Migrating pigweed.dev to Bazel#

I have been tasked with the fascinating and somewhat-overwhelming project of migrating pigweed.dev’s build system from GN to Bazel. These are my notes on the project.

Tue Sep 24 2024#

I wrote a design doc today summarizing the project and plan. Here’s the gist.

pigweed.dev is powered by Sphinx. Sphinx expects your docs to be structured under a root directory, like this:

.
├── a
│   └── a.rst
├── b
│   └── b.rst
├── c
│   └── c.rst
├── conf.py
└── index.rst

The directory containing conf.py and index.rst is the root directory. a/a.rst, b/b.rst, and c/c.rst are easy to integrate into the Sphinx system because they’re all in subdirectories relative to the root directory.

pigweed.dev on the other hand is structured like this:

.
├── a
│   └── a.rst
├── b
│   └── b.rst
├── c
│   └── c.rst
└── docs
    ├── conf.py
    └── index.rst

Some of our docs are in sibling directories relative to the root directory. Sphinx doesn’t like this.

Our GN build system mainly just gathers up all the documentation-related files that are spread across the Pigweed repo and packages them up into the directory structure that Sphinx expects. Then we just cd into that new directory and run Sphinx normally. This is the main functionality that I need to recreate in Bazel.

We’re going to use rules_python/sphinxdocs as the foundation of our Bazel-based build system. At first glance it seems to provide everything we need.

The main challenge of this project is that pigweed.dev is a fairly big docs site and it’s not easy to migrate to Bazel incrementally. For example, Sphinx has a wonderful cross-reference feature that guarantees that all internal links are correct. If you try to link to a doc that doesn’t exist, Sphinx treats this as an error and stops the build. There’s probably a way to downgrade this particular error to just a warning where Sphinx complains about the problem but continues building the site. My main point is that there are lots of things like this that make incremental migration difficult.

Fri Sep 24 2027#

Dependency Hell#

https://sourcegraphstatic.com/blog/nine-circles-of-dependency-hell.jpg

Credit: The Nine Circles of Dependency Hell#

Yesterday I made an unexpected visit to Dependency Hell.

rules_python/sphinxdocs is going to be the foundation of our Bazel-based build system. The Sphinx features in rules_python that I need are brand new. They were introduced in the last release, v0.36.0. Upgrading the Pigweed repo to v0.36.0 was a simple one-line change. But then all (dependency) hell broke loose. sphinx_build.py (the script in rules_python that starts the Sphinx build process) started having fatal errors around unexpected arguments. Eventually I figured out that rules_python v0.36.0 was developed against Sphinx v8, whereas we’re on Sphinx v7. So I tried updating the Pigweed repo to use Sphinx v8 and eventually hit a wall. Some of the Sphinx Extensions that we use in turn have dependencies on other PyPI libraries, and those dependencies don’t support Sphinx v8 yet. Luckily I figured out that I could just upgrade the Pigweed repo to the last v7 version that was released. We were on Sphinx v7.1.2 (released over a year ago); I updated us to Sphinx v7.4.7 (released two months ago). This unblocked me from using rules_python v0.36.0 within the Pigweed repo.

Once rules_python v0.36.0 started working I hit another snag:

