resource: Use MD5 to identify image files

But only a set of byte chunks spread around in the image file to calculate the fingerprint, which is much faster than reading the whole file:

```bash
BenchmarkMD5FromFileFast/full=false-4         	  300000	      4356 ns/op	     240 B/op	       5 allocs/op
BenchmarkMD5FromFileFast/full=true-4          	   30000	     42899 ns/op	   32944 B/op	       5 allocs/op
```

Fixes #4186

3 files changed, 29 insertions, 22 deletions

diff --git a/resource/image.go b/resource/image.go
index c039f68b6..916f89a48 100644
--- a/resource/image.go
+++ b/resource/image.go
@@ -112,6 +112,8 @@ type Image struct {
 
 	imaging *Imaging
 
+	hash string
+
 	*genericResource
 }
 
@@ -129,6 +131,7 @@ func (i *Image) Height() int {
 func (i *Image) WithNewBase(base string) Resource {
 	return &Image{
 		imaging:         i.imaging,
+		hash:            i.hash,
 		genericResource: i.genericResource.WithNewBase(base).(*genericResource)}
 }
 
@@ -490,6 +493,7 @@ func (i *Image) clone() *Image {
 
 	return &Image{
 		imaging:         i.imaging,
+		hash:            i.hash,
 		genericResource: &g}
 }
 
@@ -497,20 +501,11 @@ func (i *Image) setBasePath(conf imageConfig) {
 	i.rel = i.filenameFromConfig(conf)
 }
 
-// We need to set this to something static during tests.
-var fiModTimeFunc = func(fi os.FileInfo) int64 {
-	return fi.ModTime().Unix()
-}
-
 func (i *Image) filenameFromConfig(conf imageConfig) string {
 	p1, p2 := helpers.FileAndExt(i.rel)
-	sizeModeStr := fmt.Sprintf("_S%d_T%d", i.osFileInfo.Size(), fiModTimeFunc(i.osFileInfo))
-	// 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.
-	if strings.Contains(p1, sizeModeStr) {
-		sizeModeStr = ""
-	}
+	idStr := fmt.Sprintf("_H%s_%d", i.hash, i.osFileInfo.Size())
 
+	// Do not change for no good reason.
 	const md5Threshold = 100
 
 	key := conf.key()
@@ -518,12 +513,16 @@ func (i *Image) filenameFromConfig(conf imageConfig) string {
 	// It is useful to have the key in clear text, but when nesting transforms, it
 	// can easily be too long to read, and maybe even too long
 	// for the different OSes to handle.
-	if len(p1)+len(sizeModeStr)+len(p2) > md5Threshold {
+	if len(p1)+len(idStr)+len(p2) > md5Threshold {
 		key = helpers.MD5String(p1 + key + p2)
-		p1 = p1[:strings.Index(p1, "_S")]
+		p1 = p1[:strings.Index(p1, "_H")]
+	} 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.
+		idStr = ""
 	}
 
-	return fmt.Sprintf("%s%s_%s%s", p1, sizeModeStr, key, p2)
+	return fmt.Sprintf("%s%s_%s%s", p1, idStr, key, p2)
 }
 
 func decodeImaging(m map[string]interface{}) (Imaging, error) {
diff --git a/resource/image_test.go b/resource/image_test.go
index 3543abb37..61a9ef844 100644
--- a/resource/image_test.go
+++ b/resource/image_test.go
@@ -15,7 +15,6 @@ package resource
 
 import (
 	"fmt"
-	"os"
 	"testing"
 
 	"github.com/stretchr/testify/require"
@@ -52,9 +51,6 @@ func TestParseImageConfig(t *testing.T) {
 }
 
 func TestImageTransform(t *testing.T) {
-	fiModTimeFunc = func(fi os.FileInfo) int64 {
-		return int64(10111213)
-	}
 
 	assert := require.New(t)
 
@@ -86,13 +82,13 @@ func TestImageTransform(t *testing.T) {
 	assert.Equal(200, resizedAndRotated.Height())
 	assertFileCache(assert, image.spec.Fs, resizedAndRotated.RelPermalink(), 125, 200)
 
-	assert.Equal("/a/sunset_S90587_T10111213_300x200_resize_q75_box_center.jpg", resized.RelPermalink())
+	assert.Equal("/a/sunset_H47566bb0ca0462db92c65f4033d77175_90587_300x200_resize_q75_box_center.jpg", resized.RelPermalink())
 	assert.Equal(300, resized.Width())
 	assert.Equal(200, resized.Height())
 
 	fitted, err := resized.Fit("50x50")
 	assert.NoError(err)
-	assert.Equal("/a/sunset_S90587_T10111213_300x200_resize_q75_box_center_50x50_fit_q75_box_center.jpg", fitted.RelPermalink())
+	assert.Equal("/a/sunset_H47566bb0ca0462db92c65f4033d77175_90587_9b37eba4e4e6ea0cc56a59bb5aa98143.jpg", fitted.RelPermalink())
 	assert.Equal(50, fitted.Width())
 	assert.Equal(31, fitted.Height())
 
@@ -100,13 +96,13 @@ func TestImageTransform(t *testing.T) {
 	fittedAgain, _ := fitted.Fit("10x20")
 	fittedAgain, err = fittedAgain.Fit("10x20")
 	assert.NoError(err)
-	assert.Equal("/a/sunset_f1fb715a17c42d5d4602a1870424d590.jpg", fittedAgain.RelPermalink())
+	assert.Equal("/a/sunset_H47566bb0ca0462db92c65f4033d77175_90587_9a8be1402216c385e0dfd73e267c6827.jpg", fittedAgain.RelPermalink())
 	assert.Equal(10, fittedAgain.Width())
 	assert.Equal(6, fittedAgain.Height())
 
 	filled, err := image.Fill("200x100 bottomLeft")
 	assert.NoError(err)
-	assert.Equal("/a/sunset_S90587_T10111213_200x100_fill_q75_box_bottomleft.jpg", filled.RelPermalink())
+	assert.Equal("/a/sunset_H47566bb0ca0462db92c65f4033d77175_90587_200x100_fill_q75_box_bottomleft.jpg", filled.RelPermalink())
 	assert.Equal(200, filled.Width())
 	assert.Equal(100, filled.Height())
 	assertFileCache(assert, image.spec.Fs, filled.RelPermalink(), 200, 100)
diff --git a/resource/resource.go b/resource/resource.go
index 2c934d031..19392f3d3 100644
--- a/resource/resource.go
+++ b/resource/resource.go
@@ -153,7 +153,19 @@ func (r *Spec) newResource(
 	gr := r.newGenericResource(linker, fi, absPublishDir, absSourceFilename, filepath.ToSlash(relTargetFilename), mimeType)
 
 	if mimeType == "image" {
+		f, err := r.Fs.Source.Open(absSourceFilename)
+		if err != nil {
+			return nil, err
+		}
+		defer f.Close()
+
+		hash, err := helpers.MD5FromFileFast(f)
+		if err != nil {
+			return nil, err
+		}
+
 		return &Image{
+			hash:            hash,
 			imaging:         r.imaging,
 			genericResource: gr}, nil
 	}