Allow remote refs

Author: pcj

README.md

@@ -1,6 +1,6 @@
 # layer
 
-Print info about container image tarballs.
+Print info about container images.
 
 ## Installation
 
@@ -10,17 +10,30 @@ go install github.com/stackb/layer@latest
 
 ## Usage
 
-Show layers in an image:
+Show layers in an image (tarball filename):
 
 ```sh
-layer info myimage.tar
+$ layer info image.tar
+```
+
+Show layers in an image (ref):
+
+```
+$ layer info index.docker.io/nginx:latest
+N  Layer                                                                    Size
+1  sha256:9c1b6dd6c1e6be9fdd2b1987783824670d3b0dd7ae8ad6f57dc3cea5739ac71e  31 MB
+2  sha256:4b7fffa0f0a4a72b2f901c584c1d4ffb67cce7f033cc7969ee7713995c4d2610  25 MB
+3  sha256:f5ab86d69014270bcf4d5ce819b9f5c882b35527924ffdd11fecf0fc0dde81a4  604 B
+4  sha256:c876aa251c80272eb01eec011d50650e1b8af494149696b80a606bbeccf03d68  893 B
+5  sha256:7046505147d7f3edbf7c50c02e697d5450a2eebe5119b62b7362b10662899d85  667 B
+6  sha256:b6812e8d56d65d296e21a639b786e7e793e8b969bd2b109fd172646ce5ebe951  1.4 kB
 ```
 
 List files in an image:
 
 ```sh
-layer ls myimage.tar    # all layers
-layer ls -S myimage.tar # sorted
-layer ls myimage.tar 1  # first layer
-layer ls myimage.tar sha256:8fdc131ec4308d2b9196a38855550dc347e83cc0f47d739754ddeb6e03ac2cbe # by diff ID
+layer ls image.tar    # all layers
+layer ls -S image.tar # sorted
+layer ls image.tar 1  # first layer
+layer ls image.tar sha256:8fdc131ec4308d2b9196a38855550dc347e83cc0f47d739754ddeb6e03ac2cbe # by diff ID
 ```

main.go

@@ -10,7 +10,11 @@ import (
 	"text/tabwriter"
 
 	"github.com/dustin/go-humanize"
+	"github.com/google/go-containerregistry/pkg/authn"
+	"github.com/google/go-containerregistry/pkg/crane"
+	"github.com/google/go-containerregistry/pkg/name"
 	v1 "github.com/google/go-containerregistry/pkg/v1"
+	"github.com/google/go-containerregistry/pkg/v1/remote"
 	"github.com/google/go-containerregistry/pkg/v1/tarball"
 	"github.com/urfave/cli/v2"
 )
@@ -68,17 +72,49 @@ func main() {
 	app.Run(os.Args)
 }
 
-func getImage(filename string) (v1.Image, error) {
-	if filename == "" {
-		return nil, fmt.Errorf("no filename provided")
+func getImage(r string) (v1.Image, error) {
+	if r == "" {
+		return nil, fmt.Errorf("no image ref provided")
 	}
-	image, err := tarball.ImageFromPath(filename, nil)
+
+	image, _, err := getRemoteImage(r)
+	if err == nil {
+		return image, nil
+	}
+
+	image, err = tarball.ImageFromPath(r, nil)
 	if err != nil {
 		return nil, err
 	}
+
 	return image, nil
 }
 
+func makeOptions(opts ...crane.Option) crane.Options {
+	opt := crane.Options{
+		Remote: []remote.Option{
+			remote.WithAuthFromKeychain(authn.DefaultKeychain),
+		},
+	}
+	for _, o := range opts {
+		o(&opt)
+	}
+	return opt
+}
+
+func getRemoteImage(r string, opt ...crane.Option) (v1.Image, name.Reference, error) {
+	o := makeOptions(opt...)
+	ref, err := name.ParseReference(r, o.Name...)
+	if err != nil {
+		return nil, nil, fmt.Errorf("parsing reference %q: %w", r, err)
+	}
+	img, err := remote.Image(ref, o.Remote...)
+	if err != nil {
+		return nil, nil, fmt.Errorf("reading image %q: %w", ref, err)
+	}
+	return img, ref, nil
+}
+
 // info returns the config file for the given filename.
 func info(cfg *config) error {
 	image, err := getImage(cfg.filename)