Add basic "post resource publish support"

Fixes #7146

resources/post_publish.go

@@ -0,0 +1,51 @@
+// Copyright 2020 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package resources
+
+import (
+	"github.com/gohugoio/hugo/resources/postpub"
+	"github.com/gohugoio/hugo/resources/resource"
+)
+
+type transformationKeyer interface {
+	TransformationKey() string
+}
+
+// PostProcess wraps the given Resource for later processing.
+func (spec *Spec) PostProcess(r resource.Resource) (postpub.PostPublishedResource, error) {
+	key := r.(transformationKeyer).TransformationKey()
+	spec.postProcessMu.RLock()
+	result, found := spec.PostProcessResources[key]
+	spec.postProcessMu.RUnlock()
+	if found {
+		return result, nil
+	}
+
+	spec.postProcessMu.Lock()
+	defer spec.postProcessMu.Unlock()
+
+	// Double check
+	result, found = spec.PostProcessResources[key]
+	if found {
+		return result, nil
+	}
+
+	result = postpub.NewPostPublishResource(spec.incr.Incr(), r)
+	if result == nil {
+		panic("got nil result")
+	}
+	spec.PostProcessResources[key] = result
+
+	return result, nil
+}