xl: Simplify reading metadata and add a new fileMetadata type. (#1346)

xl-v1-metadata.go

@@ -0,0 +1,78 @@
+/*
+ * Minio Cloud Storage, (C) 2016 Minio, Inc.
+ *
+ * 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 main
+
+import (
+	"encoding/json"
+	"io"
+)
+
+// This code is built on similar ideas of http.Header.
+// Ref - https://golang.org/pkg/net/http/#Header
+
+// A fileMetadata represents a metadata header mapping
+// keys to sets of values.
+type fileMetadata map[string][]string
+
+// Add adds the key, value pair to the header.
+// It appends to any existing values associated with key.
+func (f fileMetadata) Add(key, value string) {
+	f[key] = append(f[key], value)
+}
+
+// Set sets the header entries associated with key to
+// the single element value. It replaces any existing
+// values associated with key.
+func (f fileMetadata) Set(key, value string) {
+	f[key] = []string{value}
+}
+
+// Get gets the first value associated with the given key.
+// If there are no values associated with the key, Get returns "".
+// Get is a convenience method.  For more complex queries,
+// access the map directly.
+func (f fileMetadata) Get(key string) []string {
+	if f == nil {
+		return nil
+	}
+	v, ok := f[key]
+	if !ok {
+		return nil
+	}
+	return v
+}
+
+// Write writes a metadata in wire format.
+func (f fileMetadata) Write(writer io.Writer) error {
+	metadataBytes, err := json.Marshal(f)
+	if err != nil {
+		return err
+	}
+	_, err = writer.Write(metadataBytes)
+	return err
+}
+
+// fileMetadataDecode - file metadata decode.
+func fileMetadataDecode(reader io.Reader) (fileMetadata, error) {
+	metadata := make(fileMetadata)
+	decoder := json.NewDecoder(reader)
+	// Unmarshalling failed, file possibly corrupted.
+	if err := decoder.Decode(&metadata); err != nil {
+		return nil, err
+	}
+	return metadata, nil
+}