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/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 --resources ./path/to/local/manifests`,
+	ValidArgsFunction: resourceNamesCompletionFunc(kustomizev1.GroupVersion.WithKind(kustomizev1.KustomizationKind)),
+	RunE:              diffKsCmdRun,
+}
+
+type diffKsFlags struct {
+	resources string
+}
+
+var diffKsArgs diffKsFlags
+
+func init() {
+	diffKsCmd.Flags().StringVar(&diffKsArgs.resources, "resources", "", "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.resources == "" {
+		return fmt.Errorf("invalid resource path %q", diffKsArgs.resources)
+	}
+
+	if fs, err := os.Stat(diffKsArgs.resources); err != nil || !fs.IsDir() {
+		return fmt.Errorf("invalid resource path %q", diffKsArgs.resources)
+	}
+
+	builder, err := kustomization.NewBuilder(rootArgs.kubeconfig, rootArgs.kubecontext, rootArgs.namespace, name, diffKsArgs.resources, kustomization.WithTimeout(rootArgs.timeout))
+	if err != nil {
+		return err
+	}
+
+	err = builder.Diff(rootArgs.kubeconfig, rootArgs.kubecontext)
+	if err != nil {
+		return err
+	}
+
+	return nil
+
+}

go.mod

@@ -16,24 +16,25 @@ require (
 	github.com/fluxcd/pkg/apis/kustomize v0.2.0
 	github.com/fluxcd/pkg/apis/meta v0.10.1
 	github.com/fluxcd/pkg/runtime v0.12.2
-	github.com/fluxcd/pkg/ssa v0.3.1
+	github.com/fluxcd/pkg/ssa v0.5.0
 	github.com/fluxcd/pkg/ssh v0.0.5
 	github.com/fluxcd/pkg/untar v0.1.0
 	github.com/fluxcd/pkg/version v0.0.1
 	github.com/fluxcd/source-controller/api v0.19.0
 	github.com/go-errors/errors v1.4.0 // indirect
 	github.com/go-git/go-git/v5 v5.4.2
+	github.com/gonvenience/ytbx v1.4.2 // indirect
 	github.com/google/go-cmp v0.5.6
 	github.com/google/go-containerregistry v0.2.0
 	github.com/hashicorp/go-retryablehttp v0.7.0 // indirect
+	github.com/homeport/dyff v1.4.6
 	github.com/manifoldco/promptui v0.9.0
 	github.com/mattn/go-shellwords v1.0.12
 	github.com/olekukonko/tablewriter v0.0.4
-	github.com/spf13/cobra v1.1.3
+	github.com/spf13/cobra v1.2.1
 	github.com/spf13/pflag v1.0.5
 	golang.org/x/crypto v0.0.0-20210817164053-32db794688a5
-	golang.org/x/sys v0.0.0-20210903071746-97244b99971b // indirect
-	golang.org/x/term v0.0.0-20210615171337-6886f2dfbf5b
+	golang.org/x/term v0.0.0-20210916214954-140adaaadfaf
 	k8s.io/api v0.22.2
 	k8s.io/apiextensions-apiserver v0.22.2
 	k8s.io/apimachinery v0.22.2