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Modernize Sphinx docs toolchain, add RTD build, and expand API docs#1089

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Modernize Sphinx docs toolchain, add RTD build, and expand API docs#1089
adcroft wants to merge 10 commits intoNOAA-GFDL:dev/gfdlfrom
adcroft:fixes-sphinx-readthedocs

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@adcroft adcroft commented Apr 19, 2026

Modernizes the docs build and adds new capabilities.

  • Toolchain upgrade (pieces 1–4 of Notes-sphinx-upgrade.md) — replaces four stale 2020-era forks
    (sphinxcontrib-autodoc_doxygen, sphinx, sphinx-fortran, flint) that had pinned the build to Sphinx 3.2.1 and a growing
    list of transitive version ceilings. End state: stock Sphinx 8 from PyPI, upstream VACUMM sphinx-fortran at a pinned
    commit, autodoc_doxygen vendored in-tree at docs/_ext/ (expanding the original additions from jr3cermak). Two small
    compatibility gaps are handled by commented monkey-patches in conf.py with TODO markers for upstream PRs: i) a math-wrap behavior from the sphinx fork and ii) a parallel-merge bug in upstream sphinx-fortran.
  • Read the Docs build: .readthedocs.yml overrides RTD's default sphinx runner with sphinx-build -M html -j auto (RTD's
    sphinx: key has no parallelism option); html_static_path is enabled so custom CSS actually deploys; doxygen_xml paths
    are resolved against app.confdir so they work under RTD's cwd.
  • API documentation improvements: Fortran type declarations now appear on function/subroutine parameters and
    derived-type members; [source] links moved below the description; a bug that dropped functions (vs subroutines) from
    the sphinx-fortran domain is fixed; new Functions and Source Files index pages; upstream CVMix sources excluded from
    doxygen input.
  • Source code browser: one HTML page per Fortran source file with syntax highlighting and clickable cross-references,
    bi-directionally linked with the API pages. Includes a multi-line LaTeX math indentation fix that dropped LaTeX errors
    from 164 to 10 on the Notation page.
  • Performance fixes in the vendored extension: two O(N²) XPath scans of the merged doxygen tree (1230 files, 109 MB):
    scanNode used // where it should have used .// (6m33s → 48.8s); visit_ref linearly scanned all elements to resolve
    each , replaced with a lazy id→element dict (911s → 676s serial). End-to-end build with the new source browser is
    115s parallel.

Commits are structured so each piece stands alone but this PR can be squashed. I have the necessary commits to create PRs for sphinx-fortran and sphinx which will later let us remove the patches in conf.py.

Testing:

  • make html succeeds (122 warnings, 48 f-modules, 538 f-objects, f-modindex.html generated, 36 cross-ref links on
    mom.html)
  • make latexpdf produces ≤10 LaTeX errors (down from 164) and 0 double-wrapped equation environments
  • RTD build succeeds on the PR preview and shows the source browser pages, Functions index, and Source Files index
  • Clicking an identifier in a source-browser page navigates to its API entry, and [source] links on API entries
    navigate back
  • Fresh-venv install from docs/requirements.txt resolves with no manual intervention

There's no way I could have fixed this in a short time: this PR was mostly debugged and implemented by AI.

Todo:

  • Some doxygen errors and formatting problems have crept into the MOM6 source while the readthedocs website has been out of date.

adcroft added 10 commits April 19, 2026 15:12
This is piece 1 of the sphinx toolchain upgrade.  The docs build
previously depended on four forks which haven't been updated since 2020.

The chain pinned the entire build to Sphinx 3.2.1, requiring a growing
list of transitive version ceilings (jinja2<3.1, sphinxcontrib_*<1.0.x,
alabaster<0.7.14, setuptools<82).

This commit handles the sphinxcontrib-autodoc_doxygen dependency. The
upstream (rmcgibbo) has been dormant since June 2021; the fork was
effectively a rewrite (~90% of xmlutils.py changed, both documenters
replaced, the whole domain switched from cpp to f). Monkey-patching
the upstream package was rejected because there was nothing stable to
patch against. Instead the fork's code is vendored in-tree at
docs/_ext/autodoc_doxygen/ so it can be debugged, blamed, and edited
like any other project source.

