resource: Fix SVG and similar resource handling

The validation of if we could process the image (resize etc.) was moved up in Hugo 0.37, which meant SVG and other "non-processable" images would fail.

This commit fixes that by creating a regular resource for these image formats. They will not have `.Resize` or any of the other image methods.

Fixes #4455

resource/image_test.go

@@ -278,6 +278,13 @@ func TestImageResize8BitPNG(t *testing.T) {
 
 }
 
+func TestSVGImage(t *testing.T) {
+	assert := require.New(t)
+	spec := newTestResourceSpec(assert)
+	svg := fetchResourceForSpec(spec, assert, "circle.svg")
+	assert.NotNil(svg)
+}
+
 func BenchmarkResizeParallel(b *testing.B) {
 	assert := require.New(b)
 	img := fetchSunset(assert)

resource/resource.go

@@ -23,8 +23,6 @@ import (
 	"strings"
 	"sync"
 
-	"github.com/disintegration/imaging"
-
 	"github.com/spf13/cast"
 
 	"github.com/gobwas/glob"
@@ -288,6 +286,15 @@ func (r *Spec) newResource(
 	gr := r.newGenericResource(targetPathBuilder, fi, absPublishDir, absSourceFilename, filepath.ToSlash(relTargetFilename), mimeType)
 
 	if mimeType == "image" {
+		ext := strings.ToLower(helpers.Ext(absSourceFilename))
+
+		imgFormat, ok := imageFormats[ext]
+		if !ok {
+			// This allows SVG etc. to be used as resources. They will not have the methods of the Image, but
+			// that would not (currently) have worked.
+			return gr, nil
+		}
+
 		f, err := r.Fs.Source.Open(absSourceFilename)
 		if err != nil {
 			return nil, err
@@ -299,13 +306,6 @@ func (r *Spec) newResource(
 			return nil, err
 		}
 
-		ext := strings.ToLower(helpers.Ext(absSourceFilename))
-
-		imgFormat, ok := imageFormats[ext]
-		if !ok {
-			return nil, imaging.ErrUnsupportedFormat
-		}
-
 		return &Image{
 			hash:            hash,
 			format:          imgFormat,

resource/testdata/circle.svg

@@ -0,0 +1,5 @@
+<svg height="100" width="100">
+  <circle cx="50" cy="50" r="40" stroke="black" stroke-width="3" fill="red" />
+  Sorry, your browser does not support inline SVG.  
+</svg> 
+ 

resource/testhelpers_test.go

@@ -88,6 +88,12 @@ func fetchImage(assert *require.Assertions, name string) *Image {
 }
 
 func fetchImageForSpec(spec *Spec, assert *require.Assertions, name string) *Image {
+	r := fetchResourceForSpec(spec, assert, name)
+	assert.IsType(&Image{}, r)
+	return r.(*Image)
+}
+
+func fetchResourceForSpec(spec *Spec, assert *require.Assertions, name string) Resource {
 	src, err := os.Open("testdata/" + name)
 	assert.NoError(err)
 
@@ -107,10 +113,9 @@ func fetchImageForSpec(spec *Spec, assert *require.Assertions, name string) *Ima
 
 	r, err := spec.NewResourceFromFilename(factory, "/public", f, name)
 	assert.NoError(err)
-	assert.IsType(&Image{}, r)
-	return r.(*Image)
-}
 
+	return r
+}
 func assertFileCache(assert *require.Assertions, fs *hugofs.Fs, filename string, width, height int) {
 	f, err := fs.Source.Open(filepath.Join("/res/_gen/images", filename))
 	assert.NoError(err)