LogDog: Add Storage-layer data caching.

logdog/common/storage/archive/cache.go

+// Copyright 2016 The LUCI Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
+package archive
+
+import (
+        "github.com/luci/luci-go/common/gcloud/gs"
+        "github.com/luci/luci-go/logdog/common/storage/caching"
+
+        "golang.org/x/net/context"
+)
+
+// cacheSchema represents the cache schema used by this version of the tail
+// cache. If the underlying data format changes, this value must also be
+// updated.
+const cacheSchema = "v1"
+
+func getCachedLogIndexData(c context.Context, cache caching.Cache, path gs.Path) []byte {
+        itm := mkCachedLogIndexDataItem(path)
+        cache.Get(c, itm)
+        return itm.Data
+}
+
+func putCachedLogIndexData(c context.Context, cache caching.Cache, path gs.Path, indexData []byte) {
+        itm := mkCachedLogIndexDataItem(path)
+        itm.Data = indexData
+        cache.Put(c, 0, itm)
+}
+
+func mkCachedLogIndexDataItem(path gs.Path) *caching.Item {
+        return &caching.Item{
+                Schema: cacheSchema,
+                Type:   "archive_log_index",
+                Key:    caching.HashKey(string(path)),
+        }
+}