Libtool is called with wrong name on Darwin if paths are absolute

[depp]

Libtool is called as libtool instead of llvm-libtool-darwin if you use an LLVM that is installed locally on your system, on Darwin.

How to Reproduce

• On a Mac,
• Install Clang+LLVM at some location on your filesystem (in my example, /opt/llvm-18),
• Use llvm_toolchain with the toolchain root set to that path,
• Build a static library.

Here is a Gist that can reproduce the issue: https://gist.github.com/depp/712aea59c1699c19a658852db53511a1

1. Install LLVM + Clang 18.1.8 at /opt/llvm-18.
2. Run bazel build //:example in the Gist.

What hapens

Bazel will invoke /opt/lvm-18/bin/libtool. This fails, because libtool should be llvm-libtool-darwin.

Linking libexample.a failed: (Exit 1): libtool failed: error executing CppArchive command (from target //:example) /opt/llvm-18/bin/libtool @bazel-out/darwin_arm64-fastbuild/bin/libexample.a-2.params

What causes this?

I see in toolchain/internal/configure.bzl, this code:

def llvm_config_impl(rctx):
    ...
    if not use_absolute_paths_llvm:
        ...
        for tool_name, symlink_name in tools.items():
            rctx.symlink(llvm_dist_rel_path + "bin/" + tool_name, tools_path_prefix + symlink_name)
        ...
    else:
        ...

In use_absolute_paths_llvm is false, then the libtool symlink is created. However, symlinks are not created if the paths are absolute. Unfortunately, this means that libtool is not symlinked to llvm-libtool-darwin.

This is as far as I got. I think the correct solution here is probably to call llvm-libtool-darwin directly, rather than symlinking it.

Workaround

ln -s llvm-libtool-darwin /opt/llvm-18/bin/libtool

Or, you know, don’t use a local version of LLVM. But this is a difficult approach—the LLVM + Clang download is large (1 GB) and seems to fail (maybe just due to its size), and the extracted archive is over 5 GB.