feat(engine): add debug log when placeholder lookup runs without cluster connection

Signed-off-by: SamuZad <samuzadori@opensignal.com>

pkg/engine/funcs.go

@@ -21,6 +21,7 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
+	"log/slog"
 	"maps"
 	"math"
 	"reflect"
@@ -88,8 +89,15 @@ func funcMap() template.FuncMap {
 		"tpl":      func(string, any) any { return "not implemented" },
 		"required": func(string, any) (any, error) { return "not implemented", nil },
 		// Provide a placeholder for the "lookup" function, which requires a kubernetes
-		// connection.
-		"lookup": func(string, string, string, string) (map[string]any, error) {
+		// connection. Emits a debug log so `helm template --debug` surfaces the fact
+		// that a chart's `lookup` calls are being silently short-circuited.
+		"lookup": func(apiVersion, kind, namespace, name string) (map[string]any, error) {
+			slog.Debug("lookup: no Kubernetes client available, returning empty result",
+				slog.String("apiVersion", apiVersion),
+				slog.String("kind", kind),
+				slog.String("namespace", namespace),
+				slog.String("name", name),
+			)
 			return map[string]any{}, nil
 		},
 	}

pkg/engine/funcs_test.go

@@ -17,6 +17,8 @@ limitations under the License.
 package engine
 
 import (
+	"bytes"
+	"log/slog"
 	"math"
 	"strings"
 	"testing"
@@ -195,6 +197,30 @@ keyInElement1 = "valueInElement1"`,
 	}
 }
 
+// TestPlaceholderLookupEmitsDebugLog verifies that the offline `lookup`
+// placeholder (used by `helm template` when there is no cluster connection)
+// emits a debug log identifying the requested resource, so users running
+// `helm template --debug` can see that `lookup` was silently short-circuited.
+func TestPlaceholderLookupEmitsDebugLog(t *testing.T) {
+	var buf bytes.Buffer
+	handler := slog.NewTextHandler(&buf, &slog.HandlerOptions{Level: slog.LevelDebug})
+	prev := slog.Default()
+	slog.SetDefault(slog.New(handler))
+	t.Cleanup(func() { slog.SetDefault(prev) })
+
+	tpl := `{{ lookup "v1" "ConfigMap" "some-ns" "some-name" }}`
+	var out strings.Builder
+	require.NoError(t, template.Must(template.New("test").Funcs(funcMap()).Parse(tpl)).Execute(&out, nil))
+	assert.Equal(t, "map[]", out.String())
+
+	logged := buf.String()
+	assert.Contains(t, logged, "lookup: no Kubernetes client available")
+	assert.Contains(t, logged, `apiVersion=v1`)
+	assert.Contains(t, logged, `kind=ConfigMap`)
+	assert.Contains(t, logged, `namespace=some-ns`)
+	assert.Contains(t, logged, `name=some-name`)
+}
+
 func TestDurationHelpers(t *testing.T) {
 	tests := []struct {
 		name   string