server/auth: Add in-process LRU-based cache.

server/auth/config.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 auth
 
 import (
         "net/http"
         "time"
 
@@ skipping 29 matching lines @@
         // expected to live for at least 1-2 mins.
         AccessTokenProvider func(c context.Context, scopes []string) (auth.Token, error)
 
         // AnonymousTransport returns http.RoundTriper that can make unauthenticated
         // HTTP requests.
         //
         // The returned round tripper is assumed to be bound to the context and won't
         // outlive it.
         AnonymousTransport func(c context.Context) http.RoundTripper
 
-        // GlobalCache implements service-global strongly consistent cache.
+        // Cache implements a strongly consistent cache.
         //
         // Usually backed by memcache. Should do namespacing itself (i.e. the auth
         // library assumes full ownership of the keyspace).
-        GlobalCache GlobalCache
+        Cache Cache
 }
 
-// GlobalCache implements service-global strongly consistent cache.
-type GlobalCache interface {
+// Cache implements a strongly consistent cache.
+type Cache interface {
         // Get returns a cached item or (nil, nil) if it's not in the cache.
         //
         // Any returned error is transient error.
         Get(c context.Context, key string) ([]byte, error)
 
         // Set unconditionally overwrites an item in the cache.
         //
         // If 'exp' is zero, the item will have no expiration time.
         //
         // Any returned error is transient error.
@@ skipping 45 matching lines @@
         return authdb.ErroringDB{Error: ErrNotConfigured}, nil
 }
 
 // GetSigner extracts Signer from the context. Returns nil if no Signer is set.
 func GetSigner(c context.Context) signing.Signer {
         if cfg := GetConfig(c); cfg != nil {
                 return cfg.Signer
         }
         return nil
 }