Improve error message when adoption-fields annotation is missing

When using `adoption-policy: adopt` without the `adoption-fields`
annotation, `ExtractAdoptionFields` calls `json.Unmarshal` on an empty
string, producing a generic "unexpected end of JSON input" error that
gives no indication of the actual problem.

This change adds an explicit empty-string check that returns a
descriptive error explaining that the `adoption-fields` annotation is
required, and wraps JSON parse errors with context about which
annotation failed.

Fixes aws-controllers-k8s/community#2824

Author: shinichitazawa

pkg/runtime/util.go

@@ -209,11 +209,18 @@ func NeedAdoption(res acktypes.AWSResource) bool {
 
 func ExtractAdoptionFields(res acktypes.AWSResource) (map[string]string, error) {
 	fields := getAdoptionFields(res)
+	if fields == "" {
+		return nil, fmt.Errorf(
+			"services.k8s.aws/adoption-fields annotation is required when " +
+				"adoption-policy is set to 'adopt'; please provide the resource " +
+				"identifier fields as a JSON object (e.g. '{\"name\": \"my-resource\"}')",
+		)
+	}
 
 	extractedFields := &map[string]string{}
 	err := json.Unmarshal([]byte(fields), extractedFields)
 	if err != nil {
-		return nil, err
+		return nil, fmt.Errorf("failed to parse services.k8s.aws/adoption-fields annotation as JSON: %w", err)
 	}
 
 	return *extractedFields, nil

pkg/runtime/util_test.go

@@ -14,6 +14,7 @@
 package runtime
 
 import (
+	"strings"
 	"testing"
 
 	"github.com/stretchr/testify/require"
@@ -118,3 +119,33 @@ func TestExtractAdoptionFields(t *testing.T) {
 	require.NoError(err)
 	require.Equal(expected, actual)
 }
+
+func TestExtractAdoptionFields_EmptyFields(t *testing.T) {
+	require := require.New(t)
+
+	// No adoption-fields annotation at all
+	res := &mocks.AWSResource{}
+	res.On("MetaObject").Return(&metav1.ObjectMeta{
+		Annotations: map[string]string{},
+	})
+
+	_, err := ExtractAdoptionFields(res)
+	require.Error(err)
+	require.True(strings.Contains(err.Error(), "services.k8s.aws/adoption-fields annotation is required"))
+	require.True(strings.Contains(err.Error(), "adoption-policy is set to 'adopt'"))
+}
+
+func TestExtractAdoptionFields_InvalidJSON(t *testing.T) {
+	require := require.New(t)
+
+	res := &mocks.AWSResource{}
+	res.On("MetaObject").Return(&metav1.ObjectMeta{
+		Annotations: map[string]string{
+			ackv1alpha1.AnnotationAdoptionFields: `not valid json`,
+		},
+	})
+
+	_, err := ExtractAdoptionFields(res)
+	require.Error(err)
+	require.True(strings.Contains(err.Error(), "failed to parse services.k8s.aws/adoption-fields annotation as JSON"))
+}