resources: Move resource interfaces into its own package

resources/resource_transformers/integrity/integrity.go

@@ -0,0 +1,106 @@
+// Copyright 2018 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package integrity
+
+import (
+	"crypto/md5"
+	"crypto/sha256"
+	"crypto/sha512"
+	"encoding/base64"
+	"encoding/hex"
+	"fmt"
+	"hash"
+	"html/template"
+	"io"
+
+	"github.com/gohugoio/hugo/resources"
+	"github.com/gohugoio/hugo/resources/resource"
+)
+
+const defaultHashAlgo = "sha256"
+
+// Client contains methods to fingerprint (cachebusting) and other integrity-related
+// methods.
+type Client struct {
+	rs *resources.Spec
+}
+
+// New creates a new Client with the given specification.
+func New(rs *resources.Spec) *Client {
+	return &Client{rs: rs}
+}
+
+type fingerprintTransformation struct {
+	algo string
+}
+
+func (t *fingerprintTransformation) Key() resources.ResourceTransformationKey {
+	return resources.NewResourceTransformationKey("fingerprint", t.algo)
+}
+
+// Transform creates a MD5 hash of the Resource content and inserts that hash before
+// the extension in the filename.
+func (t *fingerprintTransformation) Transform(ctx *resources.ResourceTransformationCtx) error {
+	algo := t.algo
+
+	var h hash.Hash
+
+	switch algo {
+	case "md5":
+		h = md5.New()
+	case "sha256":
+		h = sha256.New()
+	case "sha512":
+		h = sha512.New()
+	default:
+		return fmt.Errorf("unsupported crypto algo: %q, use either md5, sha256 or sha512", algo)
+	}
+
+	io.Copy(io.MultiWriter(h, ctx.To), ctx.From)
+	d, err := digest(h)
+	if err != nil {
+		return err
+	}
+
+	ctx.Data["Integrity"] = integrity(algo, d)
+	ctx.AddOutPathIdentifier("." + hex.EncodeToString(d[:]))
+	return nil
+}
+
+// Fingerprint applies fingerprinting of the given resource and hash algorithm.
+// It defaults to sha256 if none given, and the options are md5, sha256 or sha512.
+// The same algo is used for both the fingerprinting part (aka cache busting) and
+// 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) {
+	if algo == "" {
+		algo = defaultHashAlgo
+	}
+
+	return c.rs.Transform(
+		res,
+		&fingerprintTransformation{algo: algo},
+	)
+}
+
+func integrity(algo string, sum []byte) template.HTMLAttr {
+	encoded := base64.StdEncoding.EncodeToString(sum)
+	return template.HTMLAttr(algo + "-" + encoded)
+}
+
+func digest(h hash.Hash) ([]byte, error) {
+	sum := h.Sum(nil)
+	return sum, nil
+}