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CMake 3.16 and below allow multiple `export()` calls with the same output
file even without using `APPEND`. The implementation worked by accident
by leaking memory. Refactoring in commit 5444a8095d (cmGlobalGenerator:
modernize memrory managemenbt, 2019-12-29, v3.17.0-rc1~239^2) cleaned up
that memory leak and converted it to a use-after-free instead.
The problem is caused by using the `cmGlobalGenerator::BuildExportSets`
map to own `cmExportBuildFileGenerator` instances. It can own only
one instance per output FILE name at a time, so repeating use of the
same file now frees the old `cmExportBuildFileGenerator` instance
and leaves the pointer in the `cmMakefile::ExportBuildFileGenerators`
vector dangling. Move ownership of the instances into `cmMakefile`'s
vector since its entries are not replaced on a repeat output FILE.
In future work we should introduce a policy to error out on this case.
For now simply fix the use-after-free to restore CMake <= 3.16 behavior.
Fixes: #20469
CMake Tests Directory ********************* This directory contains the CMake test suite. See also the `CMake Source Code Guide`_. .. _`CMake Source Code Guide`: ../Help/dev/source.rst Many tests exist as immediate subdirectories, but some tests are organized as follows. * ``CMakeLib/``: Source code, used for tests, that links to the ``CMakeLib`` library defined over in ``Source/``. * ``CMakeOnly/``: Deprecated. Tests that run CMake to generate a project but not build it. Superseded by ``Tests/RunCMake/``. * ``Find*/``: Tests for specific find modules that can only be run on machines with the corresponding packages installed. They are enabled in ``CMakeLists.txt`` by undocumented options used on CI builds. * ``Module/``: Tests for specific CMake modules. * ``RunCMake/``: Tests that run CMake and/or other tools while precisely checking their return code and stdout/stderr content. Useful for testing error cases and diagnostic output.