cipd: Implement local cache for ResolveVersion(...) call with tags.

go/src/infra/tools/cipd/internal/checksum.go

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+package internal
+
+import (
+        "bytes"
+        "crypto/sha1"
+        "errors"
+
+        "github.com/golang/protobuf/proto"
+
+        "infra/tools/cipd/internal/messages"
+)
+
+// MarshalWithSHA1 serializes proto message to bytes, calculates SHA1 checksum
+// of it, and returns serialized envelope that contains both. UnmarshalWithSHA1
+// can then be used to verify SHA1 and deserialized the original object.
+func MarshalWithSHA1(pm proto.Message) ([]byte, error) {
+        blob, err := proto.Marshal(pm)
+        if err != nil {
+                return nil, err
+        }
+        sum := sha1.Sum(blob)
+        envelope := messages.BlobWithSHA1{Blob: blob, Sha1: sum[:]}
+        return proto.Marshal(&envelope)
+}
+
+// UnmarshalWithSHA1Check is reverse of MarshalWithSHA1. It checks SHA1 checksum
+// and deserializes the object if it matches the blob.
+func UnmarshalWithSHA1(buf []byte, pm proto.Message) error {
+        envelope := messages.BlobWithSHA1{}

[nodir, 2015/09/30 19:08:23]

I prefer 

  var envelope messages.BlobWithSHA1

but it doesn't matter

[Vadim Sh., 2015/09/30 22:50:59]

And I prefer var := type{} :)

+        if err := proto.Unmarshal(buf, &envelope); err != nil {
+                return err
+        }
+        sum := sha1.Sum(envelope.GetBlob())
+        if !bytes.Equal(sum[:], envelope.GetSha1()) {
+                return errors.New("check sum of tag cache file is invalid")
+        }
+        return proto.Unmarshal(envelope.GetBlob(), pm)
+}