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

go/src/infra/tools/cipd/local/fs.go

 // 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 local
 
 import (
         "fmt"
         "io/ioutil"
         "os"
@@ skipping 63 matching lines @@
                 return &fsImplErr{err}
         }
         return &fsImpl{abs, logger}
 }
 
 // fsImplErr implements FileSystem by returning given error from all methods.
 type fsImplErr struct {
         err error
 }
 
-// Root returns absolute path to a directory FileSystem operates in. All FS
-// actions are restricted to this directory.
 func (f *fsImplErr) Root() string                                                { return "" }
 func (f *fsImplErr) CwdRelToAbs(path string) (string, error)                     { return "", f.err }
 func (f *fsImplErr) RootRelToAbs(path string) (string, error)                    { return "", f.err }
 func (f *fsImplErr) EnsureDirectory(path string) (string, error)                 { return "", f.err }
 func (f *fsImplErr) EnsureSymlink(path string, target string) error              { return f.err }
 func (f *fsImplErr) EnsureFile(path string, body []byte, perm os.FileMode) error { return f.err }
 func (f *fsImplErr) EnsureFileGone(path string) error                            { return f.err }
 func (f *fsImplErr) EnsureDirectoryGone(path string) error                       { return f.err }
 func (f *fsImplErr) Replace(oldpath, newpath string) error                       { return f.err }
 
@@ skipping 214 matching lines @@
 func pseudoRand() string {
         ts := time.Now().UnixNano()
         lastUsedTimeLock.Lock()
         if ts <= lastUsedTime {
                 ts = lastUsedTime + 1
         }
         lastUsedTime = ts
         lastUsedTimeLock.Unlock()
         return fmt.Sprintf("%v_%v", os.Getpid(), ts)
 }