Use a two-part data format version in IndexedDB metadata.

content/common/indexed_db/indexed_db_data_format_version.h

+// Copyright 2017 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.
+
+#ifndef CONTENT_COMMON_INDEXED_DB_INDEXED_DB_DATA_FORMAT_VERSION_H_
+#define CONTENT_COMMON_INDEXED_DB_INDEXED_DB_DATA_FORMAT_VERSION_H_
+
+#include <cstdint>
+#include <limits>
+
+#include "base/logging.h"
+
+namespace content {
+
+// Contains version data for the wire format used for encoding IndexedDB values.
+// A version tuple (a, b) is at least as new as (a', b')
+// iff a >= a' and b >= b'.
+class IndexedDBDataFormatVersion {
+ public:
+  IndexedDBDataFormatVersion() {}
+  IndexedDBDataFormatVersion(uint32_t v8_version, uint32_t blink_version)
+      : v8_version_(v8_version), blink_version_(blink_version) {}
+
+  uint32_t v8_version() const { return v8_version_; }
+  uint32_t blink_version() const { return blink_version_; }
+
+  bool operator==(const IndexedDBDataFormatVersion& other) const {
+    return v8_version_ == other.v8_version_ &&
+           blink_version_ == other.blink_version_;
+  }
+  bool operator!=(const IndexedDBDataFormatVersion& other) const {
+    return !operator==(other);
+  }
+
+  bool IsAtLeast(const IndexedDBDataFormatVersion& other) const {
+    return v8_version_ >= other.v8_version_ &&
+           blink_version_ >= other.blink_version_;
+  }
+
+  // Encodes and decodes the tuple from an int64_t.
+  // Since negative values are considered invalid, this scheme will only work

[dcheng, 2017/04/04 19:59:21]

Nit: I think there's a missing word or two here.

+  // the v8 version would overflow int32_t. We check both, to be consistent.
+  //
+  // This scheme is chosen so that earlier versions (before we reported both the
+  // Blink and V8 versions) decode properly, with a V8 version of 0.
+  int64_t Encode() const {
+    DCHECK_GE(static_cast<int32_t>(v8_version_), 0);
+    DCHECK_GE(static_cast<int32_t>(blink_version_), 0);
+    return (static_cast<int64_t>(v8_version_) << 32) | blink_version_;
+  }
+  static IndexedDBDataFormatVersion Decode(int64_t encoded) {
+    DCHECK_GE(encoded, 0);
+    return IndexedDBDataFormatVersion(encoded >> 32, encoded);
+  }
+
+ private:
+  uint32_t v8_version_ = 0;
+  uint32_t blink_version_ = 0;
+};
+
+}  // namespace content
+
+#endif  // CONTENT_COMMON_INDEXED_DB_INDEXED_DB_DATA_FORMAT_VERSION_H_