image/cas/put: Add a PutJSON helper

A fair amount of image setup involves pushing JSON objects to CAS, so
provide a convenient wrapper around that.  This implementation could
be improved, with at least:

* Consistent key sorts, etc. to increase the chances of matching an
  existing CAS blob.
* Streaming the marshaled JSON into the engine to avoid serializing it
  in memory before passing it into Engine.Put.

But the API is fine, and we can improve the implementation as we go.

Signed-off-by: W. Trevor King <[email protected]>

Author: wking

image/cas/put.go

@@ -0,0 +1,47 @@
+// Copyright 2016 The Linux Foundation
+//
+// 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 cas
+
+import (
+	"bytes"
+	"encoding/json"
+
+	"github.com/opencontainers/image-spec/specs-go"
+	"golang.org/x/net/context"
+)
+
+// PutJSON writes a generic JSON object to content-addressable storage
+// and returns a Descriptor referencing it.
+func PutJSON(ctx context.Context, engine Engine, data interface{}, mediaType string) (descriptor *specs.Descriptor, err error) {
+	jsonBytes, err := json.Marshal(data)
+	if err != nil {
+		return nil, err
+	}
+	size := len(jsonBytes)
+	size64 := int64(size) // panics on overflow
+
+	reader := bytes.NewReader(jsonBytes)
+	digest, err := engine.Put(ctx, reader)
+	if err != nil {
+		return nil, err
+	}
+
+	descriptor = &specs.Descriptor{
+		MediaType: mediaType,
+		Digest:    digest,
+		Size:      size64,
+	}
+	return descriptor, nil
+}