[WIP] llvmPackages_{[12, 17] + git}.libcxx: re-export libcxxabi new, delete, exceptions

[ghost]

Fixes #166205

if patching to master need to apply #278945 first.

This change installs the list of new / delete / exception symbols from libcxxabi into $dev/share/libcxxabi/ and then imports them into the libcxx build where libcxx will re-export them.

on darwin this should fix having to link with libcxxabi when libcxx is already specified.

Note: if libcxxabi and libcxx were built together and not as separate components this shouldn't be necessary.

Description of changes

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 24.05 Release Notes (or backporting 23.05 and 23.11 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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[reckenrode]

Does only Darwin need this, or is it also needed by other platforms using libc++ (such as pkgsLLVM or FreeBSD)?

[ghost]

just darwin. APPLE must be defined for it to be used. the patch uses it in https://github.com/llvm/llvm-project/blob/bc82cfb38d83f1afeb2c290aa472c2e2e88919cb/libcxx/src/CMakeLists.txt#L233-L247 and the patched code looks like.

  # Maybe re-export symbols from libc++abi
  # In particular, we don't re-export the symbols if libc++abi is merged statically
  # into libc++ because in that case there's no dylib to re-export from.
  if (APPLE AND LIBCXX_CXX_ABI MATCHES "libcxxabi$"
            AND NOT DEFINED LIBCXX_OSX_REEXPORT_LIBCXXABI_SYMBOLS
            AND NOT LIBCXX_STATICALLY_LINK_ABI_IN_SHARED_LIBRARY)
    set(LIBCXX_OSX_REEXPORT_LIBCXXABI_SYMBOLS ON)
  endif()

  if (LIBCXX_OSX_REEXPORT_LIBCXXABI_SYMBOLS)
    target_link_libraries(cxx_shared PRIVATE
      "-Wl,-unexported_symbols_list,${CMAKE_CURRENT_SOURCE_DIR}/../lib/libc++unexp.exp"
      "-Wl,-reexported_symbols_list,${CMAKE_CURRENT_SOURCE_DIR}/../lib/libc++abi.exp"
      "-Wl,-force_symbols_not_weak_list,${CMAKE_CURRENT_SOURCE_DIR}/../lib/notweak.exp"
      "-Wl,-force_symbols_weak_list,${CMAKE_CURRENT_SOURCE_DIR}/../lib/weak.exp")

+    if (NOT LIBCXX_ENABLE_NEW_DELETE_DEFINITIONS
+        AND DEFINED LIBCXXABI_REEXPORT_NEW_DELETE)
+      target_link_libraries(cxx_shared PRIVATE
+              "-Wl,-reexported_symbols_list,${LIBCXXABI_REEXPORT_NEW_DELETE}")
+    endif()
+    if (DEFINED LIBCXXABI_REEXPORT_EXCEPTIONS)
+      target_link_libraries(cxx_shared PRIVATE
+              "-Wl,-reexported_symbols_list,${LIBCXXABI_REEXPORT_EXCEPTIONS}")
+    endif()

    target_link_libraries(cxx_shared PRIVATE $<TARGET_NAME_IF_EXISTS:cxxabi-reexports>)
  endif()

[ghost]

Does only Darwin need this, or is it also needed by other platforms using libc++ (such as pkgsLLVM or FreeBSD)?

I haven't tried pkgsLLVM or FreeBSD but we should provide whatever experience the provided system tools do. i've read that they still require libc++abi but random internet sources are not reliable. however, if they are true just fixing this for darwin seems sufficient as that will provide the same development / build experience using nixpkgs tools or the system tools.

[reckenrode]

My understanding is FreeBSD doesn’t use libc++abi for its ABI library. I don’t know what FreeBSD in nixpkgs does, but I assume it intends to use the same one (libcxxrt).

https://wiki.freebsd.org/NewC%2B%2BStack

[reckenrode]

As for Linux (pkgsLLVM), I assume it uses libc++abi. I’ll kick off a build on a Linux system with my branch (featuring this PR with the workarounds for #166205 removed) to see whether it needs any of its own patches.

[reckenrode]

As discussed on Matrix, I applied this PR to staging and reverted all workarounds for #166205. After fixing a few unrelated issues (see #282764 and #282777), I was able to build the following flake successfully on Darwin (see below gist).

• Testing branch: https://github.com/reckenrode/nixpkgs/tree/cc-wrapper-libcxx
• Gist: https://gist.github.com/reckenrode/b2fd338cc3b07096d13ea57360eeaf8d

[ghost]

I am likely going to implement this change differently -- eg: build libcxx and libcxxabi together -- but since the changes to llvm 12-17 + git are tested and verified to work (simple smoke test of compiling + linking a hello-world.cxx for all except for llvm 16 which was tested more thoroughly by @reckenrode) i figure they can be posted.