Start to build up a mutate package. (#729)

* Start to build up a `mutate` package.

This is not yet used, but start to lay out some useful utilities for walking over and manipulating Images and Indices.

The first bit is `mutate.AppendManifests`, which builds around the utility with the same name in GGCR.  This utility produces an `oci.SignedImageIndex`, where (because of the mutation) the signatures are necessarily empty, but which provides access to the signatures of the entities contained within it.  This was mostly useful for constructing images for tests.

The second bit is `mutate.Map`, which surfaces a way to apply a `Mutator` function to the `oci.SignedEntity`s contained within a particular `oci.SignedEntity`.  Notable features:
 * `Mutator`s can control whether they descend into the children of indices (useful for recursive signing or signature verification).
 * `Mutator`s can filter entities by returning `nil` without `error`.
 * `Mutator`s are called before and (if changed) after its children are walked, which is detectable via helpers on `ctx`.
 * `Mutator`s can return their input entity if they are just a simple readonly walk.

Signed-off-by: Matt Moore <[email protected]>

* Inline GGCR interface, presere annotations.

Signed-off-by: Matt Moore <[email protected]>

internal/oci/mutate/map.go

@@ -0,0 +1,171 @@
+//
+// Copyright 2021 The Sigstore Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package mutate
+
+import (
+	"context"
+	"errors"
+	"fmt"
+
+	v1 "github.com/google/go-containerregistry/pkg/v1"
+	"github.com/google/go-containerregistry/pkg/v1/empty"
+	"github.com/google/go-containerregistry/pkg/v1/mutate"
+	"github.com/google/go-containerregistry/pkg/v1/types"
+	"github.com/sigstore/cosign/internal/oci"
+)
+
+// Mutator is the signature of the callback supplied to Map.
+// The oci.SignedEntity is either an oci.SignedImageIndex or an oci.SignedImage.
+// This callback is called on oci.SignedImageIndex *before* its children are
+// processed with a context that returns IsBeforeChildren(ctx) == true.
+// If the images within the SignedImageIndex change after the Before pass, then
+// the Mutator will be invoked again on the new SignedImageIndex with a context
+// that returns IsAfterChildren(ctx) == true.
+// If the returned entity is nil, it is filtered from the result of Map.
+type Mutator func(context.Context, oci.SignedEntity) (oci.SignedEntity, error)
+
+// ErrSkipChildren is a special error that may be returned from a Mutator
+// to skip processing of an index's child entities.
+var ErrSkipChildren = errors.New("skip child entities")
+
+// Map calls `fn` on the signed entity and each of its constituent entities (`SignedImageIndex`
+// or `SignedImage`) transitively.
+// Any errors returned by an `fn` are returned by `Map`.
+func Map(ctx context.Context, parent oci.SignedEntity, fn Mutator) (oci.SignedEntity, error) {
+	parent, err := fn(before(ctx), parent)
+	switch {
+	case errors.Is(err, ErrSkipChildren):
+		return parent, nil
+	case err != nil:
+		return nil, err
+	case parent == nil:
+		// If the function returns nil, it filters it.
+		return nil, nil
+	}
+
+	sii, ok := parent.(oci.SignedImageIndex)
+	if !ok {
+		return parent, nil
+	}
+	im, err := sii.IndexManifest()
+	if err != nil {
+		return nil, err
+	}
+
+	// Track whether any of the child entities change.
+	changed := false
+
+	adds := []IndexAddendum{}
+	for _, desc := range im.Manifests {
+		switch desc.MediaType {
+		case types.OCIImageIndex, types.DockerManifestList:
+			x, err := sii.SignedImageIndex(desc.Digest)
+			if err != nil {
+				return nil, err
+			}
+
+			se, err := Map(ctx, x, fn)
+			if err != nil {
+				return nil, err
+			} else if se == nil {
+				// If the function returns nil, it filters it.
+				changed = true
+				continue
+			}
+
+			changed = changed || (x != se)
+			adds = append(adds, IndexAddendum{
+				Add: se.(oci.SignedImageIndex), // Must be an image index.
+				Descriptor: v1.Descriptor{
+					URLs:        desc.URLs,
+					MediaType:   desc.MediaType,
+					Annotations: desc.Annotations,
+					Platform:    desc.Platform,
+				},
+			})
+
+		case types.OCIManifestSchema1, types.DockerManifestSchema2:
+			x, err := sii.SignedImage(desc.Digest)
+			if err != nil {
+				return nil, err
+			}
+
+			se, err := fn(ctx, x)
+			if err != nil {
+				return nil, err
+			} else if se == nil {
+				// If the function returns nil, it filters it.
+				changed = true
+				continue
+			}
+
+			changed = changed || (x != se)
+			adds = append(adds, IndexAddendum{
+				Add: se.(oci.SignedImage), // Must be an image
+				Descriptor: v1.Descriptor{
+					URLs:        desc.URLs,
+					MediaType:   desc.MediaType,
+					Annotations: desc.Annotations,
+					Platform:    desc.Platform,
+				},
+			})
+
+		default:
+			return nil, fmt.Errorf("unknown mime type: %v", desc.MediaType)
+		}
+	}
+
+	if !changed {
+		return parent, nil
+	}
+
+	// Preserve the key attributes from the base IndexManifest.
+	e := mutate.IndexMediaType(empty.Index, im.MediaType)
+	e = mutate.Annotations(e, im.Annotations).(v1.ImageIndex)
+
+	// Construct a new ImageIndex from the new consituent signed images.
+	result := AppendManifests(e, adds...)
+
+	// Since the children changed, give the callback a crack at the new image index.
+	return fn(after(ctx), result)
+}
+
+// This is used to associate which pass of the Map a particular
+// callback is being invoked for.
+type mapPassKey struct{}
+
+// before decorates the context such that IsBeforeChildren(ctx) is true.
+func before(ctx context.Context) context.Context {
+	return context.WithValue(ctx, mapPassKey{}, "before")
+}
+
+// after decorates the context such that IsAfterChildren(ctx) is true.
+func after(ctx context.Context) context.Context {
+	return context.WithValue(ctx, mapPassKey{}, "after")
+}
+
+// IsBeforeChildren is true within a Mutator when it is called before the children
+// have been processed.
+func IsBeforeChildren(ctx context.Context) bool {
+	return ctx.Value(mapPassKey{}) == "before"
+}
+
+// IsAfterChildren is true within a Mutator when it is called after the children
+// have been processed; however, this call is only made if the set of children
+// changes since the Before call.
+func IsAfterChildren(ctx context.Context) bool {
+	return ctx.Value(mapPassKey{}) == "after"
+}