Prefetch dimension record cache in Butler factory

In DM-42324, the dimension record cache was changed so that it is no longer shared between Butler instances.  This cache is somewhat expensive to fetch and is used for almost every Butler operation.

LabeledButlerFactory now downloads this cache into its 'template' DirectButler instances when they are first created, so it is available for copying into clones.

python/lsst/daf/butler/_labeled_butler_factory.py

@@ -147,7 +147,16 @@ def _get_config_uri(self, label: str) -> ResourcePathExpression:
 
 
 def _create_direct_butler_factory(config: ButlerConfig) -> _FactoryFunction:
+    import lsst.daf.butler.direct_butler
+
+    # Create a 'template' Butler that will be cloned when callers request an
+    # instance.
     butler = Butler.from_config(config)
+    assert isinstance(butler, lsst.daf.butler.direct_butler.DirectButler)
+
+    # Load caches so that data is available in cloned instances without
+    # needing to refetch it from the database for every instance.
+    butler._preload_cache()
 
     def create_butler(access_token: str | None) -> Butler:
         # Access token is ignored because DirectButler does not use Gafaelfawr

python/lsst/daf/butler/dimensions/record_cache.py

@@ -137,6 +137,12 @@ def load_from(self, other: DimensionRecordCache) -> None:
         """
         self._records = copy.deepcopy(other._records)
 
+    def preload_cache(self) -> None:
+        """Fetch the cache from the DB if it has not already been fetched."""
+        if self._records is None:
+            self._records = self._fetch()
+            assert self._records.keys() == set(self._keys), "Logic bug in fetch callback."
+
     def __contains__(self, key: object) -> bool:
         if not isinstance(key, str):
             return False
@@ -145,9 +151,8 @@ def __contains__(self, key: object) -> bool:
         return False
 
     def __getitem__(self, element: str) -> DimensionRecordSet:
-        if self._records is None:
-            self._records = self._fetch()
-            assert self._records.keys() == set(self._keys), "Logic bug in fetch callback."
+        self.preload_cache()
+        assert self._records is not None
         return self._records[element]
 
     def __iter__(self) -> Iterator[str]:

python/lsst/daf/butler/direct_butler.py

@@ -2196,6 +2196,10 @@ def _query_dimension_records(
             raise EmptyQueryResultError(list(result.explain_no_results()))
         return data_ids
 
+    def _preload_cache(self) -> None:
+        """Immediately load caches that are used for common operations."""
+        self._registry.preload_cache()
+
     _config: ButlerConfig
     """Configuration for this Butler instance."""
 

python/lsst/daf/butler/registry/sql_registry.py

@@ -2581,6 +2581,10 @@ def make_datastore_tables(self, tables: Mapping[str, DatastoreOpaqueTable]) -> N
                 pass
         self._datastore_record_classes = datastore_record_classes
 
+    def preload_cache(self) -> None:
+        """Immediately load caches that are used for common operations."""
+        self.dimension_record_cache.preload_cache()
+
     @property
     def obsCoreTableManager(self) -> ObsCoreTableManager | None:
         """The ObsCore manager instance for this registry