🤖 Set build config via Sphinx ext

This is an initial change allowing for restructuring how the config
settings are computed and putting that logic into a dedicated Sphinx
extension. The idea is that this extension may have multiple callbacks
that set configuration for Sphinx based on tags and the environment
state.

As an example, this in-tree extension implements setting the `is_eol`
variable in Jinja2 context very early in Sphinx life cycle.
It does this based on inspecting the state of current Git checkout as
well as reading a config file listing EOL and supported versions of
`ansible-core`.

The configuration format is TOML as it's gained a lot of the ecosystem
adoption over the past years and its parser made its way into the
standard library of Python, while PyYAML remains a third-party
dependency.

Supersedes ansible#2251.

docs/docsite/_ext/build_context.py

@@ -0,0 +1,110 @@
+"""Sphinx extension for setting up the build settings."""
+
+import subprocess
+from dataclasses import dataclass
+from functools import cache
+from pathlib import Path
+from tomllib import loads as parse_toml_string
+from typing import Literal
+
+from sphinx.application import Sphinx
+from sphinx.util import logging
+
+
+logger = logging.getLogger(__name__)
+
+
+DOCSITE_ROOT_DIR = Path(__file__).parents[1].resolve()
+DOCSITE_EOL_CONFIG_PATH = DOCSITE_ROOT_DIR / 'end_of_life.toml'
+
+
+@dataclass(frozen=True)
+class Distribution:
+    end_of_life: list[str]
+    supported: list[str]
+
+    @classmethod
+    def from_dict(cls, raw_dist: dict[str, list[str]]) -> 'Distribution':
+        return cls(
+            **{
+                kind.replace('-', '_'): versions
+                for kind, versions in raw_dist.items()
+            },
+        )
+
+
+EOLConfigType = dict[str, Distribution]
+
+
+@cache
+def _read_eol_data() -> EOLConfigType:
+    raw_config_dict = parse_toml_string(DOCSITE_EOL_CONFIG_PATH.read_text())
+
+    return {
+        dist_name: Distribution.from_dict(dist_data)
+        for dist_name, dist_data in raw_config_dict['distribution'].items()
+    }
+
+
+@cache
+def _is_eol_build(git_branch: str, kind: str) -> bool:
+    return git_branch in _read_eol_data()[kind].end_of_life
+
+
+@cache
+def _get_current_git_branch():
+    git_branch_cmd = 'git', 'rev-parse', '--abbrev-ref', 'HEAD'
+
+    try:
+        return subprocess.check_output(git_branch_cmd, text=True).strip()
+    except subprocess.CalledProcessError as proc_err:
+        raise LookupError(
+            f'Failed to locate current Git branch: {proc_err !s}',
+        ) from proc_err
+
+
+def _set_global_j2_context(app, config):
+    if 'is_eol' in config.html_context:
+        raise ValueError(
+            '`is_eol` found in `html_context` unexpectedly. '
+            'It should not be set in `conf.py`.',
+        ) from None
+
+    dist_name = (
+        'ansible-core' if app.tags.has('core')
+        else 'ansible' if app.tags.has('ansible')
+        else None
+    )
+
+    if dist_name is None:
+        return
+
+    try:
+        git_branch = _get_current_git_branch()
+    except LookupError as lookup_err:
+        logger.info(str(lookup_err))
+        return
+
+    config.html_context['is_eol'] = _is_eol_build(
+        git_branch=git_branch, kind=dist_name,
+    )
+
+
+def setup(app: Sphinx) -> dict[str, bool | str]:
+    """Initialize the extension.
+
+    :param app: A Sphinx application object.
+    :returns: Extension metadata as a dict.
+    """
+
+    # NOTE: `config-inited` is used because it runs once as opposed to
+    # NOTE: `html-page-context` that runs per each page. The data we
+    # NOTE: compute is immutable throughout the build so there's no need
+    # NOTE: to have a callback that would be executed hundreds of times.
+    app.connect('config-inited', _set_global_j2_context)
+
+    return {
+        'parallel_read_safe': True,
+        'parallel_write_safe': True,
+        'version': app.config.release,
+    }

docs/docsite/end_of_life.toml

@@ -0,0 +1,25 @@
+[distribution.ansible]
+end-of-life = [
+  'stable-2.15',
+  'stable-2.14',
+  'stable-2.13',
+]
+supported = [
+  'devel',
+  'stable-2.18',
+  'stable-2.17',
+  'stable-2.16',
+]
+
+[distribution.ansible-core]
+end-of-life = [
+  'stable-2.15',
+  'stable-2.14',
+  'stable-2.13',
+]
+supported = [
+  'devel',
+  'stable-2.18',
+  'stable-2.17',
+  'stable-2.16',
+]

docs/docsite/rst/conf.py

@@ -18,6 +18,17 @@
 
 import sys
 import os
+from pathlib import Path
+
+
+DOCS_ROOT_DIR = Path(__file__).parent.resolve()
+
+
+# Make in-tree extension importable in non-tox setups/envs, like RTD.
+# Refs:
+# https://github.com/readthedocs/readthedocs.org/issues/6311
+# https://github.com/readthedocs/readthedocs.org/issues/7182
+sys.path.insert(0, str(DOCS_ROOT_DIR.parent / '_ext'))
 
 # If your extensions are in another directory, add it here. If the directory
 # is relative to the documentation root, use os.path.abspath to make it
@@ -65,6 +76,9 @@
     'notfound.extension',
     'sphinx_antsibull_ext',  # provides CSS for the plugin/module docs generated by antsibull
     'sphinx_copybutton',
+
+    # In-tree extensions:
+    'build_context',  # computes build settings for env context
 ]
 
 # Later on, add 'sphinx.ext.viewcode' to the list if you want to have
@@ -227,7 +241,6 @@
 html_context = {
     'display_github': 'True',
     'show_sphinx': False,
-    'is_eol': False,
     'github_user': 'ansible',
     'github_repo': 'ansible-documentation',
     'github_version': 'devel',