support building from a scratch image

pkg/build/scratch.go

@@ -0,0 +1,59 @@
+/*
+Copyright 2023 Google LLC All Rights Reserved.
+
+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 build
+
+import (
+	"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"
+)
+
+// ScratchImage returns a scratch image manifest with scratch images for each of the specified platforms
+func ScratchImage(platforms []string) (Result, error) {
+	idx := mutate.IndexMediaType(empty.Index, types.OCIImageIndex)
+	for _, pf := range platforms {
+		p, err := v1.ParsePlatform(pf)
+		if err != nil {
+			return nil, err
+		}
+		img, err := mutate.ConfigFile(empty.Image, &v1.ConfigFile{
+			RootFS: v1.RootFS{
+				// Some clients check this.
+				Type: "layers",
+			},
+			Architecture: p.Architecture,
+			OS:           p.OS,
+			Variant:      p.Variant,
+			OSVersion:    p.OSVersion,
+			OSFeatures:   p.OSFeatures,
+		},
+		)
+		if err != nil {
+			return nil, fmt.Errorf("setting config file on empty image, %w", err)
+		}
+		idx = mutate.AppendManifests(idx, mutate.IndexAddendum{
+			Add: img,
+			Descriptor: v1.Descriptor{
+				Platform: p,
+			},
+		})
+	}
+	return idx, nil
+}

pkg/build/scratch_test.go

@@ -0,0 +1,66 @@
+/*
+Copyright 2023 Google LLC All Rights Reserved.
+
+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 build
+
+import (
+	"testing"
+
+	v1 "github.com/google/go-containerregistry/pkg/v1"
+	"github.com/google/go-containerregistry/pkg/v1/types"
+)
+
+func TestScratchImage(t *testing.T) {
+	img, err := ScratchImage([]string{"linux/s390x", "plan9/386"})
+	if err != nil {
+		t.Fatalf("expected to create the image, %s", err)
+	}
+	mt, err := img.MediaType()
+	if err != nil {
+		t.Fatalf("expected to get a mediatype, %s", err)
+	}
+	expMT := types.OCIImageIndex
+	if mt != expMT {
+		t.Errorf("expected media type = %s, got %s", expMT, mt)
+	}
+
+	imgIdx, ok := img.(v1.ImageIndex)
+	if !ok {
+		t.Fatalf("expected to have an image index")
+	}
+	mf, err := imgIdx.IndexManifest()
+	if mt != expMT {
+		t.Errorf("expected a manifest, got %s", err)
+	}
+	if len(mf.Manifests) != 2 {
+		t.Fatalf("expected two manifests, got %d", len(mf.Manifests))
+	}
+	for _, m := range mf.Manifests {
+		switch m.Platform.OS {
+		case "linux":
+			if m.Platform.Architecture != "s390x" {
+				t.Errorf("expected arch = s390x, got %s", m.Platform.Architecture)
+			}
+		case "plan9":
+			if m.Platform.Architecture != "386" {
+				t.Errorf("expected arch = 386, got %s", m.Platform.Architecture)
+			}
+		default:
+			t.Errorf("unexpected OS %s", m.Platform.OS)
+		}
+	}
+
+}

pkg/commands/config.go

@@ -94,6 +94,7 @@ func getBaseImage(bo *options.BuildOptions) build.GetBase {
 		if !ok || baseImage == "" {
 			baseImage = bo.BaseImage
 		}
+
 		var nameOpts []name.Option
 		if bo.InsecureRegistry {
 			nameOpts = append(nameOpts, name.Insecure)
@@ -103,6 +104,15 @@ func getBaseImage(bo *options.BuildOptions) build.GetBase {
 			return nil, nil, fmt.Errorf("parsing base image (%q): %w", baseImage, err)
 		}
 
+		if baseImage == "scratch" {
+			log.Printf("Using base %s for %s", ref, s)
+			si, err := build.ScratchImage(bo.Platforms)
+			if err != nil {
+				return nil, nil, fmt.Errorf("constructing scratch image: %w", err)
+			}
+			return ref, si, nil
+		}
+
 		if v, ok := cache.Load(ref.String()); ok {
 			return ref, v.(build.Result), nil
 		}