Support saving more than one image at a time

- The previous method would loop over the image references and try to
save each one. Even if this did work, the tar archive would overwrite
the previous instance on each invocation meaning that the last image
specified on the CLI would be the only one in the archive. Even then,
this method would hang because a filesystem lock was acquired on the
underlying BoltDB during the first invocation of the loop
(within `client.createWorkerOpt(...)`) but never freed. Because of this, every
subsequent call would only hang as seen in genuinetools#332

- This refactor properly passes all image references to the underlying
buildkit type to let it construct the image archive with all references
specified.

README.md

@@ -463,7 +463,7 @@ Successfully tagged jess/thing as jess/otherthing
 
 ```console
 $ img save -h
-save -  Save an image to a tar archive (streamed to STDOUT by default).
+save -  Save one or more images to a tar archive (streamed to STDOUT by default).
 
 Usage: img save [OPTIONS] IMAGE [IMAGE...]
 

client/save.go

@@ -10,16 +10,10 @@ import (
 	"github.com/docker/distribution/reference"
 )
 
-// SaveImage exports an image as a tarball which can then be imported by docker.
-func (c *Client) SaveImage(ctx context.Context, image, format string, writer io.WriteCloser) error {
-	// Parse the image name and tag.
-	named, err := reference.ParseNormalizedNamed(image)
-	if err != nil {
-		return fmt.Errorf("parsing image name %q failed: %v", image, err)
-	}
-	// Add the latest lag if they did not provide one.
-	named = reference.TagNameOnly(named)
-	image = named.String()
+// SaveImages exports a list of images as a tarball which can then be imported by docker.
+func (c *Client) SaveImages(ctx context.Context, images []string, format string, writer io.WriteCloser) error {
+
+	exportOpts := []archive.ExportOpt{}
 
 	// Create the worker opts.
 	opt, err := c.createWorkerOpt(false)
@@ -31,8 +25,18 @@ func (c *Client) SaveImage(ctx context.Context, image, format string, writer io.
 		return errors.New("image store is nil")
 	}
 
-	exportOpts := []archive.ExportOpt{
-		archive.WithImage(opt.ImageStore, image),
+	for _, image := range images {
+		// Parse the image name and tag.
+		named, err := reference.ParseNormalizedNamed(image)
+		if err != nil {
+			return fmt.Errorf("parsing image name %q failed: %v", image, err)
+		}
+
+		// Add the latest lag if they did not provide one.
+		named = reference.TagNameOnly(named)
+		image = named.String()
+
+		exportOpts = append(exportOpts, archive.WithImage(opt.ImageStore, image))
 	}
 
 	switch format {
@@ -46,7 +50,7 @@ func (c *Client) SaveImage(ctx context.Context, image, format string, writer io.
 	}
 
 	if err := archive.Export(ctx, opt.ContentStore, writer, exportOpts...); err != nil {
-		return fmt.Errorf("exporting image %s failed: %v", image, err)
+		return fmt.Errorf("exporting images %v failed: %v", images, err)
 	}
 
 	return writer.Close()

save.go

@@ -3,10 +3,11 @@ package main
 import (
 	"context"
 	"fmt"
-	"github.com/spf13/cobra"
 	"io"
 	"os"
 
+	"github.com/spf13/cobra"
+
 	"github.com/containerd/containerd/namespaces"
 	"github.com/docker/docker/pkg/term"
 	"github.com/genuinetools/img/client"
@@ -15,8 +16,8 @@ import (
 )
 
 // TODO(AkihiroSuda): support OCI archive
-const saveUsageShortHelp = `Save an image to a tar archive (streamed to STDOUT by default).`
-const saveUsageLongHelp = `Save an image to a tar archive (streamed to STDOUT by default).`
+const saveUsageShortHelp = `Save one or more images to a tar archive (streamed to STDOUT by default).`
+const saveUsageLongHelp = `Save one or more images to a tar archive (streamed to STDOUT by default).`
 
 func newSaveCommand() *cobra.Command {
 
@@ -76,11 +77,9 @@ func (cmd *saveCommand) Run(args []string) (err error) {
 		return err
 	}
 
-	// Loop over the arguments as images and run save.
-	for _, image := range args {
-		if err := c.SaveImage(ctx, image, cmd.format, writer); err != nil {
-			return err
-		}
+	// Assume that the arguments are all image references
+	if err := c.SaveImages(ctx, args, cmd.format, writer); err != nil {
+		return err
 	}
 
 	return nil

save_test.go

@@ -54,3 +54,47 @@ func TestSaveImageInvalid(t *testing.T) {
 		t.Fatalf("expected invalid format to fail but did not: %s", string(out))
 	}
 }
+
+func TestSaveMultipleImages(t *testing.T) {
+	runBuild(t, "multiimage1", withDockerfile(`
+    FROM busybox
+	RUN echo multiimage1
+    `))
+
+	runBuild(t, "multiimage2", withDockerfile(`
+    FROM busybox
+	RUN echo multiimage2
+    `))
+
+	tmpf := filepath.Join(os.TempDir(), "save-multiple-images.tar")
+	defer os.RemoveAll(tmpf)
+
+	run(t, "save", "-o", tmpf, "multiimage1", "multiimage2")
+
+	// Make sure the file exists
+	if _, err := os.Stat(tmpf); os.IsNotExist(err) {
+		t.Fatalf("%s should exist after saving the image but it didn't", tmpf)
+	}
+}
+
+func TestSaveMultipleImagesOCI(t *testing.T) {
+	runBuild(t, "multiimage1", withDockerfile(`
+    FROM busybox
+	RUN echo multiimage1
+    `))
+
+	runBuild(t, "multiimage2", withDockerfile(`
+    FROM busybox
+	RUN echo multiimage2
+    `))
+
+	tmpf := filepath.Join(os.TempDir(), "save-multiple-oci.tar")
+	defer os.RemoveAll(tmpf)
+
+	run(t, "save", "--format", "oci", "-o", tmpf, "multiimage1", "multiimage2")
+
+	// Make sure the file exists
+	if _, err := os.Stat(tmpf); os.IsNotExist(err) {
+		t.Fatalf("%s should exist after saving the image but it didn't", tmpf)
+	}
+}