kayce@kayce0 ~/p/pigweed (pw_docgen)> bazelisk build //pw_docgen/...
WARNING: Option 'remote_default_platform_properties' is deprecated: --remote_default_platform_properties has been deprecated in favor of --remote_default_exec_properties.
WARNING: Option 'remote_default_platform_properties' is deprecated: --remote_default_platform_properties has been deprecated in favor of --remote_default_exec_properties.
ERROR: /home/kayce/.cache/bazel/_bazel_kayce/9659373b1552c281136de1c8eeb3080d/external/rules_python++pip+python_packages/sphinx/BUILD.bazel:10:6: in alias rule @@rules_python++pip+python_packages//sphinx:pkg: cycle in dependency graph:
    //pw_docgen:_docs_html (8bf5217cd2199c4037e0ca161ab823102dd08eb93eeb7d4325e1d09666f3d863)
    //pw_docgen:_docs_html (2ccb10a0925868f764725cd74e410c636f5bbc18edb2d446d5c2ff1eb85e7e25)
    //pw_docgen:docs (2ccb10a0925868f764725cd74e410c636f5bbc18edb2d446d5c2ff1eb85e7e25)
    //pw_docgen:sphinx-build (bef2812dd1082aa39f3bcaa89343101b37ad5ee2cf77f8448d2ef909b33c5108)
    @@rules_python++pip+python_packages//pydata_sphinx_theme:pydata_sphinx_theme (bef2812dd1082aa39f3bcaa89343101b37ad5ee2cf77f8448d2ef909b33c5108)
    @@rules_python++pip+python_packages//pydata_sphinx_theme:pkg (bef2812dd1082aa39f3bcaa89343101b37ad5ee2cf77f8448d2ef909b33c5108)
    @@rules_python++pip+python_packages_311_pydata_sphinx_theme//:pkg (bef2812dd1082aa39f3bcaa89343101b37ad5ee2cf77f8448d2ef909b33c5108)
.-> @@rules_python++pip+python_packages//sphinx:pkg (bef2812dd1082aa39f3bcaa89343101b37ad5ee2cf77f8448d2ef909b33c5108)
|   @@rules_python++pip+python_packages_311_sphinx//:pkg (bef2812dd1082aa39f3bcaa89343101b37ad5ee2cf77f8448d2ef909b33c5108)
|   @@rules_python++pip+python_packages//sphinxcontrib_serializinghtml:pkg (bef2812dd1082aa39f3bcaa89343101b37ad5ee2cf77f8448d2ef909b33c5108)
|   @@rules_python++pip+python_packages_311_sphinxcontrib_serializinghtml//:pkg (bef2812dd1082aa39f3bcaa89343101b37ad5ee2cf77f8448d2ef909b33c5108)
`-- @@rules_python++pip+python_packages//sphinx:pkg (bef2812dd1082aa39f3bcaa89343101b37ad5ee2cf77f8448d2ef909b33c5108)
Target //pw_docgen:_docs/_sources up-to-date:
  bazel-bin/pw_docgen/_docs/_sources/pw_docgen/conf.py
  bazel-bin/pw_docgen/_docs/_sources/pw_docgen/index.rst
ERROR: Analysis of target '//pw_docgen:_docs_html' failed; build aborted
INFO: Elapsed time: 0.124s, Critical Path: 0.01s
INFO: 1 process: 1 internal.
ERROR: Build did NOT complete successfully

This problem was beyond my pay grade so I asked my resident Bazel expert, Ted Pudlik, for help. Ted pointed me to this issue: Circular dependencies between Sphinx and sphinxcontrib-*

And the rules_python docs on circular dependencies.

The fixes here were also pretty easy. I mainly just needed to upgrade sphinxcontrib-serializinghtml to v1.1.10 to eliminate the circular dependency. And then I had to bump the minor revisions on babel and pygments.

After that, my unexpected tour of Dependency Hell was finished (for now?) and I was able to proceed with my prototyping. Phew.

Mon Sep 30 2027#

We have a custom Sphinx extension that pulls in data from a file in a faraway directory. E.g. the script is at //pw_docgen/py/py_docgen/sphinx/modules_index.py and it needs data from //docs/module_metadata.json. In the GN build it’s easy to access the data file from modules_index.py:

with open(f'{os.environ["PW_ROOT"]}/docs/module_metadata.json', 'r') as f:

PW_ROOT gives you the absolute path to the Pigweed repo. Bazel on the other hand uses sandboxing so you can’t access absolute paths like this. Well, maybe it’s not related to sandboxing; I’m not sure about those details. All I know is that the simple approach that works in GN doesn’t work in our Bazel system.

The Bazel solution is also not too bad, but I definitely would not have figured it out without Ted’s help again. You just add the data files to the data list in your py_library rule and depend on bazel-runfiles:

py_library(
    # ...
    data = [
        "//:PIGWEED_MODULES",
        "//docs:module_metadata.json",
    ],
    # ...
    deps = [
        "@rules_python//python/runfiles",
    ],
)

And then you tweak your Python script so that it changes the path to the data files depending on whether the script is getting executed from a Bazel build or a GN build:

try:  # Bazel location for the data
    from python.runfiles import runfiles  # type: ignore
    r = runfiles.Create()
    modules_file = r.Rlocation('pigweed/PIGWEED_MODULES')
    r = runfiles.Create()
    metadata_file = r.Rlocation('pigweed/docs/module_metadata.json')
except ImportError:  # GN location for the data
    modules_file = f'{os.environ["PW_ROOT"]}/PIGWEED_MODULES'
    metadata_file = f'{os.environ["PW_ROOT"]}/docs/module_metadata.json'
with open(modules_file, 'r') as f:
    # The complete, authoritative list of modules.
    complete_pigweed_modules_list = f.read().splitlines()
with open(metadata_file, 'r') as f:
    # Module metadata such as supported languages and status.
    metadata = json.load(f)