WIP INCOMPLETE: Sketch digest choice approach in transports

Signed-off-by: Miloslav Trmač <mitr@redhat.com>

Author: mtrmac

image/copy/blob.go

@@ -98,10 +98,11 @@ func (ic *imageCopier) copyBlobFromStream(ctx context.Context, srcReader io.Read
 
 	// === Finally, send the layer stream to dest.
 	options := private.PutBlobOptions{
-		Cache:      ic.c.blobInfoCache,
-		IsConfig:   isConfig,
-		EmptyLayer: emptyLayer,
-		Digests:    ic.c.options.DigestOptions,
+		Cache:              ic.c.blobInfoCache,
+		IsConfig:           isConfig,
+		EmptyLayer:         emptyLayer,
+		Digests:            ic.c.options.DigestOptions,
+		CannotChangeDigest: ic.cannotModifyManifestReason != "", // FIXME: does this structurally guarantee that stream.info.Digest is known?
 	}
 	if !isConfig {
 		options.LayerIndex = &layerIndex

image/directory/directory_dest.go

@@ -11,6 +11,7 @@ import (
 
 	"github.com/opencontainers/go-digest"
 	"github.com/sirupsen/logrus"
+	"go.podman.io/image/v5/internal/digests"
 	"go.podman.io/image/v5/internal/imagedestination/impl"
 	"go.podman.io/image/v5/internal/imagedestination/stubs"
 	"go.podman.io/image/v5/internal/private"
@@ -150,7 +151,11 @@ func (d *dirImageDestination) PutBlobWithOptions(ctx context.Context, stream io.
 		}
 	}()
 
-	digester, stream := putblobdigest.DigestIfUnknown(stream, inputInfo)
+	algorithm, err := options.Digests.Choose(digests.Situation{Preexisting: inputInfo.Digest, CannotChangeAlgorithm: options.CannotChangeDigest})
+	if err != nil {
+		return private.UploadedBlob{}, err
+	}
+	digester, stream := putblobdigest.DigestIfAlgorithmUnknown(stream, inputInfo, algorithm)
 
 	// TODO: This can take quite some time, and should ideally be cancellable using ctx.Done().
 	size, err := io.Copy(blobFile, stream)

image/internal/private/private.go

@@ -112,9 +112,10 @@ type PutBlobOptions struct {
 	// if they use internal/imagedestination/impl.Compat;
 	// in that case, they will all be consistently zero-valued.
 
-	EmptyLayer bool            // True if the blob is an "empty"/"throwaway" layer, and may not necessarily be physically represented.
-	LayerIndex *int            // If the blob is a layer, a zero-based index of the layer within the image; nil otherwise.
-	Digests    digests.Options // Also a new private field, but mandatory requests are enforced by the compatibility wrapper.
+	EmptyLayer         bool            // True if the blob is an "empty"/"throwaway" layer, and may not necessarily be physically represented.
+	LayerIndex         *int            // If the blob is a layer, a zero-based index of the layer within the image; nil otherwise.
+	Digests            digests.Options // Also a new private field, but mandatory requests are enforced by the compatibility wrapper.
+	CannotChangeDigest bool            // True if PutBlobWithOptions is provided with a digest and the destination must use precisely that one (in particular, use that algorithm).
 }
 
 // PutBlobPartialOptions are used in PutBlobPartial.