Add 'kpod pull' command

Signed-off-by: umohnani8 <umohnani@redhat.com>

cmd/kpod/pull.go

@@ -0,0 +1,125 @@
+package main
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/Sirupsen/logrus"
+	cp "github.com/containers/image/copy"
+	"github.com/containers/image/docker/reference"
+	"github.com/containers/image/signature"
+	is "github.com/containers/image/storage"
+	"github.com/containers/image/transports/alltransports"
+	"github.com/containers/image/types"
+	"github.com/containers/storage"
+	"github.com/pkg/errors"
+	"github.com/urfave/cli"
+)
+
+const (
+	// DefaultRegistry is a prefix that we apply to an image name
+	// to check docker hub first for the image
+	DefaultRegistry = "docker://"
+)
+
+var (
+	pullFlags = []cli.Flag{
+		cli.BoolFlag{
+			// all-tags is hidden since it has not been implemented yet
+			Name:   "all-tags, a",
+			Hidden: true,
+			Usage:  "Download all tagged images in the repository",
+		},
+	}
+
+	pullDescription = "Pulls an image from a registry and stores it locally.\n" +
+		"An image can be pulled using its tag or digest. If a tag is not\n" +
+		"specified, the image with the 'latest' tag (if it exists) is pulled."
+	pullCommand = cli.Command{
+		Name:        "pull",
+		Usage:       "pull an image from a registry",
+		Description: pullDescription,
+		Flags:       pullFlags,
+		Action:      pullCmd,
+		ArgsUsage:   "",
+	}
+)
+
+// pullCmd gets the data from the command line and calls pullImage
+// to copy an image from a registry to a local machine
+func pullCmd(c *cli.Context) error {
+	args := c.Args()
+	if len(args) == 0 {
+		logrus.Errorf("an image name must be specified")
+		return nil
+	}
+	if len(args) > 1 {
+		logrus.Errorf("too many arguments. Requires exactly 1")
+		return nil
+	}
+	image := args[0]
+
+	store, err := getStore(c)
+	if err != nil {
+		return err
+	}
+
+	allTags := false
+	if c.IsSet("all-tags") {
+		allTags = c.Bool("all-tags")
+	}
+
+	systemContext := getSystemContext("")
+
+	err = pullImage(store, image, allTags, systemContext)
+	if err != nil {
+		return errors.Errorf("error pulling image from %q: %v", image, err)
+	}
+	return nil
+}
+
+// pullImage copies the image from the source to the destination
+func pullImage(store storage.Store, imgName string, allTags bool, sc *types.SystemContext) error {
+	defaultName := DefaultRegistry + imgName
+	var fromName string
+	var tag string
+
+	srcRef, err := alltransports.ParseImageName(defaultName)
+	if err != nil {
+		srcRef2, err2 := alltransports.ParseImageName(imgName)
+		if err2 != nil {
+			return errors.Wrapf(err2, "error parsing image name %q", imgName)
+		}
+		srcRef = srcRef2
+	}
+
+	ref := srcRef.DockerReference()
+	if ref != nil {
+		imgName = srcRef.DockerReference().Name()
+		fromName = imgName
+		tagged, ok := srcRef.DockerReference().(reference.NamedTagged)
+		if ok {
+			imgName = imgName + ":" + tagged.Tag()
+			tag = tagged.Tag()
+		}
+	}
+
+	destRef, err := is.Transport.ParseStoreReference(store, imgName)
+	if err != nil {
+		return errors.Wrapf(err, "error parsing full image name %q", imgName)
+	}
+
+	policy, err := signature.DefaultPolicy(sc)
+	if err != nil {
+		return err
+	}
+
+	policyContext, err := signature.NewPolicyContext(policy)
+	if err != nil {
+		return err
+	}
+
+	fmt.Println(tag + ": pulling from " + fromName)
+
+	return cp.Image(policyContext, destRef, srcRef, getCopyOptions(os.Stdout))
+}