Add symlink and new file structure to loader.go binary (bazelbuild#951)

* Add symlink to puller's OCI intermediate format

* modify symlink generator to account for new pulled image file structure

* amend puller to take in general output directory so as to maintain image convention and add image-oci

* simplify directory logic for new file structure and add error checking for safe os operations

* refactor utility function into a utility package

* rename functions in package and export consts

* export ImageDir for consistency and expand error checking

* refactor puller binary with const and integrate compat package

* add docs for symlink function

* Add symlink to puller's OCI intermediate format

* modify symlink generator to account for new pulled image file structure

* amend puller to take in general output directory so as to maintain image convention and add image-oci

* simplify directory logic for new file structure and add error checking for safe os operations

* refactor utility function into a utility package

* rename functions in package and export consts

* export ImageDir for consistency and expand error checking

* refactor puller binary with const and integrate compat package

* add docs for symlink function

* Add symlink and new file structure to loader.go binary

* rebase on puller_go to add dependency to loader binary

* rebase on puller_go branch and use exported constant in loader binary

* update loader binary to be compatible with updated compat package

* rename symlink naming to convention src to dst

* clarify symlink in docs

* Add symlink and new file structure to loader.go binary

* rebase on puller_go to add dependency to loader binary

* rebase on puller_go branch and use exported constant in loader binary

* update loader binary to be compatible with updated compat package

* rename variables and remove comments mentioning symlinks

container/go/cmd/loader/BUILD

@@ -22,6 +22,7 @@ go_library(
     importpath = "github.com/bazelbuild/rules_docker/container/go/cmd/loader",
     visibility = ["//visibility:private"],
     deps = [
+        "//container/go/pkg/compat:go_default_library",
         "//container/go/pkg/oci:go_default_library",
         "@com_github_google_go_containerregistry//pkg/v1/tarball:go_default_library",
         "@com_github_pkg_errors//:go_default_library",

container/go/cmd/loader/loader.go

@@ -22,8 +22,10 @@ package main
 import (
 	"flag"
 	"log"
+	"path"
 
 	"github.com/bazelbuild/rules_docker/container/go/pkg/oci"
+	"github.com/bazelbuild/rules_docker/container/go/pkg/compat"
 	"github.com/google/go-containerregistry/pkg/v1/tarball"
 	"github.com/pkg/errors"
 )
@@ -33,17 +35,29 @@ var (
 	imgTarball = flag.String("tarball", "", "The path to the tarball to load.")
 )
 
+// Directory containing the pulled OCI image artifacts without modification.
+const ociImageDir = "image-oci"
+
+ // Directory containing an image and image.tar to be consumed by container_import.
+const legacyDir = "image"
+
 // load loads a docker image tarball generated by docker save from path <dir>.
 func load(tar, dir string) error {
 	img, err := tarball.ImageFromPath(tar, nil)
 	if err != nil {
 		return errors.Wrapf(err, "failed to read docker image tarball from %q", dir)
 	}
 
-	if err = oci.Write(img, dir); err != nil {
+	ociPath := path.Join(dir, ociImageDir)
+	legacyPath := path.Join(dir, legacyDir)
+	if err = oci.Write(img, ociPath); err != nil {
 		return errors.Wrapf(err, "failed to write image to OCI layout at path %s", dir)
 	}
 
+	if err := compat.LegacyFromOCIImage(img, ociPath, legacyPath); err != nil {
+		return errors.Wrapf(err, "failed to generate symbolic links to pulled image at %s", dir)
+	}
+
 	return nil
 }
 

container/go/pkg/compat/ociToLegacy.go

@@ -80,7 +80,7 @@ func LegacyFromOCIImage(img v1.Image, srcDir, dstDir string) error {
 		layerDigest, err := layer.Digest()
 		if err != nil {
 			return errors.Wrap(err, "failed to fetch the layer's digest")
-		}
+	}
 
 		layerPath = path.Join(targetDir, layerDigest.Hex)
 		out := strconv.Itoa(i) + targzExt