fix: Avoid pickling processed credentials (#1016)

Author: andrewsg

google/cloud/storage/client.py

@@ -130,10 +130,11 @@ def __init__(
         if project is _marker:
             project = None
 
-        # Save the initial value of client_info and client_options before they
+        # Save the initial value of constructor arguments before they
         # are passed along, for use in __reduce__ defined elsewhere.
         self._initial_client_info = client_info
         self._initial_client_options = client_options
+        self._initial_credentials = credentials
 
         kw_args = {"client_info": client_info}
 

google/cloud/storage/transfer_manager.py

@@ -879,7 +879,7 @@ def _reduce_client(cl):
 
     client_object_id = id(cl)
     project = cl.project
-    credentials = cl._credentials
+    credentials = cl._initial_credentials
     _http = None  # Can't carry this over
     client_info = cl._initial_client_info
     client_options = cl._initial_client_options