[libcxxabi] Add LIBCXXABI_HAS_WIN32_THREAD_API build option

[zero9178]

Summary

A few files in libc++abi make use of libc++ headers and a few of those use threading primitives provided by libc++. Since libc++ has multiple threading APIs it may be necessary to override auto-detection.

This patch adds the LIBCXXABI_HAS_WIN32_THREAD_API which does roughly the same as LIBCXXABI_HAS_PTHREAD_API and the similarly named LIBCXX_HAS_WIN32_THREAD_API from libc++. Instead of using autodetection it will force the use of win32 threads instead of pthreads in headers included from libc++.

Without this patch, libc++abi may depend on pthreads if present on the users build environment, even if win32 threading was selected for libc++.

[mstorsjo]

This LGTM (but I'm not a libcxx/libcxxabi approver, so you'll need to wait for the green checkmark for the libc++abi reviewer group), given the existing predecent with similar options for all other threading configurations. I've ran into this myself in various constellations, and in those cases worked around it by just passing -D_LIBCPP_HAS_THREAD_API_WIN32 in CMAKE_CXX_FLAGS, which is clearly a hack.

FWIW, there's work ongoing (see D97572) to make __config_site available directly which should avoid the issue if I remember correctly - and that would make these options in libcxxabi that mirror libcxx options redundant. But until that's in place, and since we already have these other ones, I think this looks sensible.

[ldionne]

I'm really unhappy about this, but we do have LIBCXX_HAS_WIN32_THREAD_API so let's add this for consistency. When we find a way to replace these explicit settings for picking the threading API, we can remove them in both libc++ and libc++abi.

NB: The reason why I don't like this is that we normally never bake in this sort of detection in the CMake. I think it's a layering violation to do so. But like I said above, this is a pre-existing condition and this patch at least makes the layering violation consistent between libcxx and libcxxabi.

[zero9178]

Fully agree, making me even more excited for when the patch Martin mentioned lands.

libcxxabi/CMakeLists.txt

 option(LIBCXXABI_ENABLE_PEDANTIC "Compile with pedantic enabled." ON)
 option(LIBCXXABI_ENABLE_PIC "Build Position-Independent Code, even in static library" ON)
 option(LIBCXXABI_ENABLE_WERROR "Fail and stop if a warning is triggered." OFF)
 option(LIBCXXABI_USE_LLVM_UNWINDER "Build and use the LLVM unwinder." OFF)
 option(LIBCXXABI_ENABLE_STATIC_UNWINDER "Statically link the LLVM unwinder." OFF)
 option(LIBCXXABI_USE_COMPILER_RT "Use compiler-rt instead of libgcc" OFF)
 option(LIBCXXABI_ENABLE_THREADS "Build with threads enabled" ON)
 option(LIBCXXABI_HAS_PTHREAD_API "Ignore auto-detection and force use of pthread API" OFF)
+option(LIBCXXABI_HAS_WIN32_THREAD_API "Ignore auto-detection and force use of win32 thread API" OFF)
 option(LIBCXXABI_HAS_EXTERNAL_THREAD_API
   "Build libc++abi with an externalized threading API.
   This option may only be set to ON when LIBCXXABI_ENABLE_THREADS=ON." OFF)
 option(LIBCXXABI_BUILD_EXTERNAL_THREAD_LIBRARY
   "Build libc++abi with an externalized threading library.
    This option may only be set to ON when LIBCXXABI_ENABLE_THREADS=ON" OFF)
 option(LIBCXXABI_ENABLE_FORGIVING_DYNAMIC_CAST
 "Make dynamic_cast more forgiving when type_info's mistakenly have hidden \

