debug/quite_long_name_for_fetchcontent_project-populate-done): Filename longer than 260 characters

CMake Error at C:/Program Files/Microsoft Visual Studio/2022/Professional/Common7/IDE/CommonExtensions/Microsoft/CMake/CMake/share/cmake-3.26/Modules/FetchContent.cmake:1622 (message):
  Build step for quite_long_name_for_fetchcontent_project failed: 1
Call Stack (most recent call first):
  C:/Program Files/Microsoft Visual Studio/2022/Professional/Common7/IDE/CommonExtensions/Microsoft/CMake/CMake/share/cmake-3.26/Modules/FetchContent.cmake:1762:EVAL:2 (__FetchContent_directPopulate)
  C:/Program Files/Microsoft Visual Studio/2022/Professional/Common7/IDE/CommonExtensions/Microsoft/CMake/CMake/share/cmake-3.26/Modules/FetchContent.cmake:1762 (cmake_language)
  C:/Program Files/Microsoft Visual Studio/2022/Professional/Common7/IDE/CommonExtensions/Microsoft/CMake/CMake/share/cmake-3.26/Modules/FetchContent.cmake:1976 (FetchContent_Populate)
  CMakeLists.txt:19 (FetchContent_MakeAvailable)


-- Configuring incomplete, errors occurred!

The error even gets worse if you increase the length of the name given to FetchContent, because then somehow retrieving the source code does not even work.