CMake build fails with Xcode 12 and CMAKE_IOS_INSTALL_COMBINED=YES - cmake

Question

EDIT This is a known CMake issue.
With Xcode 11 (specifically Xcode 11.2.1) I used to be able to cross-compile my project for iOS with this command:
cd /path/to/project
mkdir build
cd build
cmake .. -G Xcode -DCMAKE_SYSTEM_NAME=iOS \
"-DCMAKE_OSX_ARCHITECTURES=arm64;x86_64" \
-DCMAKE_XCODE_ATTRIBUTE_ONLY_ACTIVE_ARCH=NO \
-DCMAKE_IOS_INSTALL_COMBINED=YES \
-DCMAKE_INSTALL_PREFIX=install
cmake --build . --config Release --target install
This created a build with two slices, one slice for the arm64 architecture (for running on a real device) and one slice for the x86_64 architecture (for running in a simulator on an Intel-based dev environment).
After upgrading to Xcode 12 (specifically Xcode 12.3) this no longer works. The build consistently fails with this error message (line break added by me):
error: unable to attach DB: error: accessing build database "/path/to/project/build/ios/build/XCBuildData/build.db":
database is locked Possibly there are two concurrent builds running in the same filesystem location.
The issue seems to be a post-build rule that CMake creates for the install target. In that post-build rule a second build is initiated while the first one is still in progress.
if test "$CONFIGURATION" = "Release"; then :
cd /path/to/project/build/ios
/usr/local/Cellar/cmake/3.19.2/bin/cmake -DBUILD_TYPE=$CONFIGURATION -DEFFECTIVE_PLATFORM_NAME=$EFFECTIVE_PLATFORM_NAME -P cmake_install.cmake
fi
The build succeeds when I no longer set CMAKE_IOS_INSTALL_COMBINED to YES, but then the resulting build only contains one slice (arm64 in my case, presumably because this is the first architecture that is listed in CMAKE_OSX_ARCHITECTURES).
I'm considering making a separate build for each architecture, and then stitching the slices together manually. Before I go down that road, has anyone been able to find a more elegant solution?
Environment: macOS 11.1, Xcode 12.3, CMake 3.19.2

Answer 1

The CMake 3.19 release notes contain this hint:
The Xcode generator now uses the Xcode “new build system” when generating for Xcode 12.0 or higher. See the CMAKE_XCODE_BUILD_SYSTEM variable. One may use -T buildsystem=1 to switch to the legacy build system.
The workaround, for the moment, is therefore to add the -T option to the build system generation command line:
cmake .. -G Xcode -T buildsystem=1
-DCMAKE_SYSTEM_NAME=iOS \
"-DCMAKE_OSX_ARCHITECTURES=arm64;x86_64" \
-DCMAKE_XCODE_ATTRIBUTE_ONLY_ACTIVE_ARCH=NO \
-DCMAKE_IOS_INSTALL_COMBINED=YES \
-DCMAKE_INSTALL_PREFIX=install
This should work as long as Xcode still supports the legacy build system. The Xcode 12 Release Notes have this to say on the matter:
The legacy build system is deprecated, and will be removed in a future release. (62742902)
So the -T buildsystem=1 option can only be considered a temporary workaround.