Changes vs the fork at tag 0.7.13:

 - Renamed doxynamespace.rst template to doxymodule.rst and dropped
   the unused C++ doxyclass.rst (DoxygenClassDocumenter was already
   unregistered in the fork).
 - Removed ~80 lines of dead code: commented-out pdb.set_trace lines,
   the unregistered DoxygenClassDocumenter class, the dead
   visit_ref_angus alternative, _import_by_name_original, and the
   unused try-import of flint.
 - Dropped Python 2 compatibility cruft: from __future__ imports,
   from six import itervalues (replaced with dict.values()).
 - Ported four Sphinx 3 -> 8 API changes:
     * DoxygenAutosummary.get_items now passes self.bridge (the
       DocumenterBridge) to documenter constructors instead of self,
       so Documenter.__init__ finds directive.genopt.
     * self.warn / self.directive.warn calls (removed in Sphinx 8)
       replaced with sphinx.util.logging.getLogger(...).warning.
     * Wrapped env.doc2path in os.fspath to silence the
       RemovedInSphinx90Warning about str paths.

docs/conf.py: prepend _ext to sys.path and rename the extension in
the extensions list from sphinxcontrib.autodoc_doxygen to
autodoc_doxygen.

docs/conf.py also carries a monkey-patch for a parallel-build bug in
upstream VACUMM/sphinx-fortran's FortranDomain.merge_domaindata. The
upstream method references an undefined `outNames` (typo for
`ourNames`) and also unpacks the wrong tuple shape from modules and
objects dicts. With -j > 1 the merge silently fails and the f domain
ends up empty, losing every Fortran cross-reference. The patch is
heavily commented with a TODO to submit an upstream PR and drop the
workaround during piece 2 of the upgrade.

docs/requirements.txt: drop the sphinxcontrib-autodoc_doxygen git
dependency. The other three forks remain pending pieces 2, 3, 4.

Validation: built against a new venv.sph8 (Python 3.11 + Sphinx 8.2.3 +
sphinx-rtd-theme 3.1.0 + sphinxcontrib-bibtex 2.6.5 + upstream
VACUMM/sphinx-fortran master). `make html` produces 220 warnings vs
224 on the Sphinx 3 baseline. The f domain indexes 48 modules and
538 objects. Equation pages are effectively byte-identical (off only
by Sphinx 8's section-id hoist optimization and the pilcrow-as-CSS
theme change). Module pages are slightly larger because more
cross-references now resolve than in the baseline build.
Piece 2 of the sphinx toolchain upgrade (see Notes-sphinx-upgrade.md).

The jr3cermak/sphinx-fortran@1.2.2 fork existed to add Sphinx 3 API
compatibility in 2020. Upstream VACUMM/sphinx-fortran has continued
evolving since then (Sphinx logging API migration, parallel read
support, modern setuptools, removal of the future dependency) and now
works with Sphinx 8 directly. Upstream has not cut a PyPI release past
1.1.1 so we pin to a specific master commit for reproducibility.

The pinned commit ships a broken FortranDomain.merge_domaindata that
loses every f-domain object in parallel builds; the monkey-patch in
conf.py from the previous commit handles that. We will revisit that
patch and the pin together once an upstream PR is merged (see the
TODO(piece-2) marker in conf.py setup).

Validation: rebuilt docs with the pinned commit installed into
venv.sph8. `make html` produces 220 warnings (identical to the
pre-swap sph8 build), the f domain indexes 48 modules and 538 objects,
mom.html has 36 internal cross-reference links, and f-modindex.html is
generated. No behavior change from the sph8 build validated in the
previous commit.
Piece 3 of the docs toolchain upgrade plan in Notes-sphinx-upgrade.md.
This commit removes the last need for the patched sphinx fork and
collapses the long list of transitive version ceilings it forced.

Changes:

 - requirements.txt: drop git+https://github.com/jr3cermak/sphinx.git
   @v3.2.1mom6.4. Pin sphinx>=8,<9 from PyPI. Drop the eight
   ceiling lines (jinja2<3.1, sphinxcontrib_*<1.0.x, alabaster<0.7.14,
   setuptools<82.0.0) and the workaround comment around sphinx-fortran's
   broken requirements.txt; those existed only because Sphinx 3.2.1 dragged
   them in. Add lxml as an explicit dep (it is required by the vendored
   _ext/autodoc_doxygen extension to parse Doxygen XML). Keep six because
   the pinned sphinx-fortran commit imports it at module load time.

 - conf.py: add a monkey-patch for sphinx.util.math.wrap_displaymath that
   replicates the functional changes in the jr3cermak fork. The patch
   detects parts containing `\begin{equation}`, `\begin{eqnarray}`, or
   `\begin{align}` and emits them verbatim (no outer
   `\begin{equation}\begin{split}...\end{split} \end{equation}` wrapping).
   Without this, MOM6's math-heavy sources produce nested LaTeX
   environments that pdflatex chokes on. The patch is heavily commented
   with the rationale and a TODO marker for a possible upstream
   contribution. Verified by unit-testing the function directly (plain
   math still wrapped, explicit-environment math passed through verbatim)
   and by inspecting the generated _build/latex/ MOM6.tex: 0 double-wrapped
   equation/eqnarray/align blocks, 100 + 18 + 17 explicit environments
   preserved as top-level blocks.

