Add dry-run mode to copy skopeo-copy

Dry-run functionality was merged into the sync command in #1608.
Here, the same sort of functionality is added to the copy command
as well.

Signed-off-by: Ryan Egesdahl <deriamis@gmail.com>

cmd/skopeo/copy.go

@@ -19,6 +19,7 @@ import (
 	"github.com/containers/image/v5/transports/alltransports"
 	encconfig "github.com/containers/ocicrypt/config"
 	enchelpers "github.com/containers/ocicrypt/helpers"
+	"github.com/sirupsen/logrus"
 	"github.com/spf13/cobra"
 )
 
@@ -39,6 +40,7 @@ type copyOptions struct {
 	format                   commonFlag.OptionalString // Force conversion of the image to a specified format
 	quiet                    bool                      // Suppress output information when copying images
 	all                      bool                      // Copy all of the images if the source is a list
+	dryRun                   bool                      // Don't actually copy anything, just output what it would have done
 	multiArch                commonFlag.OptionalString // How to handle multi architecture images
 	preserveDigests          bool                      // Preserve digests during copy
 	encryptLayer             []int                     // The list of layers to encrypt
@@ -82,6 +84,7 @@ See skopeo(1) section "IMAGE NAMES" for the expected format
 	flags.StringSliceVar(&opts.additionalTags, "additional-tag", []string{}, "additional tags (supports docker-archive)")
 	flags.BoolVarP(&opts.quiet, "quiet", "q", false, "Suppress output information when copying images")
 	flags.BoolVarP(&opts.all, "all", "a", false, "Copy all images if SOURCE-IMAGE is a list")
+	flags.BoolVar(&opts.dryRun, "dry-run", false, "Run without actually copying images")
 	flags.Var(commonFlag.NewOptionalStringValue(&opts.multiArch), "multi-arch", `How to handle multi-architecture images (system, all, or index-only)`)
 	flags.BoolVar(&opts.preserveDigests, "preserve-digests", false, "Preserve digests of images and lists")
 	flags.BoolVar(&opts.removeSignatures, "remove-signatures", false, "Do not copy signatures from SOURCE-IMAGE")
@@ -126,6 +129,10 @@ func (opts *copyOptions) run(args []string, stdout io.Writer) (retErr error) {
 	opts.deprecatedTLSVerify.warnIfUsed([]string{"--src-tls-verify", "--dest-tls-verify"})
 	imageNames := args
 
+	if opts.dryRun {
+		logrus.Warn("Running in dry-run mode")
+	}
+
 	if err := reexecIfNecessaryForImages(imageNames...); err != nil {
 		return err
 	}
@@ -282,6 +289,11 @@ func (opts *copyOptions) run(args []string, stdout io.Writer) (retErr error) {
 
 	opts.destImage.warnAboutIneffectiveOptions(destRef.Transport())
 
+	if opts.dryRun {
+		logrus.Info(fmt.Sprintf("Would have copied from=%s to=%s", imageNames[0], imageNames[1]))
+		return nil
+	}
+
 	return retry.IfNecessary(ctx, func() error {
 		manifestBytes, err := copy.Image(ctx, policyContext, destRef, srcRef, &copy.Options{
 			RemoveSignatures:                 opts.removeSignatures,

docs/skopeo-copy.1.md

@@ -32,6 +32,10 @@ If _source-image_ refers to a list of images, instead of copying just the image
 architecture (subject to the use of the global --override-os, --override-arch and --override-variant options), attempt to copy all of
 the images in the list, and the list itself.
 
+**--dry-run**
+
+Run the sync without actually copying data to the destination.
+
 **--authfile** _path_
 
 Path of the authentication file. Default is ${XDG_RUNTIME\_DIR}/containers/auth.json, which is set using `skopeo login`.

integration/copy_test.go

@@ -6,6 +6,7 @@ import (
 	"crypto/x509"
 	"encoding/json"
 	"fmt"
+	"io"
 	"io/fs"
 	"log"
 	"net/http"
@@ -127,6 +128,13 @@ func (s *copySuite) TestCopyAllWithManifestListRoundTrip() {
 	assert.Equal(t, "", out)
 }
 
+func (s *copySuite) TestCopyDryRun() {
+	t := s.T()
+	dir := t.TempDir()
+	assertSkopeoSucceeds(t, "", "copy", "--dry-run", knownListImage, "dir:"+dir)
+	assertDirIsEmpty(t, dir)
+}
+
 func (s *copySuite) TestCopyAllWithManifestListConverge() {
 	t := s.T()
 	oci1 := t.TempDir()
@@ -662,6 +670,14 @@ func assertSchema1DirImagesAreEqualExceptNames(t *testing.T, dir1, ref1, dir2, r
 	assert.Equal(t, "", out)
 }
 
+func assertDirIsEmpty(t *testing.T, dir string) {
+	d, err := os.Open(dir)
+	require.NoError(t, err)
+	defer d.Close()
+	_, err = d.Readdirnames(1)
+	assert.Equal(t, err, io.EOF)
+}
+
 // Streaming (skopeo copy)
 func (s *copySuite) TestCopyStreaming() {
 	t := s.T()

systemtest/020-copy.bats

@@ -158,6 +158,15 @@ function setup() {
     expect_output "amd64"
 }
 
+@test "copy: --dry-run" {
+    local remote_image=docker://quay.io/libpod/busybox:latest
+    local dir=$TESTDIR/dir
+
+    run_skopeo copy --dry-run $remote_image oci:$dir:latest
+    expect_output --substring "Running in dry-run mode"
+    expect_output --substring "Would have copied from=${remote_image} to=oci:${dir}:latest"
+}
+
 teardown() {
     podman rm -f reg