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Support zstd compression in image commit #5452

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Support zstd compression in image commit #5452

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6 changes: 3 additions & 3 deletions image.go
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Original file line number Diff line number Diff line change
Expand Up @@ -155,9 +155,9 @@ func computeLayerMIMEType(what string, layerCompression archive.Compression) (om
// how to decompress them, we can't try to compress layers with xz.
return "", "", errors.New("media type for xz-compressed layers is not defined")
case archive.Zstd:
// Until the image specs define a media type for zstd-compressed layers, even if we know
// how to decompress them, we can't try to compress layers with zstd.
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@mtrmac mtrmac Apr 2, 2024 •
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This comment is accurate: Docker images can’t represent Zstd images layers.

We can’t very well just silently ignore that problem and create something that doesn’t work.

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I'm happy to defer to you and refuse to create schema2 images with zstd compression. But I'm curious to understand why this is a problem - BuildKit seems happy to create them using this media type, and they seem to work fine with Docker and kaniko (but not buildah...).

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One place is our own https://github.com/containers/image/blob/1c648b405e0763472db0bcc189136ddb6a5b61c6/manifest/docker_schema2.go#L173 . In retrospect, being that strict might have been a mistake, but either way that code refusing unexpected MIME types has been there for many years.

(IIRC, Docker is not checking MIME types at all when pulling; that’s the other extreme option.)

So we would need to think about a migration mechanism, and giving users enough tools to manage it. For a format feature which is, AFAICS, undocumented. Meanwhile, “if you want to use Zstd, migrate to OCI” is a simple story that works. for all implementations.

return "", "", errors.New("media type for zstd-compressed layers is not defined")
omediaType = v1.MediaTypeImageLayerZstd
dmediaType = "application/vnd.docker.image.rootfs.diff.tar.zstd"
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Drive-by comment: I'm uncomfortable with hardcoded magic strings. Shouldn't this be defined as a constant in c/image?

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Yes, if this should happen, it would be in c/image/v5/manifest.

(Buildkit does seem to use this constant, but it is not a part of the public API.)

logrus.Debugf("compressing %s with zstd", what)
default:
logrus.Debugf("compressing %s with unknown compressor(?)", what)
}
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