Storage backends: added exists() method

This method returns whether the file exists on the backend.

pillar/api/file_storage_backends/abstract.py

@@ -149,5 +149,9 @@ class Blob(metaclass=abc.ABCMeta):
         Only performs an actual action on backends that support temporary links.
         """
 
+    @abc.abstractmethod
+    def exists(self) -> bool:
+        """Returns True iff the file exists on the storage backend."""
+
 
 Bl = typing.TypeVar('Bl', bound=Blob)

pillar/api/file_storage_backends/gcs.py

@@ -180,6 +180,11 @@ class GoogleCloudStorageBlob(Blob):
     def make_public(self):
         self.gblob.make_public()
 
+    def exists(self) -> bool:
+        # Reload to get the actual file properties from Google.
+        self.gblob.reload()
+        return self.gblob.exists()
+
 
 def update_file_name(node):
     """Assign to the CGS blob the same name of the asset node. This way when

pillar/api/file_storage_backends/local.py

@@ -100,3 +100,6 @@ class LocalBlob(Blob):
     def make_public(self):
         # No-op on this storage backend.
         pass
+
+    def exists(self) -> bool:
+        return self.abspath().exists()