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

@@ -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)
  }
 

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 {