Fix ordering inside searchindex.js not being deterministic

CHANGES.rst

@@ -18,6 +18,8 @@ Bugs fixed
 
 * #11668: Raise a useful error when ``theme.conf`` is missing.
   Patch by Vinay Sajip.
+* #11622: Ensure that the order of keys in ``searchindex.js`` is deterministic.
+  Patch by Pietro Albini.
 
 Testing
 -------

sphinx/search/__init__.py

@@ -162,7 +162,7 @@ class _JavaScriptIndex:
     SUFFIX = ')'
 
     def dumps(self, data: Any) -> str:
-        return self.PREFIX + json.dumps(data) + self.SUFFIX
+        return self.PREFIX + json.dumps(data, sort_keys=True) + self.SUFFIX
 
     def loads(self, s: str) -> Any:
         data = s[len(self.PREFIX):-len(self.SUFFIX)]

tests/test_search.py

@@ -304,3 +304,40 @@ def test_parallel(app):
     app.build()
     index = load_searchindex(app.outdir / 'searchindex.js')
     assert index['docnames'] == ['index', 'nosearch', 'tocitem']
+
+
+@pytest.mark.sphinx(testroot='search')
+def test_search_index_is_deterministic(app):
+    lists_not_to_sort = {
+        # Each element of .titles is related to the element of .docnames in the same position.
+        # The ordering is deterministic because .docnames is sorted.
+        '.titles',
+        # Each element of .filenames is related to the element of .docnames in the same position.
+        # The ordering is deterministic because .docnames is sorted.
+        '.filenames',
+    }
+
+    # In the search index, titles inside .alltitles are stored as a tuple of
+    # (document_idx, title_anchor). Tuples are represented as lists in JSON,
+    # but their contents must not be sorted. We cannot sort them anyway, as
+    # document_idx is an int and title_anchor is a str.
+    def is_title_tuple_type(item):
+        return len(item) == 2 and isinstance(item[0], int) and isinstance(item[1], str)
+
+    def assert_is_sorted(item, path):
+        err_path = path if path else '<root>'
+        if isinstance(item, dict):
+            assert list(item.keys()) == sorted(item.keys()), f'{err_path} is not sorted'
+            for key, value in item.items():
+                assert_is_sorted(value, f'{path}.{key}')
+        elif isinstance(item, list):
+            if not is_title_tuple_type(item) and path not in lists_not_to_sort:
+                assert item == sorted(item), f'{err_path} is not sorted'
+            for i, child in enumerate(item):
+                assert_is_sorted(child, f'{path}[{i}]')
+
+    app.builder.build_all()
+    index = load_searchindex(app.outdir / 'searchindex.js')
+    # Pretty print the index. Only shown by pytest on failure.
+    print(f'searchindex.js contents:\n\n{json.dumps(index, indent=2)}')
+    assert_is_sorted(index, '')