Add retry attempts for internal error on multipart upload

Fixes #1740

storage/cloud.py

@@ -3,6 +3,7 @@ import os
 import logging
 import copy
 
+from boto.exception import S3ResponseError
 import boto.s3.connection
 import boto.s3.multipart
 import boto.gs.connection
@@ -374,7 +375,22 @@ class _CloudStorage(BaseStorageV2):
           chunk_end_offset_inclusive = chunk.length - 1
           mpu.copy_part_from_key(self.get_cloud_bucket().name, abs_chunk_path, part_num,
                                  start=0, end=chunk_end_offset_inclusive)
-        mpu.complete_upload()
+
+        # Note: Sometimes Amazon S3 simply raises an internal error when trying to complete a
+        # multipart upload. The recommendation is to simply try calling complete_upload again.
+        for remaining_retries in range(3, -1, -1):
+          try:
+            mpu.complete_upload()
+            break
+          except S3ResponseError as s3re:
+            if remaining_retries and s3re.status == 200 and s3re.error_code == 'InternalError':
+              # Weird internal error case. Retry.
+              continue
+
+            # Otherwise, raise it.
+            logger.exception('Exception trying to complete multipart upload for: %s', final_path)
+            raise s3re
+
 
       except IOError as ioe:
         # Something bad happened, log it and then give up