Avoid writing the same processed image to /public twice

Fixes #6307

hugolib/image_test.go

@@ -211,4 +211,7 @@ SUNSET2: {{ $resized2.RelPermalink }}/{{ $resized2.Width }}/Lat: {{ $resized2.Ex
 	b.AssertFileContent("resources/_gen/images/sunset_17701188623491591036.json",
 		"DateTimeDigitized|time.Time", "PENTAX")
 
+	// TODO(bep) add this as a default assertion after Build()?
+	b.AssertNoDuplicateWrites()
+
 }

hugolib/testhelpers_test.go

@@ -486,6 +486,8 @@ func (s *sitesBuilder) CreateSitesE() error {
 		return errors.Wrap(err, "failed to load config")
 	}
 
+	s.Fs.Destination = hugofs.NewCreateCountingFs(s.Fs.Destination)
+
 	depsCfg := s.depsCfg
 	depsCfg.Fs = s.Fs
 	depsCfg.Cfg = s.Cfg
@@ -680,6 +682,12 @@ func (s *sitesBuilder) AssertImage(width, height int, filename string) {
 	s.Assert(cfg.Height, qt.Equals, height)
 }
 
+func (s *sitesBuilder) AssertNoDuplicateWrites() {
+	s.Helper()
+	d := s.Fs.Destination.(hugofs.DuplicatesReporter)
+	s.Assert(d.ReportDuplicates(), qt.Equals, "")
+}
+
 func (s *sitesBuilder) FileContent(filename string) string {
 	s.T.Helper()
 	filename = filepath.FromSlash(filename)

resources/resource.go

@@ -233,19 +233,26 @@ func (l *genericResource) Permalink() string {
 }
 
 func (l *genericResource) Publish() error {
-	fr, err := l.ReadSeekCloser()
+	var err error
+	l.publishInit.Do(func() {
+		var fr hugio.ReadSeekCloser
+		fr, err = l.ReadSeekCloser()
 		if err != nil {
-		return err
+			return
 		}
 		defer fr.Close()
 
-	fw, err := helpers.OpenFilesForWriting(l.spec.BaseFs.PublishFs, l.getTargetFilenames()...)
+		var fw io.WriteCloser
+		fw, err = helpers.OpenFilesForWriting(l.spec.BaseFs.PublishFs, l.getTargetFilenames()...)
 		if err != nil {
-		return err
+			return
 		}
 		defer fw.Close()
 
 		_, err = io.Copy(fw, fr)
+
+	})
+
 	return err
 }
 
@@ -400,8 +407,10 @@ func (l genericResource) clone() *genericResource {
 	return &l
 }
 
-// returns an opened file or nil if nothing to write.
-func (l *genericResource) openDestinationsForWriting() (io.WriteCloser, error) {
+// returns an opened file or nil if nothing to write (it may already be published).
+func (l *genericResource) openDestinationsForWriting() (w io.WriteCloser, err error) {
+
+	l.publishInit.Do(func() {
 		targetFilenames := l.getTargetFilenames()
 		var changedFilenames []string
 
@@ -412,14 +421,20 @@ func (l *genericResource) openDestinationsForWriting() (io.WriteCloser, error) {
 			if _, err := l.getSpec().BaseFs.PublishFs.Stat(targetFilename); err == nil {
 				continue
 			}
+
 			changedFilenames = append(changedFilenames, targetFilename)
 		}
 
 		if len(changedFilenames) == 0 {
-		return nil, nil
+			return
 		}
 
-	return helpers.OpenFilesForWriting(l.getSpec().BaseFs.PublishFs, changedFilenames...)
+		w, err = helpers.OpenFilesForWriting(l.getSpec().BaseFs.PublishFs, changedFilenames...)
+
+	})
+
+	return
+
 }
 
 func (r *genericResource) openPublishFileForWriting(relTargetPath string) (io.WriteCloser, error) {
@@ -524,6 +539,8 @@ type permalinker interface {
 type resourceContent struct {
 	content     string
 	contentInit sync.Once
+
+	publishInit sync.Once
 }
 
 type resourceFileInfo struct {