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

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

Index: go/src/infra/tools/cipd/internal/checksum.go
diff --git a/go/src/infra/tools/cipd/internal/checksum.go b/go/src/infra/tools/cipd/internal/checksum.go
new file mode 100644
index 0000000000000000000000000000000000000000..1dc24a04ecd0b20f4e59f90ab986f497e37c6280
--- /dev/null
+++ b/go/src/infra/tools/cipd/internal/checksum.go
@@ -0,0 +1,42 @@
+// 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{}
+	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)
+}