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Swift already supports that much, at least. AWS used to not support any conditional PUTs, but that's changed somewhat recently; see - https://aws.amazon.com/about-aws/whats-new/2024/08/amazon-s3-conditional-writes/ - https://aws.amazon.com/about-aws/whats-new/2024/11/amazon-s3-functionality-conditional-writes/ Drive-By: Fix retry of a CompleteMultipartUpload with changed parts; it should 404 rather than succeed in writing the new manifest. Change-Id: I2e57dacb342b5758f16b502bb91372a2443d0182