If libc++abi library path is given, use it to reexport

[isuruf]

Summary

When building libc++ on OSX against a newer libc++abi library that is not a system library (Not in /usr/lib and not in CMAKE_OSX_SYSROOT), libc++ links against both the newer libc++abi and /usr/lib/libc++abi.dylib one. With this change, libc++ is not linked against a system library.

[ldionne]

Can you explain the problem that this change is solving?

I'll admit I don't fully understand the logic governing whether we re-export the system libc++abi symbols or use the export list: this area of the build is pretty messy.

[isuruf]

In D53797#1283049, @ldionne wrote:

Can you explain the problem that this change is solving?

Before this change libc++.dylib was linking to system libc++abi.dylib even though I gave LIBCXX_CXX_ABI_LIBRARY_PATH path to a newly built libc++abi.dylib

I'll admit I don't fully understand the logic governing whether we re-export the system libc++abi symbols or use the export list: this area of the build is pretty messy.

Me neither. I just wanted libc++.dylib to link to the given libc++abi.dylib instead of the system one.

[EricWF]

This LGTM. But adding @ldionne as this is an Apple specific change.

[ldionne]

This should be superseded by https://reviews.llvm.org/D54595. I don't think we ever want to re-export the current system's libc++abi -- we should always use an explicit list of exported symbols.

[isuruf]

Thanks

lib/CMakeLists.txt

if (NOT DEFINED LIBCXX_LIBCPPABI_VERSION)
     endif()
   endif()
 
   if ( CMAKE_OSX_DEPLOYMENT_TARGET STREQUAL "10.6" )
     message(FATAL_ERROR "Mac OSX 10.6 is not supported anymore as a deployment "
                         "target. If you need support for this, please contact "
                         "the libc++ maintainers.")
   else()
-    if (DEFINED CMAKE_OSX_SYSROOT AND NOT CMAKE_OSX_SYSROOT STREQUAL "")
+    if (DEFINED LIBCXX_CXX_ABI_LIBRARY_PATH AND NOT LIBCXX_CXX_ABI_LIBRARY_PATH STREQUAL "")
+        set (LIBCXX_OSX_REEXPORT_SYSTEM_ABI_LIBRARY_PATH "${LIBCXX_CXX_ABI_LIBRARY_PATH}")
+    elseif (DEFINED CMAKE_OSX_SYSROOT AND NOT CMAKE_OSX_SYSROOT STREQUAL "")
+        set (LIBCXX_OSX_REEXPORT_SYSTEM_ABI_LIBRARY_PATH "${CMAKE_OSX_SYSROOT}/usr/lib")
+    endif()
+    if (DEFINED LIBCXX_OSX_REEXPORT_SYSTEM_ABI_LIBRARY_PATH AND NOT LIBCXX_OSX_REEXPORT_SYSTEM_ABI_LIBRARY_PATH STREQUAL "")
       list(FIND CMAKE_OSX_ARCHITECTURES "armv7" OSX_HAS_ARMV7)
       if (NOT OSX_HAS_ARMV7 EQUAL -1)
         set(OSX_RE_EXPORT_LINE
-          "${CMAKE_OSX_SYSROOT}/usr/lib/libc++abi.dylib"
+          "${LIBCXX_OSX_REEXPORT_SYSTEM_ABI_LIBRARY_PATH}/libc++abi.dylib"
           "-Wl,-reexported_symbols_list,${CMAKE_CURRENT_SOURCE_DIR}/libc++sjlj-abi.exp")
       else()
         set(OSX_RE_EXPORT_LINE
-          "-Wl,-reexport_library,${CMAKE_OSX_SYSROOT}/usr/lib/libc++abi.dylib")
+          "-Wl,-reexport_library,${LIBCXX_OSX_REEXPORT_SYSTEM_ABI_LIBRARY_PATH}/libc++abi.dylib")
       endif()
     else()
       set(OSX_RE_EXPORT_LINE "/usr/lib/libc++abi.dylib -Wl,-reexported_symbols_list,${CMAKE_CURRENT_SOURCE_DIR}/libc++abi${LIBCXX_LIBCPPABI_VERSION}.exp")
       if (NOT LIBCXX_ENABLE_NEW_DELETE_DEFINITIONS)
         add_link_flags("/usr/lib/libc++abi.dylib -Wl,-reexported_symbols_list,${CMAKE_CURRENT_SOURCE_DIR}/libc++abi-new-delete.exp")
       endif()
     endif()
     add_link_flags(