 - conf.py: tighten exclude_patterns. Sphinx walks the entire source
   tree by default, which previously caused it to descend into local
   virtualenvs (`venv.sph3/`, `venv.sph8/`, `venv-3.11/`, etc.) and
   pick up LICENSE.rst, README.rst, and stray autosummary template
   files from inside site-packages. Each of those generated a "document
   isn't included in any toctree" warning and a corresponding stray
   .html file in the build output. The new exclusions cover venv*,
   venv-*, _build.*, and _ext/*/templates. Final build: 90 HTML files
   (down from 148, the lost 58 were all venv junk and template
   scaffolding) and 122 warnings (down from 220, with the same project-
   internal warnings as before plus the venv noise removed).

 - Makefile: add the equation post-processing hook to the html target,
   gated on UPDATEHTMLEQS=Y. The jr3cermak/sphinx fork carried this
   as a patch to sphinx/cmd/build.py so it ran after every sphinx-build
   invocation. Now invoked from the Makefile so it works against
   stock upstream Sphinx. The existing nortd target already had the
   same hook with different arguments (-p APIs -b doxygen vs -p html
   -b sphinx).

The forked sphinx-fortran fix and the upstream PR for the math wrapping
change are deferred to a follow-up.

Validation: built from a fresh venv (venv.sph8.fresh) created by
`python3.11 -m venv` and `pip install -r requirements.txt`. The
install resolves cleanly with no manual intervention. `make html`
succeeds with 122 warnings, indexes 48 modules and 538 f-domain
objects, generates f-modindex.html, and produces 36 internal cross-
reference links on mom.html. `make html UPDATEHTMLEQS=Y` runs the
post-processing script successfully. `make latex` produces 0
double-wrapped math environments.
Piece 4 of the docs toolchain upgrade plan in Notes-sphinx-upgrade.md.
With this commit the dependencies in docs/requirements.txt are only on
maintained upstream sources.

flint (marshallward/flint, vendored as jr3cermak/flint at 0.0.1) was
historically pulled into the docs build with the intent of patching
up Doxygen's incomplete parsing of Fortran functions with `result()`
clauses. The fork's sphinxcontrib-autodoc_doxygen imported it at
module load time but never actually called it; the import was a
no-op behind a `try: import flint; except: pass`. We removed that
import as part of piece 1's dead-code cleanup, so the vendored
extension no longer touches flint at all.

Verified empirically: uninstalled flint from venv.sph8.fresh and
rebuilt. The build is byte-identical to the previous one with flint
installed (122 warnings, 48 f-modules, 538 f-objects, 36 internal
cross-reference links on mom.html, f-modindex.html present, 90 html
files). Nothing in the current pipeline depends on flint.

The two surviving "flint" mentions in docs/_ext/autodoc_doxygen/ are
both in comments describing the original intent, not live code; they
are left in place as historical context. If a future enhancement
wants to actually fix Doxygen's result() clause parsing, that should
be a separate piece of work and would not necessarily resurrect
flint specifically.

docs/README.md is updated to drop the flint bullet from the
requirements list and the flint row from the credits table. Other
stale entries in that section (sphinxcontrib_autodox-doxygen, the
sphinx and sphinx-fortran fork rows) remain and will need a broader
README sweep in a follow-up.
Consolidates four small fixes that followed the Sphinx 8 upgrade:

- Switch all Makefile targets from `sphinx-build -b` to `-M` for
  consistent parallel build support.
- Update docs/README.md to match the modernized toolchain (remove
  references to the four sphinx* forks, update requirements and
  credits sections).
- Fix crash in autodoc_doxygen visit_image on empty <image>
  elements (node.text is None when doxygen produces <image/>
  without caption text).
- Add __pycache__ and *.pyc to docs/.gitignore so the vendored
  extension's bytecode does not appear in git status.
Consolidates four fixes for the Read the Docs build environment:

- Override RTD's default sphinx runner with a build.jobs.build.html
  entry in .readthedocs.yml that runs sphinx-build -M html -j auto,
  since RTD's high-level sphinx: key has no parallelism option.
- Make doxygen_xml path resolution robust to cwd changes: resolve
  relative paths against app.confdir rather than the ambient cwd,
  which differs between local builds and RTD.
- Switch Makefile html target from -j 4 to -j auto to match RTD.
- Enable html_static_path = ['_static'] in conf.py so custom CSS
  files (autodoxysource.css) are copied into the build output.
  Previously commented out, which meant app.add_css_file() added
  the <link> tag but the file was never deployed.
Two instances of the same class of bug in the vendored
autodoc_doxygen extension, both scanning the entire merged doxygen
XML tree (1230 files, 109 MB with programlisting) on every call:

