Implement extractConfig binary in Go (bazelbuild#967)

* Implement extractConfig binary in Go

* builidifier error resolution

* moved extract config binary to a package under container/go

* update layer_tools.bzl to utilize new binary

container/go/cmd/extract_config/BUILD

@@ -1,8 +1,24 @@
+# Copyright 2017 The Bazel Authors. All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+# ###
+# Build file for extractConfig binary based on go-containerregistry backend.
+
 load("@io_bazel_rules_go//go:def.bzl", "go_binary", "go_library")
 
 go_library(
     name = "go_default_library",
-    srcs = ["main.go"],
+    srcs = ["extractConfig.go"],
     importpath = "github.com/bazelbuild/rules_docker/container/go/cmd/extract_config",
     visibility = ["//visibility:private"],
     deps = ["@com_github_google_go_containerregistry//pkg/v1/tarball:go_default_library"],

container/go/cmd/extract_config/extractConfig.go

@@ -0,0 +1,74 @@
+// Copyright 2017 The Bazel Authors. All rights reserved.
+
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+
+//    http://www.apache.org/licenses/LICENSE-2.0
+
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License/
+////////////////////////////////////
+//This binary implements the ability to load a docker image tarball and
+// extract its config & manifest json to paths specified via command line
+// arguments.
+// It expects to be run with:
+//     extract_config -tarball=image.tar -output=output.confi
+package main
+
+import (
+	"flag"
+	"io/ioutil"
+	"log"
+
+	"github.com/google/go-containerregistry/pkg/v1/tarball"
+)
+
+var (
+	imageTar       = flag.String("imageTar", "", "The path to the Docker image tarball to extract the config & manifest for.")
+	outputConfig   = flag.String("outputConfig", "", "The path to the output file where the image config will be written to.")
+	outputManifest = flag.String("outputManifest", "", "The path to the output file where the image manifest will be written to.")
+)
+
+func main() {
+	flag.Parse()
+	log.Println("Running the Image Config & Manifest Extractor.")
+
+	if *imageTar == "" {
+		log.Fatalln("Required option -imageTar was not specified.")
+	}
+	if *outputConfig == "" {
+		log.Fatalln("Required option -outputConfig was not specified.")
+	}
+	if *outputManifest == "" {
+		log.Fatalln("Required option -outputManifest was not specified.")
+	}
+
+	img, err := tarball.ImageFromPath(*imageTar, nil)
+	if err != nil {
+		log.Fatalf("Unable to load docker image from %s: %v", *imageTar, err)
+	}
+
+	// Write the config file contents to the ouput file specified withh permissions 0644.
+	configContent, err := img.RawConfigFile()
+	if err != nil {
+		log.Fatalf("Failed to read config file: %v", err)
+	}
+	if err := ioutil.WriteFile(*outputConfig, configContent, 0644); err != nil {
+		log.Fatalf("Failed to write config file contents to %s: %v", *outputConfig, err)
+	}
+
+	// Write the manifest file contents to the manifestoutput file specified with permissions 0644.
+	manifestContent, err := img.RawManifest()
+	if err != nil {
+		log.Fatalf("Failed to read manifest file: %v", err)
+	}
+	if err := ioutil.WriteFile(*outputManifest, manifestContent, 0644); err != nil {
+		log.Fatalf("Failed to write manifest file contents to %s: %v", *outputManifest, err)
+	}
+
+	log.Println("Image Config & Manifest Extractor was successful.")
+}

container/layer_tools.bzl

@@ -29,11 +29,11 @@ def _extract_layers(ctx, name, artifact):
     ctx.actions.run(
         executable = ctx.executable.extract_config,
         arguments = [
-            "--tarball",
+            "-imageTar",
             artifact.path,
-            "--output",
+            "-outputConfig",
             config_file.path,
-            "--manifestoutput",
+            "-outputManifest",
             manifest_file.path,
         ],
         tools = [artifact],
@@ -235,7 +235,7 @@ def incremental_load(
 
 tools = {
     "extract_config": attr.label(
-        default = Label("//container:extract_config"),
+        default = Label("//container/go/cmd/extract_config:extract_config"),
         cfg = "host",
         executable = True,
         allow_files = True,