tpl/transform: Add transform.Unmarshal func

Fixes #5428

tpl/transform/unmarshal.go

@@ -0,0 +1,98 @@
+// 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 transform
+
+import (
+	"io/ioutil"
+
+	"github.com/gohugoio/hugo/common/hugio"
+
+	"github.com/gohugoio/hugo/helpers"
+	"github.com/gohugoio/hugo/parser/metadecoders"
+	"github.com/gohugoio/hugo/resource"
+	"github.com/pkg/errors"
+
+	"github.com/spf13/cast"
+)
+
+// Unmarshal unmarshals the data given, which can be either a string
+// or a Resource. Supported formats are JSON, TOML and YAML.
+func (ns *Namespace) Unmarshal(data interface{}) (interface{}, error) {
+
+	// All the relevant Resource types implements ReadSeekCloserResource,
+	// which should be the most effective way to get the content.
+	if r, ok := data.(resource.ReadSeekCloserResource); ok {
+		var key string
+		var reader hugio.ReadSeekCloser
+
+		if k, ok := r.(resource.Identifier); ok {
+			key = k.Key()
+		}
+
+		if key == "" {
+			reader, err := r.ReadSeekCloser()
+			if err != nil {
+				return nil, err
+			}
+			defer reader.Close()
+
+			key, err = helpers.MD5FromReader(reader)
+			if err != nil {
+				return nil, err
+			}
+
+			reader.Seek(0, 0)
+		}
+
+		return ns.cache.GetOrCreate(key, func() (interface{}, error) {
+			f := metadecoders.FormatFromMediaType(r.MediaType())
+			if f == "" {
+				return nil, errors.Errorf("MIME %q not supported", r.MediaType())
+			}
+
+			if reader == nil {
+				var err error
+				reader, err = r.ReadSeekCloser()
+				if err != nil {
+					return nil, err
+				}
+				defer reader.Close()
+			}
+
+			b, err := ioutil.ReadAll(reader)
+			if err != nil {
+				return nil, err
+			}
+
+			return metadecoders.Unmarshal(b, f)
+		})
+
+	}
+
+	dataStr, err := cast.ToStringE(data)
+	if err != nil {
+		return nil, errors.Errorf("type %T not supported", data)
+	}
+
+	key := helpers.MD5String(dataStr)
+
+	return ns.cache.GetOrCreate(key, func() (interface{}, error) {
+		f := metadecoders.FormatFromContentString(dataStr)
+		if f == "" {
+			return nil, errors.New("unknown format")
+		}
+
+		return metadecoders.Unmarshal([]byte(dataStr), f)
+	})
+}