1. scanNode used `node.xpath('//latexonly')` etc. In XPath, `//`
   starts from the document root, not from `node`. Since the
   extension merges all XML into one tree, every call scanned
   the whole tree. Fix: `//` -> `.//` (descendant-of-self).
   Recovery: 6m33s -> 48.8s wall clock at full input scale.

2. visit_ref used `get_doxygen_root().findall('.//*[@id=X]')` to
   resolve each prose <ref> by linearly scanning every element in
   the merged tree. With XML_PROGRAMLISTING=YES the tree tripled
   in size, making this the dominant cost (250s of 911s serial).
   Fix: lazy {id: element} dict built once on first use, O(1)
   lookup thereafter. Recovery: 911s -> 676s serial.
Generate one HTML page per Fortran source file with syntax
highlighting and clickable cross-references. Clicking an
identifier in the source jumps to its API documentation page;
each API entry gets a [source] link back to the source listing.

Implementation:
- Enable XML_PROGRAMLISTING in Doxyfile_rtd (standalone commit
  originally, now folded in).
- New autodoxysource directive in _ext/autodoc_doxygen/ that walks
  doxygen's <programlisting> XML, emitting per-line anchors,
  CSS-classed highlight spans, and pending_xref nodes for
  identifiers.
- Stub generator produces 329 :orphan: source pages under
  api/generated/source/.
- [source] links added to DoxygenMethodDocumenter,
  DoxygenTypeDocumenter, and DoxygenModuleDocumenter.
- CSS styled to match the sphinx_rtd_theme's code blocks (font
  stack, size, colors, line gutter).
- Node count optimization: coalesce text runs in _walk_highlight
  (10-30 nodes/line -> 3-5), set support_smartquotes=False on the
  source container to skip the smartquotes transform. Combined
  with the visit_ref fix, total build time with the source browser
  is 115s parallel (vs 360s before optimization).
Consolidates five improvements to the rendered API pages:

- Show Fortran type declarations for function/subroutine
  parameters and derived-type members. Types are rendered as
  inline code (e.g. ``real, intent(in), optional``) by looking
  up the <param> elements on the parent <memberdef>.
- Move [source] links from the top of each entry to the bottom,
  so the description and parameters appear first.
- Fix functions (as opposed to subroutines) not being registered
  in the sphinx-fortran domain. format_name() was prepending the
  return type (e.g. "real") which sphinx-fortran's f_sig_re regex
  cannot parse, leaving function entries without anchors.
- Add Functions and Source Files index pages to the API Reference,
  with cross-reference links to module pages and source browser
  pages respectively.
- Exclude upstream CVMix sources from doxygen input to avoid
  warnings from undocumented third-party code.
Multi-line math content (e.g. \begin{align}...\end{align}) from
doxygen XML was emitted as:

    .. math:: \begin{align}
    \mathbf{v}
      = \mathbf{u} + ...
     \end{align}

RST requires directive body content to be indented. The
continuation lines at column 0 were parsed as regular text by
docutils, producing garbled LaTeX output with "Runaway argument"
and "Missing $" errors — 164 LaTeX errors on the Notation page
alone.

Fix: in visit_formula, detect multi-line math text and emit it
as a properly indented block:

    .. math::

       \begin{align}
       \mathbf{v}
         = \mathbf{u} + ...
       \end{align}

Single-line math is unchanged (stays on the .. math:: line).

After fix: `make latexpdf` LaTeX errors drop from 164 to 10.
The remaining 10 are unrelated (9 Unicode Greek characters in
source comments that pdflatex cannot render, 1 duplicate label).
@adcroft adcroft added the documentation Improvements or additions to documentation label Apr 19, 2026
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codecov bot commented Apr 19, 2026

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 37.31%. Comparing base (c925265) to head (d5bc339).

Additional details and impacted files
@@             Coverage Diff              @@
##           dev/gfdl    #1089      +/-   ##
============================================
- Coverage     38.76%   37.31%   -1.45%     
============================================
  Files           274      263      -11     
  Lines         92265    91065    -1200     
  Branches      17823    17795      -28     
============================================
- Hits          35765    33984    -1781     
- Misses        49796    50357     +561     
- Partials       6704     6724      +20     

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