feat(directives): support writing kustomize output to multiple files

internal/directives/kustomize_builder.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
+	"strings"
 	"sync"
 
 	securejoin "github.com/cyphar/filepath-securejoin"
@@ -88,21 +89,57 @@ func (k *kustomizeBuilder) runPromotionStep(
 	if err != nil {
 		return PromotionStepResult{Status: PromotionStatusFailure}, err
 	}
-	if err = os.MkdirAll(filepath.Dir(outPath), 0o700); err != nil {
-		return PromotionStepResult{Status: PromotionStatusFailure}, err
-	}
 
 	// Write the built manifests to the output path.
-	b, err := rm.AsYaml()
-	if err != nil {
-		return PromotionStepResult{Status: PromotionStatusFailure}, err
-	}
-	if err = os.WriteFile(outPath, b, 0o600); err != nil {
-		return PromotionStepResult{Status: PromotionStatusFailure}, err
+	if err := k.writeResult(rm, outPath); err != nil {
+		return PromotionStepResult{Status: PromotionStatusFailure}, fmt.Errorf(
+			"failed to write built manifests to %q: %w", cfg.OutPath,
+			sanitizePathError(err, stepCtx.WorkDir),
+		)
 	}
 	return PromotionStepResult{Status: PromotionStatusSuccess}, nil
 }
 
+func (k *kustomizeBuilder) writeResult(rm resmap.ResMap, outPath string) error {
+	if ext := filepath.Ext(outPath); ext == ".yaml" || ext == ".yml" {
+		if err := os.MkdirAll(filepath.Dir(outPath), 0o700); err != nil {
+			return err
+		}
+		b, err := rm.AsYaml()
+		if err != nil {
+			return err
+		}
+		return os.WriteFile(outPath, b, 0o600)
+	}
+
+	// If the output path is a directory, write each manifest to a separate file.
+	if err := os.MkdirAll(outPath, 0o700); err != nil {
+		return err
+	}
+	for _, r := range rm.Resources() {
+		kind, namespace, name := r.GetKind(), r.GetNamespace(), r.GetName()
+		if kind == "" || name == "" {
+			return fmt.Errorf("resource kind and name of %q must be non-empty to write to a directory", r.CurId())
+		}
+
+		fileName := fmt.Sprintf("%s-%s", kind, name)
+		if namespace != "" {
+			fileName = fmt.Sprintf("%s-%s", namespace, fileName)
+		}
+
+		b, err := r.AsYAML()
+		if err != nil {
+			return fmt.Errorf("failed to convert %q to YAML: %w", r.CurId(), err)
+		}
+
+		path := filepath.Join(outPath, fmt.Sprintf("%s.yaml", strings.ToLower(fileName)))
+		if err = os.WriteFile(path, b, 0o600); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
 // kustomizeBuild builds the manifests in the given directory using Kustomize.
 func kustomizeBuild(fs filesys.FileSystem, path string) (_ resmap.ResMap, err error) {
 	kustomizeRenderMutex.Lock()

internal/directives/kustomize_builder_test.go

@@ -46,6 +46,36 @@ metadata:
 				assert.Contains(t, string(b), "test-deployment")
 			},
 		},
+		{
+			name: "successful build with output directory",
+			setupFiles: func(t *testing.T, dir string) {
+				require.NoError(t, os.WriteFile(filepath.Join(dir, "kustomization.yaml"), []byte(`
+apiVersion: kustomize.config.k8s.io/v1beta1
+kind: Kustomization
+resources:
+- deployment.yaml
+`), 0o600))
+				require.NoError(t, os.WriteFile(filepath.Join(dir, "deployment.yaml"), []byte(`---
+apiVersion: apps/v1
+kind: Deployment
+metadata:
+  name: test-deployment
+`), 0o600))
+			},
+			config: KustomizeBuildConfig{
+				Path:    ".",
+				OutPath: "output/",
+			},
+			assertions: func(t *testing.T, dir string, result PromotionStepResult, err error) {
+				require.NoError(t, err)
+				assert.Equal(t, PromotionStepResult{Status: PromotionStatusSuccess}, result)
+
+				assert.DirExists(t, filepath.Join(dir, "output"))
+				b, err := os.ReadFile(filepath.Join(dir, "output", "deployment-test-deployment.yaml"))
+				require.NoError(t, err)
+				assert.Contains(t, string(b), "test-deployment")
+			},
+		},
 		{
 			name:       "kustomization file not found",
 			setupFiles: func(*testing.T, string) {},