1 files changed, 92 insertions, 0 deletions
diff --git a/hugofs/hashing_fs.go b/hugofs/hashing_fs.go
new file mode 100644
index 000000000..94a50b960
--- /dev/null
+++ b/hugofs/hashing_fs.go
@@ -0,0 +1,92 @@
+// Copyright 2019 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 hugofs
+
+import (
+	"crypto/md5"
+	"encoding/hex"
+	"hash"
+	"os"
+
+	"github.com/spf13/afero"
+)
+
+var (
+	_ afero.Fs = (*md5HashingFs)(nil)
+)
+
+// FileHashReceiver will receive the filename an the content's MD5 sum on file close.
+type FileHashReceiver interface {
+	OnFileClose(name, md5sum string)
+}
+
+type md5HashingFs struct {
+	afero.Fs
+	hashReceiver FileHashReceiver
+}
+
+// NewHashingFs creates a new filesystem that will receive MD5 checksums of
+// any written file content on Close. Note that this is probably not a good
+// idea for "full build" situations, but when doing fast render mode, the amount
+// of files published is low, and it would be really nice to know exactly which
+// of these files where actually changed.
+// Note that this will only work for file operations that use the io.Writer
+// to write content to file, but that is fine for the "publish content" use case.
+func NewHashingFs(delegate afero.Fs, hashReceiver FileHashReceiver) afero.Fs {
+	return &md5HashingFs{Fs: delegate, hashReceiver: hashReceiver}
+}
+
+func (fs *md5HashingFs) Create(name string) (afero.File, error) {
+	f, err := fs.Fs.Create(name)
+	if err == nil {
+		f = fs.wrapFile(f)
+	}
+	return f, err
+}
+
+func (fs *md5HashingFs) OpenFile(name string, flag int, perm os.FileMode) (afero.File, error) {
+	f, err := fs.Fs.OpenFile(name, flag, perm)
+	if err == nil && isWrite(flag) {
+		f = fs.wrapFile(f)
+	}
+	return f, err
+}
+
+func (fs *md5HashingFs) wrapFile(f afero.File) afero.File {
+	return &hashingFile{File: f, h: md5.New(), hashReceiver: fs.hashReceiver}
+}
+
+func (fs *md5HashingFs) Name() string {
+	return "md5HashingFs"
+}
+
+type hashingFile struct {
+	hashReceiver FileHashReceiver
+	h            hash.Hash
+	afero.File
+}
+
+func (h *hashingFile) Write(p []byte) (n int, err error) {
+	n, err = h.File.Write(p)
+	if err != nil {
+		return
+	}
+	return h.h.Write(p)
+}
+
+func (h *hashingFile) Close() error {
+	sum := hex.EncodeToString(h.h.Sum(nil))
+	h.hashReceiver.OnFileClose(h.Name(), sum)
+	return h.File.Close()
+}