8ac71b026e
Our threading support layer is currently a huge mess. There are too many configurations with too many confusing names, and none of them are tested in the usual CI. Here's a list of names related to these configurations: LIBCXX_BUILD_EXTERNAL_THREAD_LIBRARY _LIBCPP_BUILDING_THREAD_LIBRARY_EXTERNAL LIBCXXABI_BUILD_EXTERNAL_THREAD_LIBRARY _LIBCPP_HAS_THREAD_LIBRARY_EXTERNAL LIBCXX_HAS_EXTERNAL_THREAD_API _LIBCPP_HAS_THREAD_API_EXTERNAL This patch cleans this up by removing the ability to build libc++ with an "external" threading library for testing purposes, removing 4 out of 6 "names" above. That setting was meant to be used by libc++ developers, but we don't use it in-tree and it's not part of our CI. I know the ability to use an external threading API is used by some folks out-of-tree, and this patch doesn't change that. This only changes the way they will have to test their external threading support. After this patch, the intent would be for them to set `-DLIBCXX_HAS_EXTERNAL_THREAD_API=ON` when building the library, and to provide their usual `<__external_threading>` header when they are testing the library. This can be done easily now that we support custom lit configuration files in test suites. The motivation for this patch is that our threading support layer is basically unmaintainable -- anything beyond adding a new "backend" in the slot designed for it requires incredible attention. The complexity added by this setting just doesn't pull its weigh considering the available alternatives. Concretely, this will also allow future patches to clean up `<__threading_support>` significantly. Differential Revision: https://reviews.llvm.org/D154466
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