Check that someone else has not moved a file into the cache already

If multiple processes are sharing a cache directory and caching
on read then it is possible that the other process has already
copied the dataset into the cache. We therefore need to check
before we try to copy it.

python/lsst/daf/butler/configs/datastores/fileDatastore.yaml

@@ -32,7 +32,7 @@ datastore:
       threshold: 4
     # Default cache value. This will be the default decision if no specific
     # match is found in the "cacheable" section later on.
-    default: false
+    default: true
     # Use a dict over list to simplify merging logic.
     cacheable:
       Exposure: true

python/lsst/daf/butler/core/datastoreCacheManager.py

@@ -601,6 +601,14 @@ def move_to_cache(self, uri: ResourcePath, ref: DatasetRef) -> Optional[Resource
         # item.
         self._expire_cache()
 
+        # The above reset the in-memory cache status. It's entirely possible
+        # that another process has just cached this file (if multiple
+        # processes are caching on read), so check our in-memory cache
+        # before attempting to cache the dataset.
+        path_in_cache = cached_location.relative_to(self.cache_directory)
+        if path_in_cache and path_in_cache in self._cache_entries:
+            return cached_location
+
         # Move into the cache. Given that multiple processes might be
         # sharing a single cache directory, and the file we need might have
         # been copied in whilst we were checking, allow overwrite without