Refactor new_container_pull to generate fully backward compatible for…

…mat (bazelbuild#974)

* refactor puller.go to generate fully backward compatible format

* implement manifest.json symlink creation

* refactor variable names and add error checks for manifest list length

* modify manifest list check to prevent manifest panic

* refactor error message

* add comma

container/go/cmd/puller/puller.go

@@ -16,7 +16,6 @@
 // For the format specification, if the format is:
 // 		1. 'docker': image is pulled as tarball and may be used with `docker load -i`.
 // 		2. 'oci' (default): image will be pulled as a collection of files in OCI layout to directory.
-// 		3. 'both': both formats of image are pulled.
 // Unlike regular docker pull, the format this package uses is proprietary.
 
 package main
@@ -31,8 +30,8 @@ import (
 
 	"github.com/pkg/errors"
 
-	"github.com/bazelbuild/rules_docker/container/go/pkg/oci"
 	"github.com/bazelbuild/rules_docker/container/go/pkg/compat"
+	"github.com/bazelbuild/rules_docker/container/go/pkg/oci"
 	"github.com/google/go-containerregistry/pkg/authn"
 	"github.com/google/go-containerregistry/pkg/name"
 	v1 "github.com/google/go-containerregistry/pkg/v1"

container/go/pkg/compat/BUILD

@@ -29,6 +29,7 @@ go_library(
     visibility = ["//visibility:public"],
     deps = [
         "@com_github_google_go_containerregistry//pkg/v1:go_default_library",
+        "@com_github_google_go_containerregistry//pkg/v1/layout:go_default_library",
         "@com_github_google_go_containerregistry//pkg/v1/partial:go_default_library",
         "@com_github_google_go_containerregistry//pkg/v1/types:go_default_library",
         "@com_github_google_go_containerregistry//pkg/v1/validate:go_default_library",

container/go/pkg/compat/ociToLegacy.go

@@ -19,17 +19,20 @@
 package compat
 
 import (
+	"log"
 	ospkg "os"
 	"path"
 	"strconv"
 
 	v1 "github.com/google/go-containerregistry/pkg/v1"
+	"github.com/google/go-containerregistry/pkg/v1/layout"
 	"github.com/pkg/errors"
 )
 
 // Extension for layers and config files that are made symlinks
-const targzExt = ".tar.gz"
-const configExt = "config.json"
+const compressedLayerExt = ".tar.gz"
+const legacyConfigFile = "config.json"
+const legacyManifestFile = "manifest.json"
 
 // generateSymlinks safely generates a symbolic link at dst pointing to src.
 func generateSymlinks(src, dst string) error {
@@ -63,7 +66,7 @@ func LegacyFromOCIImage(img v1.Image, srcDir, dstDir string) error {
 		return errors.Wrap(err, "failed to get the config file's hash information for image")
 	}
 	configPath := path.Join(targetDir, config.Hex)
-	dstLink := path.Join(dstDir, configExt)
+	dstLink := path.Join(dstDir, legacyConfigFile)
 	if err = generateSymlinks(configPath, dstLink); err != nil {
 		return errors.Wrap(err, "failed to generate config.json symlink")
 	}
@@ -80,15 +83,35 @@ 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
+		out := strconv.Itoa(i) + compressedLayerExt
 		dstLink = path.Join(dstDir, out)
 		if err = generateSymlinks(layerPath, dstLink); err != nil {
 			return errors.Wrapf(err, "failed to generate legacy symlink for layer %d with digest %s", i, layerDigest)
 		}
 	}
 
+	// symlink for the image manifest pulled into OCI layout to .json, so must rename the file from being
+	// named after its sha256 digest under blobs/sha256 to manifest.json
+	imgIndex, err := layout.ImageIndexFromPath(srcDir)
+	if err != nil {
+		return errors.Wrapf(err, "unable to open image as index from %s", srcDir)
+	}
+	manifest, err := imgIndex.IndexManifest()
+	if err != nil {
+		return errors.Wrap(err, "unable to get manifest from image index")
+	}
+	if len(manifest.Manifests) != 1 {
+		log.Fatalf("Image index read from %s had unexpected number of manifests: got %d, want 1", srcDir, len(manifest.Manifests))
+	}
+	manifestHex := manifest.Manifests[0].Digest.Hex
+	dstLink = path.Join(dstDir, legacyManifestFile)
+	manifestPath := path.Join(targetDir, manifestHex)
+	if err = generateSymlinks(manifestPath, dstLink); err != nil {
+		return errors.Wrapf(err, "failed to generate %s symlink", legacyManifestFile)
+	}
+
 	return nil
 }