 
 # Threading
 if (NOT LIBCXXABI_ENABLE_THREADS)
   if (LIBCXXABI_HAS_PTHREAD_API)
     message(FATAL_ERROR "LIBCXXABI_HAS_PTHREAD_API can only"
                         " be set to ON when LIBCXXABI_ENABLE_THREADS"
                         " is also set to ON.")
   endif()
+  if (LIBCXXABI_HAS_WIN32_THREAD_API)
+    message(FATAL_ERROR "LIBCXXABI_HAS_WIN32_THREAD_API can only"
+                        " be set to ON when LIBCXXABI_ENABLE_THREADS"
+                        " is also set to ON.")
+  endif()
   if (LIBCXXABI_HAS_EXTERNAL_THREAD_API)
     message(FATAL_ERROR "LIBCXXABI_HAS_EXTERNAL_THREAD_API can only"
                         " be set to ON when LIBCXXABI_ENABLE_THREADS"
                         " is also set to ON.")
   endif()
   if (LIBCXXABI_BUILD_EXTERNAL_THREAD_LIBRARY)
     message(FATAL_ERROR "LIBCXXABI_BUILD_EXTERNAL_THREAD_LIBRARY can only"
                         " be set to ON when LIBCXXABI_ENABLE_THREADS"
                         " is also set to ON.")
   endif()
   add_definitions(-D_LIBCXXABI_HAS_NO_THREADS)
   add_definitions(-D_LIBCPP_HAS_NO_THREADS)
 endif()
 
 if (LIBCXXABI_HAS_EXTERNAL_THREAD_API)
   if (LIBCXXABI_HAS_PTHREAD_API)
     message(FATAL_ERROR "The options LIBCXXABI_HAS_EXTERNAL_THREAD_API"
                         " and LIBCXXABI_HAS_PTHREAD_API cannot be both"
                         " set to ON at the same time.")
   endif()
+  if (LIBCXXABI_HAS_WIN32_THREAD_API)
+    message(FATAL_ERROR "The options LIBCXXABI_HAS_EXTERNAL_THREAD_API"
+                        " and LIBCXXABI_HAS_WIN32_THREAD_API cannot be both"
+                        " set to ON at the same time.")
+  endif()
   if (LIBCXXABI_BUILD_EXTERNAL_THREAD_LIBRARY)
     message(FATAL_ERROR "The options LIBCXXABI_BUILD_EXTERNAL_THREAD_LIBRARY"
                         " and LIBCXXABI_HAS_EXTERNAL_THREAD_API cannot be both"
                         " set to ON at the same time.")
   endif()
 endif()
 
+if (LIBCXXABI_HAS_PTHREAD_API)
+  if (LIBCXXABI_HAS_WIN32_THREAD_API)
+    message(FATAL_ERROR "The options LIBCXXABI_HAS_PTHREAD_API"
+            "and LIBCXXABI_HAS_WIN32_THREAD_API cannot be both"
+            "set to ON at the same time.")
+  endif()
+endif()
+
 if (LLVM_ENABLE_MODULES)
   # Ignore that the rest of the modules flags are now unused.
   add_compile_flags_if_supported(-Wno-unused-command-line-argument)
   add_compile_flags(-fno-modules)
 endif()
 
 set(LIBCXXABI_HAS_UNDEFINED_SYMBOLS OFF)
 if ((NOT LIBCXXABI_ENABLE_NEW_DELETE_DEFINITIONS)
else()
     string(REPLACE "-Wl,-z,defs" "" CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS}")
   endif()
 endif()
 
 if (LIBCXXABI_HAS_PTHREAD_API)
   add_definitions(-D_LIBCPP_HAS_THREAD_API_PTHREAD)
 endif()
 
+if (LIBCXXABI_HAS_WIN32_THREAD_API)
+  add_definitions(-D_LIBCPP_HAS_THREAD_API_WIN32)
+endif()
+
 if (LIBCXXABI_HAS_EXTERNAL_THREAD_API)
   add_definitions(-D_LIBCPP_HAS_THREAD_API_EXTERNAL)
 endif()
 
 if (LIBCXXABI_BUILD_EXTERNAL_THREAD_LIBRARY)
   add_definitions(-D_LIBCPP_HAS_THREAD_LIBRARY_EXTERNAL)
 endif()