Add --platform CLI option for crane (#788)

* Add --platform CLI option for crane

* Allow to set variant in platform CLI flag

* Update docs

cmd/crane/cmd/root.go

@@ -25,11 +25,13 @@ import (
 func init() {
 	Root.PersistentFlags().BoolVarP(&verbose, "verbose", "v", false, "Enable debug logs")
 	Root.PersistentFlags().BoolVar(&insecure, "insecure", false, "Allow image references to be fetched without TLS")
+	Root.PersistentFlags().Var(platform, "platform", "Specifies the platform in the form os/arch[/variant] (e.g. linux/amd64).")
 }
 
 var (
 	verbose  = false
 	insecure = false
+	platform = &platformValue{}
 
 	// Crane options for this invocation.
 	options = []crane.Option{}
@@ -48,6 +50,8 @@ var (
 				options = append(options, crane.Insecure)
 			}
 
+			options = append(options, crane.WithPlatform(platform.platform))
+
 			// Add any http headers if they are set in the config file.
 			cf, err := config.Load(os.Getenv("DOCKER_CONFIG"))
 			if err != nil {

cmd/crane/cmd/util.go

@@ -0,0 +1,67 @@
+package cmd
+
+import (
+	"fmt"
+	"strings"
+
+	v1 "github.com/google/go-containerregistry/pkg/v1"
+)
+
+type platformValue struct {
+	platform *v1.Platform
+}
+
+func (pv *platformValue) Set(platform string) error {
+	p, err := parsePlatform(platform)
+	if err != nil {
+		return err
+	}
+	pv.platform = p
+	return nil
+}
+
+func (pv *platformValue) String() string {
+	return platformToString(pv.platform)
+}
+
+func (pv *platformValue) Type() string {
+	return "platform"
+}
+
+func platformToString(p *v1.Platform) string {
+	if p == nil {
+		return "all"
+	}
+	platform := ""
+	if p.OS != "" && p.Architecture != "" {
+		platform = p.OS + "/" + p.Architecture
+	}
+	if p.Variant != "" {
+		platform += "/" + p.Variant
+	}
+	return platform
+}
+
+func parsePlatform(platform string) (*v1.Platform, error) {
+	if platform == "all" {
+		return nil, nil
+	}
+
+	p := &v1.Platform{}
+	parts := strings.Split(platform, "/")
+
+	if len(parts) < 2 {
+		return nil, fmt.Errorf("failed to parse platform '%s': expected format os/arch[/variant]", platform)
+	}
+	if len(parts) > 3 {
+		return nil, fmt.Errorf("failed to parse platform '%s': too many slashes", platform)
+	}
+
+	p.OS = parts[0]
+	p.Architecture = parts[1]
+	if len(parts) > 2 {
+		p.Variant = parts[2]
+	}
+
+	return p, nil
+}

pkg/crane/options.go

@@ -60,7 +60,9 @@ func Insecure(o *options) {
 // WithPlatform is an Option to specify the platform.
 func WithPlatform(platform *v1.Platform) Option {
 	return func(o *options) {
-		o.remote = append(o.remote, remote.WithPlatform(*platform))
+		if platform != nil {
+			o.remote = append(o.remote, remote.WithPlatform(*platform))
+		}
 		o.platform = platform
 	}
 }

pkg/v1/remote/index.go

@@ -155,7 +155,7 @@ func (r *remoteIndex) childByPlatform(platform v1.Platform) (*Descriptor, error)
 			return r.childDescriptor(childDesc, platform)
 		}
 	}
-	return nil, fmt.Errorf("no child with platform %s/%s in index %s", platform.Architecture, platform.OS, r.Ref)
+	return nil, fmt.Errorf("no child with platform %s/%s in index %s", platform.OS, platform.Architecture, r.Ref)
 }
 
 func (r *remoteIndex) childByHash(h v1.Hash) (*Descriptor, error) {