From f7526fef6e7c7bc001b5f6819945b420fcc28638 Mon Sep 17 00:00:00 2001
From: Justin Kulikauskas <jkulikau@redhat.com>
Date: Fri, 26 Apr 2024 11:58:24 -0400
Subject: [PATCH] Add resolve option to error on missing keys

By default, the template engine uses "<no value>" when a key is missing
from a lookup, and it does not return an error. That behavior can make
it less obvious when something is going wrong in a template.

Signed-off-by: Justin Kulikauskas <jkulikau@redhat.com>
---
 pkg/templates/templates.go      | 10 ++++++++++
 pkg/templates/templates_test.go |  7 +++++++
 2 files changed, 17 insertions(+)

diff --git a/pkg/templates/templates.go b/pkg/templates/templates.go
index e8d03e3d..91d52d57 100644
--- a/pkg/templates/templates.go
+++ b/pkg/templates/templates.go
@@ -59,6 +59,7 @@ var (
 	ErrCacheDisabled            = client.ErrCacheDisabled
 	ErrNoCacheEntry             = client.ErrNoCacheEntry
 	ErrContextTransformerFailed = errors.New("the context transformer failed")
+	ErrMissingKey               = errors.New("missing key while resolving template")
 )
 
 // Config is a struct containing configuration for the API.
@@ -121,6 +122,7 @@ type ResolveOptions struct {
 	DisableAutoCacheCleanUp bool
 	LookupNamespace         string
 	Watcher                 *client.ObjectIdentifier
+	ErrorOnMissingKey       bool
 }
 
 type ClusterScopedObjectIdentifier struct {
@@ -629,11 +631,19 @@ func (t *TemplateResolver) ResolveTemplate(
 		}
 	}
 
+	if options.ErrorOnMissingKey {
+		tmpl = tmpl.Option("missingkey=error")
+	}
+
 	err = tmpl.Execute(&buf, ctx)
 	if err != nil {
 		tmplRawStr := string(tmplRaw)
 		klog.Errorf("error resolving the template %v,\n template str %v,\n error: %v", tmplRawStr, templateStr, err)
 
+		if options.ErrorOnMissingKey && strings.Contains(err.Error(), "no entry for key") {
+			return resolvedResult, fmt.Errorf("%w %v: %w", ErrMissingKey, tmplRawStr, err)
+		}
+
 		return resolvedResult, fmt.Errorf("failed to resolve the template %v: %w", tmplRawStr, err)
 	}
 
diff --git a/pkg/templates/templates_test.go b/pkg/templates/templates_test.go
index 4e8c7328..8198ece1 100644
--- a/pkg/templates/templates_test.go
+++ b/pkg/templates/templates_test.go
@@ -725,6 +725,13 @@ func TestResolveTemplateErrors(t *testing.T) {
 				ErrInvalidInput,
 			),
 		},
+		"error_on_missing_key": {
+			inputTmpl: `foo: '{{ (lookup "v1" "ConfigMap" "testns" "nonexist-cm").data }}'`,
+			resolveOptions: ResolveOptions{
+				ErrorOnMissingKey: true,
+			},
+			expectedErr: ErrMissingKey,
+		},
 	}
 
 	for testName, test := range testcases {