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Since commit729d997f10(Precompile Headers: Add REUSE_FROM signature, 2019-08-30, v3.16.0-rc1~101^2), `GetPchFileObject` handles the case that it is called first for another target's `REUSE_FROM` by calling `AddSource` to make sure `GetObjectName` can produce the correct object name. However, `AddSource` causes `ClearSourcesCache` to be called, which since commita9f4f58f0c(cmGeneratorTarget: Clear AllConfigSources in ClearSourcesCache, 2020-05-15, v3.16.7~2^2) now correctly erases the `AllConfigSources` structure. This is okay during `AddPchDependencies`, but there is another code path in which it is problematic. When the Visual Studio generator's `WriteAllSources` method is looping over the sources, the `cmake_pch.cxx` source is encountered first. This causes `OutputSourceSpecificFlags` to call `GetPchCreateCompileOptions`, which calls `GetPchFile`, which under MSVC with `CMAKE_LINK_PCH` calls `GetPchFileObject`. That leads to `ClearSourcesCache` erasing the structure over which `WriteAllSources` is iterating! This bug is caught by our `RunCMake.PrecompileHeaders` test when run with the VS generator as of the commit that exposed it by fixing `ClearSourcesCache`. However, that change was backported to the CMake 3.16 series after testing only with later versions versions that contain commita55df20499(Multi-Ninja: Add precompile headers support, 2020-01-10, v3.17.0-rc1~136^2). By adding proper multi-config support for PCH, that commit taught `cmLocalGenerator::AddPchDependencies` to call `GetPchFile` with the real set of configurations instead of just the empty string. This allows the `GetPchFile` cache of PCH sources to be populated up front so that the later calls to it in the `WriteAllSources` loop as described above do not actually call `GetPchFileObject` or `ClearSourcesCache`. That hid the problem. Fix this by re-ordering calls to `AddPchDependencies` to handle `REUSE_FROM` targets only after the targets whose PCH they re-use. Remove the now-unnecessary call to `AddSource` from `GetPchFileObject` so that `ClearSourcesCache` is never called during `WriteAllSources`. Update the PchReuseFrom test case to cover an ordering of targets that causes generators to encounter a `REUSE_FROM` target before the target whose PCH it re-uses. Fixes: #20770
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.