LogDog: Add Storage-layer data caching.

logdog/common/storage/caching/cache.go

Index: logdog/common/storage/caching/cache.go
diff --git a/logdog/common/storage/caching/cache.go b/logdog/common/storage/caching/cache.go
new file mode 100644
index 0000000000000000000000000000000000000000..1662a420cbda6018111b84d9c602c31c5343da4d
--- /dev/null
+++ b/logdog/common/storage/caching/cache.go
@@ -0,0 +1,81 @@
+// 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 caching
+
+import (
+	"bufio"
+	"crypto/sha256"
+	"encoding/hex"
+	"time"
+
+	"golang.org/x/net/context"
+)
+
+// Item is a single cache item. An item is uniquely identified by its Schema and
+// Key sequence.
+type Item struct {
+	// Schema is the item's schema value. If empty, the item is schemaless.
+	Schema string
+
+	// Type is the item's type identifier. If empty, the item has no type.
+	Type string
+
+	// Key is the item's individual key. It uniquely identifies the Item within
+	// the scope of the Schema and Type.
+	Key string
+
+	// Data is the Item's data. For Put, the contents of this Data will be used to
+	// populate the cache. For Get, this value will be non-nil and populated with
+	// the retrieved data, or nil if the item was not present in the cache.
+	Data []byte
+}
+
+// Cache is a simple cache interface. It is capable of storage and retrieval.
+type Cache interface {
+	// Put caches the supplied Items into the cache. If an Item already exists in
+	// the cache, it will be overridden.
+	//
+	// If exp, the supplied expiration Duration, is >0, the cache should only
+	// store the data if it can expire it after this period of time.
+	//
+	// This method does not return whether or not the caching storage was
+	// successful.
+	Put(c context.Context, exp time.Duration, items ...*Item)
+
+	// Get retrieves a cached entry. If the entry was present, a non-nil value
+	// will be returned, even if the data length is zero. If the entry was not
+	// present, nil will be returned.
+	//
+	// This method does not distinguish between an error and missing data. Either
+	// valid data is returned, or it is not.
+	Get(c context.Context, items ...*Item)
+}
+
+// HashKey composes a hex-encoded SHA256 string from the supplied parts. The
+// local schema version and KeyType are included in the hash.
+func HashKey(parts ...string) string {
+	hash := sha256.New()
+	bio := bufio.NewWriter(hash)
+
+	// Add a full NULL-delimited key sequence segment to our hash.
+	for i, part := range parts {
+		// Write the separator, except for the first key.
+		if i > 0 {
+			if _, err := bio.WriteRune('\x00'); err != nil {
+				panic(err)
+			}
+		}
+
+		if _, err := bio.WriteString(part); err != nil {
+			panic(err)
+		}
+	}
+	if err := bio.Flush(); err != nil {
+		panic(err)
+	}
+
+	// Return the hex-encoded hash sum.
+	return hex.EncodeToString(hash.Sum(nil))
+}