tpl/transform: Add transform.Unmarshal func

Fixes #5428

3 files changed, 31 insertions, 6 deletions

diff --git a/resource/resource.go b/resource/resource.go
index a8f9dde06..0f5a43648 100644
--- a/resource/resource.go
+++ b/resource/resource.go
@@ -50,6 +50,7 @@ var (
 	_ ResourcesLanguageMerger = (*Resources)(nil)
 	_ permalinker             = (*genericResource)(nil)
 	_ collections.Slicer      = (*genericResource)(nil)
+	_ Identifier              = (*genericResource)(nil)
 )
 
 var noData = make(map[string]interface{})
@@ -76,6 +77,8 @@ type Cloner interface {
 
 // Resource represents a linkable resource, i.e. a content page, image etc.
 type Resource interface {
+	resourceBase
+
 	// Permalink represents the absolute link to this resource.
 	Permalink() string
 
@@ -87,9 +90,6 @@ type Resource interface {
 	// For content pages, this value is "page".
 	ResourceType() string
 
-	// MediaType is this resource's MIME type.
-	MediaType() media.Type
-
 	// Name is the logical name of this resource. This can be set in the front matter
 	// metadata for this resource. If not set, Hugo will assign a value.
 	// This will in most cases be the base filename.
@@ -109,6 +109,13 @@ type Resource interface {
 	Params() map[string]interface{}
 }
 
+// resourceBase pulls out the minimal set of operations to define a Resource,
+// to simplify testing etc.
+type resourceBase interface {
+	// MediaType is this resource's MIME type.
+	MediaType() media.Type
+}
+
 // ResourcesLanguageMerger describes an interface for merging resources from a
 // different language.
 type ResourcesLanguageMerger interface {
@@ -121,12 +128,17 @@ type translatedResource interface {
 	TranslationKey() string
 }
 
+// Identifier identifies a resource.
+type Identifier interface {
+	Key() string
+}
+
 // ContentResource represents a Resource that provides a way to get to its content.
 // Most Resource types in Hugo implements this interface, including Page.
 // This should be used with care, as it will read the file content into memory, but it
 // should be cached as effectively as possible by the implementation.
 type ContentResource interface {
-	Resource
+	resourceBase
 
 	// Content returns this resource's content. It will be equivalent to reading the content
 	// that RelPermalink points to in the published folder.
@@ -143,7 +155,7 @@ type OpenReadSeekCloser func() (hugio.ReadSeekCloser, error)
 
 // ReadSeekCloserResource is a Resource that supports loading its content.
 type ReadSeekCloserResource interface {
-	Resource
+	resourceBase
 	ReadSeekCloser() (hugio.ReadSeekCloser, error)
 }
 
@@ -716,6 +728,10 @@ func (l *genericResource) RelPermalink() string {
 	return l.relPermalinkFor(l.relTargetDirFile.path())
 }
 
+func (l *genericResource) Key() string {
+	return l.relTargetDirFile.path()
+}
+
 func (l *genericResource) relPermalinkFor(target string) string {
 	return l.relPermalinkForRel(target, false)
 
diff --git a/resource/resource_test.go b/resource/resource_test.go
index b76f0a604..b3f6035b6 100644
--- a/resource/resource_test.go
+++ b/resource/resource_test.go
@@ -50,6 +50,7 @@ func TestGenericResourceWithLinkFacory(t *testing.T) {
 
 	assert.Equal("https://example.com/foo/foo.css", r.Permalink())
 	assert.Equal("/foo/foo.css", r.RelPermalink())
+	assert.Equal("foo.css", r.Key())
 	assert.Equal("css", r.ResourceType())
 }
 
diff --git a/resource/transform.go b/resource/transform.go
index 796c7ee23..bd59d0658 100644
--- a/resource/transform.go
+++ b/resource/transform.go
@@ -38,6 +38,7 @@ var (
 	_ ContentResource        = (*transformedResource)(nil)
 	_ ReadSeekCloserResource = (*transformedResource)(nil)
 	_ collections.Slicer     = (*transformedResource)(nil)
+	_ Identifier             = (*transformedResource)(nil)
 )
 
 func (s *Spec) Transform(r Resource, t ResourceTransformation) (Resource, error) {
@@ -249,6 +250,13 @@ func (r *transformedResource) MediaType() media.Type {
 	return m
 }
 
+func (r *transformedResource) Key() string {
+	if err := r.initTransform(false, false); err != nil {
+		return ""
+	}
+	return r.linker.relPermalinkFor(r.Target)
+}
+
 func (r *transformedResource) Permalink() string {
 	if err := r.initTransform(false, true); err != nil {
 		return ""
@@ -481,8 +489,8 @@ func (r *transformedResource) transform(setContent, publish bool) (err error) {
 	}
 
 	return nil
-
 }
+
 func (r *transformedResource) initTransform(setContent, publish bool) error {
 	r.transformInit.Do(func() {
 		r.published = publish