resource: Fix handling of very long image file names

Fixes #4261

resource/image.go

@@ -538,7 +538,14 @@ func (i *Image) filenameFromConfig(conf imageConfig) string {
 	// for the different OSes to handle.
 	if len(p1)+len(idStr)+len(p2) > md5Threshold {
 		key = helpers.MD5String(p1 + key + p2)
-		p1 = p1[:strings.Index(p1, "_hu")]
+		huIdx := strings.Index(p1, "_hu")
+		if huIdx != -1 {
+			p1 = p1[:huIdx]
+		} else {
+			// This started out as a very long file name. Making it even longer
+			// could melt ice in the Arctic.
+			p1 = ""
+		}
 	} else if strings.Contains(p1, idStr) {
 		// On scaling an already scaled image, we get the file info from the original.
 		// Repeating the same info in the filename makes it stuttery for no good reason.

resource/image_test.go

@@ -115,6 +115,25 @@ func TestImageTransform(t *testing.T) {
 
 }
 
+// https://github.com/gohugoio/hugo/issues/4261
+func TestImageTransformLongFilename(t *testing.T) {
+	assert := require.New(t)
+
+	image := fetchImage(assert, "1234567890qwertyuiopasdfghjklzxcvbnm5to6eeeeee7via8eleph.jpg")
+	assert.NotNil(image)
+
+	resized, err := image.Resize("200x")
+	assert.NoError(err)
+	assert.NotNil(resized)
+	assert.Equal(200, resized.Width())
+	assert.Equal("/a/_hu59e56ffff1bc1d8d122b1403d34e039f_90587_fd0f8b23902abcf4092b68783834f7fe.jpg", resized.RelPermalink())
+	resized, err = resized.Resize("100x")
+	assert.NoError(err)
+	assert.NotNil(resized)
+	assert.Equal(100, resized.Width())
+	assert.Equal("/a/_hu59e56ffff1bc1d8d122b1403d34e039f_90587_5f399e62910070692b3034a925f1b2d7.jpg", resized.RelPermalink())
+}
+
 func TestDecodeImaging(t *testing.T) {
 	assert := require.New(t)
 	m := map[string]interface{}{

resource/testhelpers_test.go

@@ -37,12 +37,16 @@ func newTestResourceSpecForBaseURL(assert *require.Assertions, baseURL string) *
 }
 
 func fetchSunset(assert *require.Assertions) *Image {
-	src, err := os.Open("testdata/sunset.jpg")
+	return fetchImage(assert, "sunset.jpg")
+}
+
+func fetchImage(assert *require.Assertions, name string) *Image {
+	src, err := os.Open("testdata/" + name)
 	assert.NoError(err)
 
 	spec := newTestResourceSpec(assert)
 
-	out, err := spec.Fs.Source.Create("/b/sunset.jpg")
+	out, err := spec.Fs.Source.Create("/b/" + name)
 	assert.NoError(err)
 	_, err = io.Copy(out, src)
 	out.Close()
@@ -53,7 +57,7 @@ func fetchSunset(assert *require.Assertions) *Image {
 		return path.Join("/a", s)
 	}
 
-	r, err := spec.NewResourceFromFilename(factory, "/public", "/b/sunset.jpg", "sunset.jpg")
+	r, err := spec.NewResourceFromFilename(factory, "/public", "/b/"+name, name)
 	assert.NoError(err)
 	assert.IsType(&Image{}, r)
 	return r.(*Image)