cmake: Fix build directory exclusion for real

[RytoEX]

Description

Upon further review, the build directory is never set to build_$arch. Currently, our CMake Presets on Ubuntu only use build_ubuntu. However, we can attempt to be flexible here and simply exclude the build directory configured in CMake.

Motivation and Context

Want this to really be fixed. Follow-up to #11501.

Discovered after tagging 31.0.0-rc1 that the build directory was not excluded from the source package. Dug a bit deeper and realized that the build directory on CI is always build_ubuntu and never build_$arch//build_${CMAKE_SYSTEM_PROCESSOR}.

We could also simply say "we only support our provided presets" and hardcode this to "build_ubuntu" as we do in CMakePresets.json, but this should use that same value.

How Has This Been Tested?

Tested locally on WSL.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.

[PatTheMav]

CMAKE_BINARY_DIR should be an absolute path - I'm surprised that it will match a local relative path.

[RytoEX]

IIRC, CMake converts these paths to absolute paths if they are relative, but ultimately uses absolute paths. I'll go re-read the source later to check, but it worked on my machine.

[RytoEX]

The Ubuntu presets set binaryDir to "${sourceDir}/build_ubuntu". Let's say CMAKE_BINARY_DIR is /home/runner/work/obs-studio/obs-studio/build_ubuntu.

CMake sets CPACK_SOURCE_IGNORE_FILES to CPACK_IGNORE_FILES.
https://gitlab.kitware.com/cmake/cmake/-/blob/v3.31.1/Modules/CPack.cmake#L984

The value of CPACK_IGNORE_FILES is assigned to cpackIgnoreFiles.
https://gitlab.kitware.com/cmake/cmake/-/blob/v3.31.1/Source/CPack/cmCPackGenerator.cxx#L344

cpackIgnoreFiles is used to build ignoreFilesRegex:
https://gitlab.kitware.com/cmake/cmake/-/blob/v3.31.1/Source/CPack/cmCPackGenerator.cxx#L343-352

The top directory logged here is an absolute path.
https://gitlab.kitware.com/cmake/cmake/-/blob/v3.31.1/Source/CPack/cmCPackGenerator.cxx#L373-378

 CPack: - Install directory: /home/runner/work/obs-studio/obs-studio

The top directory is used to make findExpr:
https://gitlab.kitware.com/cmake/cmake/-/blob/v3.31.1/Source/CPack/cmCPackGenerator.cxx#L376-382

findExpr = /home/runner/work/obs-studio/obs-studio/*

findExpr is passed to cmsys::Glob::FindFiles(const std::string& inexpr, GlobMessages* messages) which assigns inexpr to expr and changes expr to an absolute path if it was not already one:
https://gitlab.kitware.com/cmake/cmake/-/blob/v3.31.1/Source/kwsys/Glob.cxx#L353-356
In this case, both expr and inexpr would be /home/runner/work/obs-studio/obs-studio/*

If the last slash in expr is not the first character, last_slash is set to that position value. If last_slash is greater than 0, skip is set to that same value:
https://gitlab.kitware.com/cmake/cmake/-/blob/v3.31.1/Source/kwsys/Glob.cxx#L359-374
In this case, last_slash and skip should be 39.

Since skip is greater than 0, expr is trimmed to erase all directories and only leave the last path component:
https://gitlab.kitware.com/cmake/cmake/-/blob/v3.31.1/Source/kwsys/Glob.cxx#L397-399
In this case, expr becomes /*.

Then, cexpr becomes * and is added to the Expressions vector:
https://gitlab.kitware.com/cmake/cmake/-/blob/v3.31.1/Source/kwsys/Glob.cxx#L401-414

Since skip is greater than zero, this branch is taken:
https://gitlab.kitware.com/cmake/cmake/-/blob/v3.31.1/Source/kwsys/Glob.cxx#L417-419

this->ProcessDirectory(0, "/home/runner/work/obs-studio/obs-studio/", messages);

In ProcessDirectory(), the first path is taken because start is 0, and the Expressions vector is size 1, so 0 = 1-1:
https://gitlab.kitware.com/cmake/cmake/-/blob/v3.31.1/Source/kwsys/Glob.cxx#L287-293

In RecurseDirectory(), the directory /home/runner/work/obs-studio/obs-studio/ is loaded, and all files in it are recursively added to the Files vector. The file paths are constructed as absolute paths:
https://gitlab.kitware.com/cmake/cmake/-/blob/v3.31.1/Source/kwsys/Glob.cxx#L205-209

In cmsys::Glob, Relative is initialized to "":
https://gitlab.kitware.com/cmake/cmake/-/blob/v3.31.1/Source/kwsys/Glob.cxx#L51

None of cmsys::Glob::FindFiles, anything in its calls, or the setup in cmCPackGenerator::InstallProjectViaInstalledDirectories changes the Relative member variable. That being the case, when files are added to cmsys::Glob's this->Internal->Files with AddFile(), they are using absolute paths:
https://gitlab.kitware.com/cmake/cmake/-/blob/v3.31.1/Source/kwsys/Glob.cxx#L447-454

These files as absolute paths are retrieved using cmsys::Glob::GetFiles():
https://gitlab.kitware.com/cmake/cmake/-/blob/v3.31.1/Source/CPack/cmCPackGenerator.cxx#L388
https://gitlab.kitware.com/cmake/cmake/-/blob/v3.31.1/Source/kwsys/Glob.cxx#L68-71

After recursively globbing all files in the source directory, files are checked to see if they should be ignored.
Directory paths are normalized to end with a slash:
https://gitlab.kitware.com/cmake/cmake/-/blob/v3.31.1/Source/CPack/cmCPackGenerator.cxx#L388-395

Paths that match any regex in ignoreFilesRegex (see above) are ignored:
https://gitlab.kitware.com/cmake/cmake/-/blob/v3.31.1/Source/CPack/cmCPackGenerator.cxx#L396-405

This does also mean that if any part of the path preceding the source directory (parent directories) matches /.git, /.ccache or /.deps, then the entire obs-studio source folder will be ignored.

I stepped through most of FindFiles() and ProcessDirectory() with Compiler Explorer to confirm their behavior.