Tests: explicitly unset SOURCE_DATE_EPOCH during 'test_html_multi_lin…

…e_copyright' (#13224)

Sphinx's copyright substitution currently allows years identified as
the current year to be downgraded to previous years when
``SOURCE_DATE_EPOCH`` is configured, to assist reproducibility [1] of
documentation builds.

However, we have a test case ``test_html_multi_line_copyright``,
written in 2024, that mentioned a future year (2025).

Now that we have reached 2025, it is eligible for substitution when
``SOURCE_DATE_EPOCH`` is configured.  Many buildsystems, such as those
used by Debian and Fedora, choose the most-recent packaging/commit
timestamp to use as `SOURCE_DATE_EPOCH`'s timestamp, since those
correspond sensibly to a time-of-build.

However, for the Sphinx 8.1.3 release including the updated
substitution logic, the year-of-release/commit was 2024. Thus, if a
commit/packaging date from that year is chosen, and
``SOURCE_DATE_EPOCH`` is configured when the unit tests run, the
``test_html_multi_line_copyright`` test will fail.

The fix suggested here is to explicitly unset ``SOURCE_DATE_EPOCH``
within ``test_html_multi_line_copyright``.

[1]: https://www.reproducible-builds.org/

Authored-by: Colin Watson <[email protected]>
Co-authored-by: Colin Watson <[email protected]>
Authored-by: James Addison <[email protected]>
Co-authored-by: James Addison <[email protected]>

CHANGES.rst

@@ -58,3 +58,6 @@ Bugs fixed
 
 Testing
 -------
+
+* #13224: Correctness fixup for ``test_html_multi_line_copyright``.
+  Patch by Colin Watson, applied by James Addison.

tests/test_builders/test_build_html_copyright.py

@@ -9,6 +9,18 @@
 LOCALTIME_2009 = type(LT)(LT_NEW)
 
 
+@pytest.fixture
+def no_source_date_year(monkeypatch):
+    """
+    Explicitly clear SOURCE_DATE_EPOCH from the environment; this
+    fixture can be used to ensure that copyright substitution logic
+    does not occur during selected test cases.
+    """
+    with monkeypatch.context() as m:
+        m.delenv('SOURCE_DATE_EPOCH', raising=False)
+        yield
+
+
 @pytest.fixture(
     params=[
         1199145600,  # 2008-01-01 00:00:00
@@ -24,7 +36,7 @@ def source_date_year(request, monkeypatch):
 
 
 @pytest.mark.sphinx('html', testroot='copyright-multiline')
-def test_html_multi_line_copyright(app):
+def test_html_multi_line_copyright(no_source_date_year, app):
     app.build(force_all=True)
 
     content = (app.outdir / 'index.html').read_text(encoding='utf-8')