2 files changed, 41 insertions, 8 deletions
diff --git a/resources/resource_transformers/integrity/integrity.go b/resources/resource_transformers/integrity/integrity.go
index 95065603d..1b74de7eb 100644
--- a/resources/resource_transformers/integrity/integrity.go
+++ b/resources/resource_transformers/integrity/integrity.go
@@ -23,6 +23,8 @@ import (
 	"html/template"
 	"io"
 
+	"github.com/gohugoio/hugo/resources/internal"
+
 	"github.com/pkg/errors"
 
 	"github.com/gohugoio/hugo/resources"
@@ -46,8 +48,8 @@ type fingerprintTransformation struct {
 	algo string
 }
 
-func (t *fingerprintTransformation) Key() resources.ResourceTransformationKey {
-	return resources.NewResourceTransformationKey("fingerprint", t.algo)
+func (t *fingerprintTransformation) Key() internal.ResourceTransformationKey {
+	return internal.NewResourceTransformationKey("fingerprint", t.algo)
 }
 
 // Transform creates a MD5 hash of the Resource content and inserts that hash before
@@ -59,7 +61,17 @@ func (t *fingerprintTransformation) Transform(ctx *resources.ResourceTransformat
 		return err
 	}
 
-	io.Copy(io.MultiWriter(h, ctx.To), ctx.From)
+	var w io.Writer
+	if rc, ok := ctx.From.(io.ReadSeeker); ok {
+		// This transformation does not change the content, so try to
+		// avoid writing to To if we can.
+		defer rc.Seek(0, 0)
+		w = h
+	} else {
+		w = io.MultiWriter(h, ctx.To)
+	}
+
+	io.Copy(w, ctx.From)
 	d, err := digest(h)
 	if err != nil {
 		return err
@@ -91,15 +103,12 @@ func newHash(algo string) (hash.Hash, error) {
 // the base64-encoded Subresource Integrity hash, so you will have to stay away from
 // md5 if you plan to use both.
 // See https://developer.mozilla.org/en-US/docs/Web/Security/Subresource_Integrity
-func (c *Client) Fingerprint(res resource.Resource, algo string) (resource.Resource, error) {
+func (c *Client) Fingerprint(res resources.ResourceTransformer, algo string) (resource.Resource, error) {
 	if algo == "" {
 		algo = defaultHashAlgo
 	}
 
-	return c.rs.Transform(
-		res,
-		&fingerprintTransformation{algo: algo},
-	)
+	return res.Transform(&fingerprintTransformation{algo: algo})
 }
 
 func integrity(algo string, sum []byte) template.HTMLAttr {
diff --git a/resources/resource_transformers/integrity/integrity_test.go b/resources/resource_transformers/integrity/integrity_test.go
index cb1caa006..3759e6313 100644
--- a/resources/resource_transformers/integrity/integrity_test.go
+++ b/resources/resource_transformers/integrity/integrity_test.go
@@ -14,9 +14,13 @@
 package integrity
 
 import (
+	"html/template"
 	"testing"
 
+	"github.com/gohugoio/hugo/resources/resource"
+
 	qt "github.com/frankban/quicktest"
+	"github.com/gohugoio/hugo/resources/resource_transformers/htesting"
 )
 
 func TestHashFromAlgo(t *testing.T) {
@@ -46,3 +50,23 @@ func TestHashFromAlgo(t *testing.T) {
 		})
 	}
 }
+
+func TestTransform(t *testing.T) {
+	c := qt.New(t)
+
+	spec, err := htesting.NewTestResourceSpec()
+	c.Assert(err, qt.IsNil)
+	client := New(spec)
+
+	r, err := htesting.NewResourceTransformerForSpec(spec, "hugo.txt", "Hugo Rocks!")
+	c.Assert(err, qt.IsNil)
+
+	transformed, err := client.Fingerprint(r, "")
+
+	c.Assert(err, qt.IsNil)
+	c.Assert(transformed.RelPermalink(), qt.Equals, "/hugo.a5ad1c6961214a55de53c1ce6e60d27b6b761f54851fa65e33066460dfa6a0db.txt")
+	c.Assert(transformed.Data(), qt.DeepEquals, map[string]interface{}{"Integrity": template.HTMLAttr("sha256-pa0caWEhSlXeU8HObmDSe2t2H1SFH6ZeMwZkYN+moNs=")})
+	content, err := transformed.(resource.ContentProvider).Content()
+	c.Assert(err, qt.IsNil)
+	c.Assert(content, qt.Equals, "Hugo Rocks!")
+}