Add dynamic completion suppport

This commit adds dynamic completion support for the following commands
and flags:

- `flux delete ...` command
- `flux export ...` command
- `flux get ...` command
- `flux reconcile ...` command
- `flux resume ...` command
- `flux suspend ...` command
- `--namespace` flag
- `--context` flag

Signed-off-by: Kazuki Suda <kazuki.suda@gmail.com>

cmd/flux/completion.go

@@ -17,7 +17,18 @@ limitations under the License.
 package main
 
 import (
+	"context"
+	"strings"
+
+	"github.com/fluxcd/flux2/internal/utils"
 	"github.com/spf13/cobra"
+	"k8s.io/apimachinery/pkg/api/meta"
+	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
+	"k8s.io/apimachinery/pkg/runtime/schema"
+	"k8s.io/client-go/discovery"
+	memory "k8s.io/client-go/discovery/cached"
+	"k8s.io/client-go/dynamic"
+	"k8s.io/client-go/restmapper"
 )
 
 var completionCmd = &cobra.Command{
@@ -29,3 +40,77 @@ var completionCmd = &cobra.Command{
 func init() {
 	rootCmd.AddCommand(completionCmd)
 }
+
+func contextsCompletionFunc(cmd *cobra.Command, args []string, toComplete string) ([]string, cobra.ShellCompDirective) {
+	rawConfig, err := utils.ClientConfig(rootArgs.kubeconfig, rootArgs.kubecontext).RawConfig()
+	if err != nil {
+		return completionError(err)
+	}
+
+	var comps []string
+
+	for name := range rawConfig.Contexts {
+		if strings.HasPrefix(name, toComplete) {
+			comps = append(comps, name)
+		}
+	}
+
+	return comps, cobra.ShellCompDirectiveNoFileComp
+}
+
+func resourceNamesCompletionFunc(gvk schema.GroupVersionKind) func(cmd *cobra.Command, args []string, toComplete string) ([]string, cobra.ShellCompDirective) {
+	return func(cmd *cobra.Command, args []string, toComplete string) ([]string, cobra.ShellCompDirective) {
+		ctx, cancel := context.WithTimeout(context.Background(), rootArgs.timeout)
+		defer cancel()
+
+		cfg, err := utils.KubeConfig(rootArgs.kubeconfig, rootArgs.kubecontext)
+		if err != nil {
+			return completionError(err)
+		}
+
+		dc, err := discovery.NewDiscoveryClientForConfig(cfg)
+		if err != nil {
+			return completionError(err)
+		}
+		mapper := restmapper.NewDeferredDiscoveryRESTMapper(memory.NewMemCacheClient(dc))
+
+		mapping, err := mapper.RESTMapping(gvk.GroupKind(), gvk.Version)
+		if err != nil {
+			return completionError(err)
+		}
+
+		client, err := dynamic.NewForConfig(cfg)
+		if err != nil {
+			return completionError(err)
+		}
+
+		var dr dynamic.ResourceInterface
+		if mapping.Scope.Name() == meta.RESTScopeNameNamespace {
+			dr = client.Resource(mapping.Resource).Namespace(rootArgs.namespace)
+		} else {
+			dr = client.Resource(mapping.Resource)
+		}
+
+		list, err := dr.List(ctx, metav1.ListOptions{})
+		if err != nil {
+			return completionError(err)
+		}
+
+		var comps []string
+
+		for _, item := range list.Items {
+			name := item.GetName()
+
+			if strings.HasPrefix(name, toComplete) {
+				comps = append(comps, name)
+			}
+		}
+
+		return comps, cobra.ShellCompDirectiveNoFileComp
+	}
+}
+
+func completionError(err error) ([]string, cobra.ShellCompDirective) {
+	cobra.CompError(err.Error())
+	return nil, cobra.ShellCompDirectiveError
+}