Add a diff kustomization feature

If implemented it will permit queriying the Kubernetes API to fetch the specified
Flux Kustomization, then uses the specified path to build the overlay.
It will then ssa-dry-run apply and output the diff using homeport/dyff

Signed-off-by: Soule BA <soule@weave.works>

cmd/flux/build_kustomization.go

@@ -30,22 +30,23 @@ var buildKsCmd = &cobra.Command{
 	Use:     "kustomization",
 	Aliases: []string{"ks"},
 	Short:   "Build Kustomization",
-	Long: `The build command queries the Kubernetes API and fetches the specified Flux Kustomization, 
-then it uses the specified files or path to build the overlay to write the resulting multi-doc YAML to stdout.`,
+	Long: `The build command queries the Kubernetes API and fetches the specified Flux Kustomization. 
+It then uses the fetched in cluster flux kustomization to perform needed transformation on the local kustomization.yaml
+pointed at by --path. The local kustomization.yaml is generated if it does not exist. Finally it builds the overlays using the local kustomization.yaml, and write the resulting multi-doc YAML to stdout.`,
 	Example: `# Create a new overlay.
-flux build kustomization my-app --resources ./path/to/local/manifests`,
+flux build kustomization my-app --path ./path/to/local/manifests`,
 	ValidArgsFunction: resourceNamesCompletionFunc(kustomizev1.GroupVersion.WithKind(kustomizev1.KustomizationKind)),
 	RunE:              buildKsCmdRun,
 }
 
 type buildKsFlags struct {
-	resources string
+	path string
 }
 
 var buildKsArgs buildKsFlags
 
 func init() {
-	buildKsCmd.Flags().StringVar(&buildKsArgs.resources, "resources", "", "Name of a file containing a file to add to the kustomization file.)")
+	buildKsCmd.Flags().StringVar(&buildKsArgs.path, "path", "", "Path to the manifests location.)")
 	buildCmd.AddCommand(buildKsCmd)
 }
 
@@ -55,15 +56,15 @@ func buildKsCmdRun(cmd *cobra.Command, args []string) error {
 	}
 	name := args[0]
 
-	if buildKsArgs.resources == "" {
-		return fmt.Errorf("invalid resource path %q", buildKsArgs.resources)
+	if buildKsArgs.path == "" {
+		return fmt.Errorf("invalid resource path %q", buildKsArgs.path)
 	}
 
-	if fs, err := os.Stat(buildKsArgs.resources); err != nil || !fs.IsDir() {
-		return fmt.Errorf("invalid resource path %q", buildKsArgs.resources)
+	if fs, err := os.Stat(buildKsArgs.path); err != nil || !fs.IsDir() {
+		return fmt.Errorf("invalid resource path %q", buildKsArgs.path)
 	}
 
-	builder, err := kustomization.NewBuilder(rootArgs.kubeconfig, rootArgs.kubecontext, rootArgs.namespace, name, buildKsArgs.resources, kustomization.WithTimeout(rootArgs.timeout))
+	builder, err := kustomization.NewBuilder(rootArgs.kubeconfig, rootArgs.kubecontext, rootArgs.namespace, name, buildKsArgs.path, kustomization.WithTimeout(rootArgs.timeout))
 	if err != nil {
 		return err
 	}

cmd/flux/diff.go

@@ -0,0 +1,31 @@
+/*
+Copyright 2021 The Flux authors
+
+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.
+*/
+
+package main
+
+import (
+	"github.com/spf13/cobra"
+)
+
+var diffCmd = &cobra.Command{
+	Use:   "diff",
+	Short: "Diff a flux resource",
+	Long:  "The diff command is used to do a server-side dry-run on flux resources, then output the diff.",
+}
+
+func init() {
+	rootCmd.AddCommand(diffCmd)
+}

cmd/flux/diff_kustomization.go

@@ -0,0 +1,77 @@
+/*
+Copyright 2021 The Flux authors
+
+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.
+*/
+
+package main
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/spf13/cobra"
+
+	"github.com/fluxcd/flux2/internal/kustomization"
+	kustomizev1 "github.com/fluxcd/kustomize-controller/api/v1beta2"
+)
+
+var diffKsCmd = &cobra.Command{
+	Use:     "kustomization",
+	Aliases: []string{"ks"},
+	Short:   "Diff Kustomization",
+	Long:    `The diff command does a build, then it performs a server-side dry-run and output the diff.`,
+	Example: `# Create a new overlay.
+flux diff kustomization my-app --path ./path/to/local/manifests`,
+	ValidArgsFunction: resourceNamesCompletionFunc(kustomizev1.GroupVersion.WithKind(kustomizev1.KustomizationKind)),
+	RunE:              diffKsCmdRun,
+}
+
+type diffKsFlags struct {
+	path string
+}
+
+var diffKsArgs diffKsFlags
+
+func init() {
+	diffKsCmd.Flags().StringVar(&diffKsArgs.path, "path", "", "Name of a file containing a file to add to the kustomization file.)")
+	diffCmd.AddCommand(diffKsCmd)
+}
+
+func diffKsCmdRun(cmd *cobra.Command, args []string) error {
+	if len(args) < 1 {
+		return fmt.Errorf("%s name is required", kustomizationType.humanKind)
+	}
+	name := args[0]
+
+	if diffKsArgs.path == "" {
+		return fmt.Errorf("invalid resource path %q", diffKsArgs.path)
+	}
+
+	if fs, err := os.Stat(diffKsArgs.path); err != nil || !fs.IsDir() {
+		return fmt.Errorf("invalid resource path %q", diffKsArgs.path)
+	}
+
+	builder, err := kustomization.NewBuilder(rootArgs.kubeconfig, rootArgs.kubecontext, rootArgs.namespace, name, diffKsArgs.path, kustomization.WithTimeout(rootArgs.timeout))
+	if err != nil {
+		return err
+	}
+
+	err = builder.Diff()
+	if err != nil {
+		return err
+	}
+
+	return nil
+
+}