👌 Add warning log for config values that cannot be cached

sphinx/config.py

@@ -465,6 +465,17 @@ def __getstate__(self) -> dict:
         for name, opt in self._options.items():
             real_value = getattr(self, name)
             if not is_serializable(real_value):
+                if opt.rebuild:
+                    # if the value is not cached, then any build that utilises this cache
+                    # will always mark the config value as changed,
+                    # and thus always invalidate the cache and perform a rebuild.
+                    logger.warning(
+                        __('cannot cache unpickable configuration value: %r'),
+                        name,
+                        type='config',
+                        subtype='cache',
+                        once=True,
+                    )
                 # omit unserializable value
                 real_value = None
             # valid_types is also omitted

tests/test_config/test_config.py

@@ -22,6 +22,7 @@
 )
 from sphinx.deprecation import RemovedInSphinx90Warning
 from sphinx.errors import ConfigError, ExtensionError, VersionRequirementError
+from sphinx.util.console import strip_colors
 
 if TYPE_CHECKING:
     from collections.abc import Iterable
@@ -292,6 +293,24 @@ def check(
     assert counters[type(x['b'])] == 2
 
 
+def test_uncacheable_config_warning(make_app, tmp_path):
+    """Test than an unpickleable config value raises a warning."""
+    srcdir = tmp_path / 'srcdir123'
+    srcdir.mkdir()
+    srcdir.joinpath('conf.py').write_text("""
+my_config = lambda: None
+show_warning_types = True
+def setup(app):
+    app.add_config_value('my_config', None, 'env')
+    """, encoding='utf-8')
+    srcdir.joinpath('index.rst').write_text('Test\n====\n', encoding='utf-8')
+    app = make_app(srcdir=srcdir)
+    app.build()
+    assert strip_colors(app.warning.getvalue()).strip() == (
+        "WARNING: cannot cache unpickable configuration value: 'my_config' [config.cache]"
+    )
+
+
 def test_extension_values():
     config = Config()