Clean up the CNAME file in sphinx.ext.githubpages (#11295)

Co-authored-by: Adam Turner <9087854+aa-turner@users.noreply.github.com>

sphinx/ext/githubpages.py

@@ -3,27 +3,52 @@
 from __future__ import annotations
 
 import os
-import urllib
+import urllib.parse
 from typing import Any
 
 import sphinx
 from sphinx.application import Sphinx
 from sphinx.environment import BuildEnvironment
 
 
+def _get_domain_from_url(url: str) -> str:
+    """Get the domain from a URL."""
+    return url and urllib.parse.urlparse(url).hostname or ''
+
+
 def create_nojekyll_and_cname(app: Sphinx, env: BuildEnvironment) -> None:
-    if app.builder.format == 'html':
-        open(os.path.join(app.builder.outdir, '.nojekyll'), 'wb').close()
-
-        html_baseurl = app.config.html_baseurl
-        if html_baseurl:
-            domain = urllib.parse.urlparse(html_baseurl).hostname
-            if domain and not domain.endswith(".github.io"):
-                with open(os.path.join(app.builder.outdir, 'CNAME'), 'w',
-                          encoding="utf-8") as f:
-                    # NOTE: don't write a trailing newline. The `CNAME` file that's
-                    # auto-generated by the Github UI doesn't have one.
-                    f.write(domain)
+    """Manage the ``.nojekyll`` and ``CNAME`` files for GitHub Pages.
+
+    For HTML-format builders (e.g. 'html', 'dirhtml') we unconditionally create
+    the ``.nojekyll`` file to signal that GitHub Pages should not run Jekyll
+    processing.
+
+    If the :confval:`html_baseurl` option is set, we also create a CNAME file
+    with the domain from ``html_baseurl``, so long as it is not a ``github.io``
+    domain.
+
+    If this extension is loaded and the domain in ``html_baseurl`` no longer
+    requires a CNAME file, we remove any existing ``CNAME`` files from the
+    output directory.
+    """
+    if app.builder.format != 'html':
+        return
+
+    open(os.path.join(app.builder.outdir, '.nojekyll'), 'wb').close()
+    cname_path = os.path.join(app.builder.outdir, 'CNAME')
+
+    domain = _get_domain_from_url(app.config.html_baseurl)
+    # Filter out GitHub Pages domains, as they do not require CNAME files.
+    if domain and not domain.endswith(".github.io"):
+        with open(cname_path, 'w', encoding="utf-8") as f:
+            # NOTE: don't write a trailing newline. The `CNAME` file that's
+            # auto-generated by the GitHub UI doesn't have one.
+            f.write(domain)
+    else:
+        try:
+            os.unlink(cname_path)
+        except FileNotFoundError:
+            pass
 
 
 def setup(app: Sphinx) -> dict[str, Any]: