Method copyLayer

copy/single.go:696–892

copyLayer copies a layer with srcInfo (with known Digest and Annotations and possibly known Size) in src to dest, perhaps (de/re/)compressing it, and returns a complete blobInfo of the copied layer, and a value for LayerDiffIDs if diffIDIsNeeded srcRef can be used as an additional hint to the destin

(ctx context.Context, srcInfo types.BlobInfo, toEncrypt bool, pool *mpb.Progress, layerIndex int, srcRef reference.Named, emptyLayer bool)

// and returns a complete blobInfo of the copied layer, and a value for LayerDiffIDs if diffIDIsNeeded
// srcRef can be used as an additional hint to the destination during checking whether a layer can be reused but srcRef can be nil.
func (ic *imageCopier) copyLayer(ctx context.Context, srcInfo types.BlobInfo, toEncrypt bool, pool *mpb.Progress, layerIndex int, srcRef reference.Named, emptyLayer bool) (types.BlobInfo, digest.Digest, error) {
	// If the srcInfo doesn't contain compression information, try to compute it from the
	// MediaType, which was either read from a manifest by way of LayerInfos() or constructed
	// by LayerInfosForCopy(), if it was supplied at all.  If we succeed in copying the blob,
	// the BlobInfo we return will be passed to UpdatedImage() and then to UpdateLayerInfos(),
	// which uses the compression information to compute the updated MediaType values.
	// (Sadly UpdatedImage() is documented to not update MediaTypes from
	//  ManifestUpdateOptions.LayerInfos[].MediaType, so we are doing it indirectly.)
	if srcInfo.CompressionOperation == types.PreserveOriginal && srcInfo.CompressionAlgorithm == nil {
		op, algo, err := compressionEditsFromBlobInfo(srcInfo)
		if err != nil {
			return types.BlobInfo{}, "", err
		}
		srcInfo.CompressionOperation = op
		srcInfo.CompressionAlgorithm = algo
	}

	ic.c.printCopyInfo("blob", srcInfo)

	diffIDIsNeeded := false
	var cachedDiffID digest.Digest = ""
	if ic.diffIDsAreNeeded {
		cachedDiffID = ic.c.blobInfoCache.UncompressedDigest(srcInfo.Digest) // May be ""
		diffIDIsNeeded = cachedDiffID == ""
	}
	// When encrypting to decrypting, only use the simple code path. We might be able to optimize more
	// (e.g. if we know the DiffID of an encrypted compressed layer, it might not be necessary to pull, decrypt and decompress again),
	// but it’s not trivially safe to do such things, so until someone takes the effort to make a comprehensive argument, let’s not.
	encryptingOrDecrypting := toEncrypt || (isOciEncrypted(srcInfo.MediaType) && ic.c.options.OciDecryptConfig != nil)
	canAvoidProcessingCompleteLayer := !diffIDIsNeeded && !encryptingOrDecrypting

	// Don’t read the layer from the source if we already have the blob, and optimizations are acceptable.
	if canAvoidProcessingCompleteLayer {
		canChangeLayerCompression := ic.src.CanChangeLayerCompression(srcInfo.MediaType)
		logrus.Debugf("Checking if we can reuse blob %s: general substitution = %v, compression for MIME type %q = %v",
			srcInfo.Digest, ic.canSubstituteBlobs, srcInfo.MediaType, canChangeLayerCompression)
		canSubstitute := ic.canSubstituteBlobs && canChangeLayerCompression

		var requiredCompression *compressiontypes.Algorithm
		if ic.requireCompressionFormatMatch {
			requiredCompression = ic.compressionFormat
		}

		var tocDigest digest.Digest

		// Check if we have a chunked layer in storage that's based on that blob.  These layers are stored by their TOC digest.
		d, err := chunkedToc.GetTOCDigest(srcInfo.Annotations)
		if err != nil {
			return types.BlobInfo{}, "", err
		}
		if d != nil {
			tocDigest = *d
		}

		reused, reusedBlob, err := ic.c.dest.TryReusingBlobWithOptions(ctx, srcInfo, private.TryReusingBlobOptions{
			Cache:                   ic.c.blobInfoCache,
			CanSubstitute:           canSubstitute,
			EmptyLayer:              emptyLayer,