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Build: Fail if included with add_subdirectory()

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Even though BUILDING.md and CONTRIBUTING.md explicitly state that the
libjpeg-turbo build system does not and will not support being
integrated into downstream build systems using add_subdirectory() (see
0565548191), people continue to file bug
reports, feature requests, and pull requests regarding that (see #265,
 #637, and #653 in addition to the issues listed in
05655481917a2d2761cf2fe19b76f639b7f159ef.)  Responding to those
issues wastes our project's limited resources.  Hopefully people will
get the hint if the build system explicitly tells them that it can't be
included using add_subdirectory(), which will prompt them to read the
comments in CMakeLists.txt explaining why.  To anyone stumbling upon
this commit message, please refer to the discussions under the issues
listed above, as well as the issues listed in
0565548191.  Our project's position on
this has been stated, explained, and defended numerous times.
This commit is contained in:
DRC
2023-02-10 09:55:27 -06:00
parent 8ecba3647e
commit 33eaf17e5b
1 changed files with 28 additions and 0 deletions
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CMakeLists.txt
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@@ -31,6 +31,34 @@ pad_number(VERSION_MINOR 3)
pad_number(VERSION_REVISION 3)
set(LIBJPEG_TURBO_VERSION_NUMBER ${VERSION_MAJOR}${VERSION_MINOR}${VERSION_REVISION})
# The libjpeg-turbo build system has never supported and will never support
# being integrated into another build system using add_subdirectory(), because
# doing so would require that we (minimally):
#
# 1. avoid using certain CMake variables, such as CMAKE_SOURCE_DIR,
# CMAKE_BINARY_DIR, and CMAKE_PROJECT_NAME;
# 2. avoid using implicit include directories and relative paths;
# 3. optionally provide a way to skip the installation of libjpeg-turbo
# components when the 'install' target is built;
# 4. optionally provide a way to postfix target names, to avoid namespace
# conflicts;
# 5. restructure the top-level CMakeLists.txt so that it properly sets the
# PROJECT_VERSION variable; and
# 6. design automated regression tests to ensure that new commits don't break
# any of the above.
#
# Even if we did all of that, issues would still arise, because it is
# impossible for an upstream build system to anticipate the widely varying
# needs of every downstream build system. That's why the CMake
# ExternalProject_Add() function exists. Downstream projects that wish to
# integrate libjpeg-turbo as a subdirectory should either use
# ExternalProject_Add() or make downstream modifications to the libjpeg-turbo
# build system to suit their specific needs. Please do not file bug reports,
# feature requests, or pull requests regarding this.
if(NOT CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
message(FATAL_ERROR "The libjpeg-turbo build system cannot be integrated into another build system using add_subdirectory(). Use ExternalProject_Add() instead.")
endif()
# CMake 3.14 and later sets CMAKE_MACOSX_BUNDLE to TRUE by default when
# CMAKE_SYSTEM_NAME is iOS, tvOS, or watchOS, which breaks the libjpeg-turbo
# build. (Specifically, when CMAKE_MACOSX_BUNDLE is TRUE, executables for