feat: also export lib symlink from python host repository

[DoDoENT]

This PR adds lib to the list of exported symlinks in the generated host toolchain. This is useful when a Python host interpreter is used as a script runner during the build of other toolchains, where simple py_binaries cannot be used.

An example where I need this can be seen in this emscripten issue.

If you think there is a better way of solving this problem, please let me know.

[gemini-code-assist]

Summary of Changes

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This pull request enhances the Python host toolchain generation by including the lib symlink in the exported files. This change facilitates scenarios where a Python host interpreter is utilized to execute scripts during the build process of other toolchains, providing necessary library access for such operations.

Highlights

• Exported lib symlink: The lib symlink is now exported from the generated Python host toolchain, making it accessible for use cases where a Python host interpreter acts as a script runner during other toolchain builds.

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Changelog

• python/private/toolchains_repo.bzl
  • Added 'lib' to the list of files exported by the host toolchain build content.

Activity

• No specific activity (comments, reviews, or progress updates) has been recorded for this pull request yet.

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Code Review

The pull request adds lib to the list of exported symlinks in the generated host toolchain, which is useful when a Python host interpreter is used as a script runner during the build of other toolchains. The change is minimal and directly addresses the described use case.

[rickeylev]

I'm pretty sure this isn't what you want to do. I don't think it work reliably, either, since "lib" is a directory, and passing those around directly isn't well supported by Bazel.

The @python_3_13_host//:python target is only appropriate for usage during the repository phase. The "host" it refers to working with is the one Bazel is running on.

It more sounds like your case is running a build tool during the normal build phase, i.e. ctx.actions.run. For that to be sound, it needs to find python through toolchain resolution, which ensures a python compatible with where bazel chooses to run the tool.

Try doing something like done here:

rules_python/python/private/common.bzl

Lines 607 to 610 in 589c7ee

	exec_runtime = ctx.toolchains[EXEC_TOOLS_TOOLCHAIN_TYPE].exec_tools.exec_runtime
	if exec_runtime.interpreter:
	action_exe = exec_runtime.interpreter
	action_inputs.add(exec_runtime.files)

• Lookup up the exec runtime via toolchain resolution (L605)
• Add the interpreter and its files to the action inputs (L610)