Glacier multipart upload - unable to send a precomputed checksum

I have an issue with higher than expected CPU usage on multipart upload with boto3. I have precomputed checksums for the data for each part, and send the checksum in the upload part request:

multipart_upload.upload_part(range=my_range, body=my_body, checksum=my_checksum)

This correctly adds the header x-amz-sha256-tree-hash. However, when we get inside botocore/handlers.py there is library code that reads the body client side and computes another checksum, adding also a x-amz-content-sha256 header.

1. How can I specify the x-amz-content-sha256 via upload_part method of a multipart upload resource? I want to pre-compute this client side, not have it automatically computed at the time of upload.
2. Why are two hashes required anyway? Isn’t just one checksum enough for uploading a part? We can still send both the payload checksum and the tree hash in the final complete upload stage of a multipart upload, of course.

Note: I’ve tried using the lower level client.upload_multipart_part but that didn’t allow to explicitly specify the content sha256 either.

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[wimglenn]

@swetashre No this does not solve the use-case. We don’t want to send unsigned payload. We want to send the checksum that are already known, rather than have the client burn a lot of CPU to recalculate them (we have them already pre-calculated per chunk).

It’s a usability matter of allowing the client to specify them explicitly.