Avoid storing build time in gzip headers

sphinxcontrib/devhelp/__init__.py

@@ -124,7 +124,7 @@ def write_index(title: str, refs: list[Any], subitems: Any) -> None:
 
         # Dump the XML file
         xmlfile = path.join(outdir, outname + '.devhelp.gz')
-        with gzip.open(xmlfile, 'w') as f:
+        with gzip.GzipFile(filename=xmlfile, mode='w', mtime=0) as f:
             tree.write(f, 'utf-8')  # type: ignore
 
 

tests/test_devhelp.py

@@ -1,8 +1,25 @@
 """Test for devhelp extension."""
+from time import sleep
 
 import pytest
 
 
 @pytest.mark.sphinx('devhelp', testroot='basic')
 def test_basic(app, status, warning):
     app.builder.build_all()
+
+
+@pytest.mark.sphinx('devhelp', testroot='basic', freshenv=True)
+def test_basic_deterministic_build(app):
+    app.config.devhelp_basename, output_filename = 'testing', 'testing.devhelp.gz'
+
+    app.builder.build_all()
+    output_initial = (app.outdir / output_filename).read_bytes()
+
+    sleep(2)
+
+    app.builder.build_all()
+    output_repeat = (app.outdir / output_filename).read_bytes()
+
+    msg = f"Content of '{output_filename}' differed between builds."
+    assert output_repeat == output_initial, msg