api: return an error when object get --encoding=protobuf is fails.

When `--encoding=protobuf` was set, `object get` used to always return a 200
response and would leave the body blank on error. Now, it returns a JSON error.

fixes ipfs#2468

License: MIT
Signed-off-by: Steven Allen <steven@stebalien.com>

commands/http/handler.go

@@ -71,6 +71,10 @@ var mimeTypes = map[string]string{
 	cmds.Text:     "text/plain",
 }
 
+var errorMimeTypes = map[string]string{
+	cmds.Protobuf: "application/json",
+}
+
 type ServerConfig struct {
 	// Headers is an optional map of headers that is written out.
 	Headers map[string][]string
@@ -206,6 +210,12 @@ func guessMimeType(res cmds.Response) (string, error) {
 		return "", errors.New("no encoding option set")
 	}
 
+	if res.Error() != nil {
+		if m, ok := errorMimeTypes[enc]; ok {
+			return m, nil
+		}
+	}
+
 	if m, ok := mimeTypes[enc]; ok {
 		return m, nil
 	}

core/commands/object/object.go

@@ -223,15 +223,25 @@ This command outputs data in the following encodings:
 	Type: Node{},
 	Marshalers: cmds.MarshalerMap{
 		cmds.Protobuf: func(res cmds.Response) (io.Reader, error) {
-			node := res.Output().(*Node)
-			object, err := deserializeNode(node)
-			if err != nil {
-				return nil, err
-			}
-
-			marshaled, err := object.Marshal()
-			if err != nil {
-				return nil, err
+			var marshaled []byte
+
+			if res.Error() != nil {
+				output, err := json.Marshal(res.Error())
+				if err != nil {
+					return nil, err
+				}
+				marshaled = append(output, '\n')
+			} else {
+				node := res.Output().(*Node)
+				object, err := deserializeNode(node)
+				if err != nil {
+					return nil, err
+				}
+				output, err := object.Marshal()
+				if err != nil {
+					return nil, err
+				}
+				marshaled = output
 			}
 			return bytes.NewReader(marshaled), nil
 		},