Motivation

We've got a report in our Community Slack with a small sample (as represented by the new test) where Checksum handling did not work well with the UploadPartCopy operation: https://docs.aws.amazon.com/AmazonS3/latest/API/API_UploadPartCopy.html

This was due because we simply did not handle checksums as this was not implemented in our previous implementation.

We now properly handle checksums when copying an object into a part, via our storage engine.

This was the error when calling ListParts:

An error occurred (InternalError) when calling the ListParts operation (reached max retries: 4): exception while calling s3.ListParts: 'NoneType' object has no attribute 'upper'

Changes

• add new test covering the case of multipart uploads with checksum and UploadPartCopy
• fix the behavior to retrieve the new checksum from the stored object, calculated when being stored
• return the checksum field when the conditions are filled for UploadPartCopy
• add a better check in ListParts to avoid unhandled exceptions