fix: prevent a 404 error when serving Sphinx docs and Bazel is configured with a --symlink_prefix option

[laurenshobert]

When Bazel is configured with the --symlink_prefix option, the sphinxdocs .serve target fails to serve files from the correct directory. This happens because the directory layout under execroot and the workspace no longer match, causing the target to miss the generated HTML files.

This change updates the logic to use rpathlocation instead, ensuring the correct path is resolved regardless of symlink configuration.

Fixes #3410

[gemini-code-assist]

Summary of Changes

Hello @laurenshobert, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a critical issue where serving Sphinx documentation via Bazel's .serve target would fail with a 404 error when the --symlink_prefix flag was enabled. The core problem stemmed from incorrect path resolution within Bazel's execution environment. The changes introduce the use of Bazel's rlocationpath and the Runfiles API to robustly locate generated HTML files, ensuring that the documentation server correctly identifies and serves the content, thereby enhancing compatibility and reliability for users employing various Bazel configurations.

Highlights

• Fix for Sphinx Docs Serving: Resolved a 404 error that occurred when serving Sphinx documentation with Bazel configured with the --symlink_prefix option, ensuring correct file resolution.
• Path Resolution Update: The sphinxdocs .serve target now uses $(rlocationpath) instead of $(execpath) for argument resolution, which correctly handles paths regardless of Bazel's symlink configuration.
• Runfiles API Integration: The sphinx_server.py script has been updated to utilize the python.runfiles.Runfiles API to determine the correct serving directory, replacing direct environment variable access.
• Dependency Addition: A dependency on //python/runfiles has been added to the sphinx.bzl rule to support the new Runfiles API usage.

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[gemini-code-assist]

Code Review

This pull request correctly fixes an issue with the sphinxdocs .serve target when Bazel's --symlink_prefix is used. The changes are robust, replacing a fragile path construction with the idiomatic use of rlocationpath and the python.runfiles library. The implementation is solid. I've added one suggestion to improve error handling in sphinx_server.py to make it even more robust.

[aignas]

Thanks!

[shayanhoshyari]

Not related to this PR, just some notes that others might find interesting...

Looking at this PR, I ended up going down a rabbit hole of why Bazel has so amny different $(...) macros for paths https://bazel.build/reference/be/make-variables#predefined_label_variables : $(location), $(rlocationpath), $(rootpath), $(execpath).

Digging turned out:

• First there was just location, but it turned out problematic as there was no way to distinguish when runfile path is needed vs. when path under workspace: $(location) should use runtime path when used in jvm_flags attribute bazelbuild/bazel#2475 (comment), so execpath, rootpath were added. The names are not really helpful to tell one which is which.
• Then when bazel mod came out, the existence of external repos broke things + workspace, and they had to add rlocationpath: Runfiles workspace directories use wrong workspace name if repository renaming is in effect bazelbuild/bazel#15029. Though onces workspaces go away I think we have rlocation path = os.path.join(_main, rootpath)

Hindsight is 20/20 😅

[schenker]

Thanks!