fix: custom_time decoding without microseconds

Currently custom_time is not being decoded correctly if the value
has a zero in the microseconds field. This fixes the issue by
using a different helper method to decode.

Fixes #363

google/cloud/storage/blob.py

@@ -56,6 +56,7 @@
 from google.cloud._helpers import _bytes_to_unicode
 from google.cloud._helpers import _datetime_to_rfc3339
 from google.cloud._helpers import _rfc3339_to_datetime
+from google.cloud._helpers import _rfc3339_nanos_to_datetime
 from google.cloud._helpers import _to_bytes
 from google.cloud.exceptions import NotFound
 from google.cloud.storage._helpers import _add_generation_match_parameters
@@ -3775,7 +3776,7 @@ def custom_time(self):
         """
         value = self._properties.get("customTime")
         if value is not None:
-            return _rfc3339_to_datetime(value)
+            return _rfc3339_nanos_to_datetime(value)
 
     @custom_time.setter
     def custom_time(self, value):

tests/system/test_system.py

@@ -1040,6 +1040,21 @@ def test_upload_blob_custom_time(self):
         custom_time = same_blob.custom_time.replace(tzinfo=None)
         self.assertEqual(custom_time, current_time)
 
+    def test_blob_custom_time_no_micros(self):
+        # Test that timestamps without microseconds are treated correctly by
+        # custom_time encoding/decoding.
+        blob = self.bucket.blob("CustomTimeNoMicrosBlob")
+        file_contents = b"Hello World"
+        time_without_micros = datetime.datetime(2021, 2, 10, 12, 30)
+        blob.custom_time = time_without_micros
+        blob.upload_from_string(file_contents)
+        self.case_blobs_to_delete.append(blob)
+
+        same_blob = self.bucket.blob(("CustomTimeNoMicrosBlob"))
+        same_blob.reload(projection="full")
+        custom_time = same_blob.custom_time.replace(tzinfo=None)
+        self.assertEqual(custom_time, time_without_micros)
+
     def test_blob_crc32_md5_hash(self):
         blob = self.bucket.blob("MyBuffer")
         file_contents = b"Hello World"