question: How do I cache a layer directly? about go-containerregistry HOT 4 CLOSED

remote.Layer(digest) does not fetch the layer from the registry. What is actually first fetching the layer is the call to Cache.Put when it tries to get the DiffID of the layer here.

from go-containerregistry.

A workaround is to provide a custom DiffID method that doesn't require the contents of the layer to be computed. (This effectively disables support for auto uncompressing layers though!)

In the example from the description:

type noCompression struct {
	v1.Layer
}

func (s noCompression) DiffID() (v1.Hash, error) {
	return s.Digest()
}

func main() {
	...
	layer, err := c.Get(hash)
	if err != nil {
		...

		l, err := remote.Layer(digest)
		if err != nil {
			panic(err)
		}

		l2 := noCompression{l}

		lazyLayer, err := c.Put(l2)
		...
}

from go-containerregistry.

Maybe this is a limitation of the current implementation of the FilesystemCache. Ideally, DiffID should be lazily evaluated. For example, instead of creating a file with the DiffID here, write to a temporary file, then once that's complete, compute the digest and move the file over to the expected location. (This seems like a safer approach anyways.)

However, in order to check if the uncompressed layer is already in the cache, you need to know the DiffID. This is not a problem when caching images because an image config contains the DiffIDs for its layers.

from go-containerregistry.

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from go-containerregistry.