Add a build 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.

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

cmd/flux/build.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 buildCmd = &cobra.Command{
+	Use:   "build",
+	Short: "Build a flux resource",
+	Long:  "The build command is used to build flux resources.",
+}
+
+func init() {
+	rootCmd.AddCommand(buildCmd)
+}

cmd/flux/build_kustomization.go

@@ -0,0 +1,80 @@
+/*
+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 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.`,
+	Example: `# Create a new overlay.
+flux build kustomization my-app --resources ./path/to/local/manifests`,
+	ValidArgsFunction: resourceNamesCompletionFunc(kustomizev1.GroupVersion.WithKind(kustomizev1.KustomizationKind)),
+	RunE:              buildKsCmdRun,
+}
+
+type buildKsFlags struct {
+	resources 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.)")
+	buildCmd.AddCommand(buildKsCmd)
+}
+
+func buildKsCmdRun(cmd *cobra.Command, args []string) error {
+	if len(args) < 1 {
+		return fmt.Errorf("%s name is required", kustomizationType.humanKind)
+	}
+	name := args[0]
+
+	if buildKsArgs.resources == "" {
+		return fmt.Errorf("invalid resource path %q", buildKsArgs.resources)
+	}
+
+	if fs, err := os.Stat(buildKsArgs.resources); err != nil || !fs.IsDir() {
+		return fmt.Errorf("invalid resource path %q", buildKsArgs.resources)
+	}
+
+	builder, err := kustomization.NewBuilder(rootArgs.kubeconfig, rootArgs.kubecontext, rootArgs.namespace, name, buildKsArgs.resources, kustomization.WithTimeout(rootArgs.timeout))
+	if err != nil {
+		return err
+	}
+
+	manifests, err := builder.Build()
+	if err != nil {
+		return err
+	}
+
+	cmd.Print(string(manifests))
+
+	return nil
+
+}