Use locale-independent HTTP dates for conditional requests

noxfile.py

@@ -71,7 +71,7 @@ def typing(session):
     """
     Check static typing.
     """
-    session.install("mypy", "types-docutils", "types-lxml", ROOT)
+    session.install("mypy", "pytest", "types-docutils", "types-lxml", ROOT)
     session.run("python", "-m", "mypy", PACKAGE)
 
 

sphinx_json_schema_spec/__init__.py

@@ -4,6 +4,7 @@
 
 from contextlib import suppress
 from datetime import UTC, datetime
+from email.utils import format_datetime
 from importlib import metadata
 from pathlib import Path
 from urllib.parse import urljoin
@@ -76,7 +77,7 @@ def fetch_or_load(cache_path, url):
 
     with suppress(FileNotFoundError):
         modified = datetime.fromtimestamp(cache_path.stat().st_mtime, UTC)
-        date = modified.strftime("%a, %d %b %Y %I:%M:%S UTC")
+        date = format_datetime(modified, usegmt=True)
         headers["If-Modified-Since"] = date
 
     request = urllib.request.Request(url, headers=headers)

sphinx_json_schema_spec/tests/test_fetching.py

@@ -0,0 +1,41 @@
+from datetime import UTC, datetime
+from io import BytesIO
+import locale
+import os
+
+import pytest
+
+from sphinx_json_schema_spec import fetch_or_load
+
+
+@pytest.mark.parametrize("time_locale", ["C", ""])
+def test_conditional_request_uses_http_date(
+    tmp_path,
+    monkeypatch,
+    time_locale,
+):
+    cache_path = tmp_path / "spec.html"
+    cache_path.write_bytes(b"<html><body>cached</body></html>")
+    modified = datetime(2025, 1, 2, 15, 4, 5, tzinfo=UTC).timestamp()
+    os.utime(cache_path, (modified, modified))
+    requests = []
+
+    class Response(BytesIO):
+        code = 200
+
+    def urlopen(request, context):
+        requests.append(request)
+        return Response(b"<html><body>fresh</body></html>")
+
+    monkeypatch.setattr("urllib.request.urlopen", urlopen)
+    previous_locale = locale.setlocale(locale.LC_TIME)
+    try:
+        locale.setlocale(locale.LC_TIME, time_locale)
+        result = fetch_or_load(cache_path, "https://example.com/spec.html")
+    finally:
+        locale.setlocale(locale.LC_TIME, previous_locale)
+
+    assert requests[0].get_header("If-modified-since") == (
+        "Thu, 02 Jan 2025 15:04:05 GMT"
+    )
+    assert result.find("body").text == "fresh"