Originally reported in containers/podman#20611 (comment) : pull a zstd:chunked image, then podman save (to docker-archive) fails with

Error: creating an updated image manifest: Error during manifest conversion: "application/vnd.oci.image.layer.v1.tar+zstd": zstd compression is not supported for docker images

What happens here:

• The original manifest is recoded, with a zstd MIME type
• Because the data is actually stored uncompressed, LayerInfosForCopy returns an uncompressed MIME type
• imageCopier.copyLayers does account for the LayerInfosForCopy changes by setting srcInfosUpdated = true. But that isn’t sufficient, because it is documented that ManifestUpdateOptions.LayerInfos ignores the MIME type, the edit must be represented by CompressionOperation+CompressionAlgorithm.
• imageCopier.copyLayer has code to compensate for this, by setting CompressionAlgorithm based on MIME type. But it doesn’t set CompressionOperation: Decompress for uncompressed MIME types; and it only applies on the TryReusingBlob code path.
• On the primary “full copy” code path, CompressionOperation+CompressionAlgorithm are set separately, based just on the characteristics of the incoming blob. And when the blob is uncompressed, and the desired state is also uncompressed, there is “nothing to do”, and the code sets CompressionOperation: PreserveOriginal, CompressionAlgorithm: nil, i.e. “do no edits”.
• As a result, the original manifest with a zstd MIME type is not updated to correctly represent an uncompressed layer, and a conversion to schema2 then fails.

Ugh. This is going to be a mess to untangle.

• Maybe it’s time to finally deprecate the edit fields of BlobInfo, and build some internal-only edit interface afresh? (A lot of work, and what would that look like?)
• Or the approach from the top of copyLayer, and have copyBlobFromStream preserve the supplied operation if it makes no changes?