Declared the interface for a file system to help with testing caching.

For the testing work in #1.

samples/cachingfs/caching_fs.go

+// Copyright 2015 Google Inc. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package cachingfs
+
+import (
+	"time"
+
+	"github.com/jacobsa/fuse"
+	"github.com/jacobsa/fuse/fuseutil"
+)
+
+// Constants that define the relative offsets of the inodes exported by the
+// file system. See notes on the RenumberInodes method.
+const (
+	FooInodeOffset = iota
+	DirInodeOffset
+	BarInodeOffset
+)
+
+// A file system with a fixed structure that looks like this:
+//
+//     foo
+//     dir/
+//         bar
+//
+// The file system is configured with durations that specify how long to allow
+// inode entries and attributes to be cached, used when responding to fuse
+// requests. It also exposes methods for renumbering inodes and updating mtimes
+// that are useful in testing that these durations are honored.
+type CachingFS struct {
+	fuseutil.NotImplementedFileSystem
+}
+
+var _ fuse.FileSystem = &CachingFS{}
+
+// Create a file system that issues cacheable responses according to the
+// following rules:
+//
+//  *  LookUpInodeResponse.Entry.EntryExpiration is set according to
+//     lookupEntryTimeout.
+//
+//  *  GetInodeAttributesResponse.AttributesExpiration is set according to
+//     getattrTimeout.
+//
+//  *  Nothing else is marked cacheable. (In particular, the attributes
+//     returned by LookUpInode are not cacheable.)
+//
+func NewCachingFS(
+	lookupEntryTimeout time.Duration,
+	getattrTimeout time.Duration) (fs *CachingFS, err error)
+
+// Cause inodes to receive IDs according to the following rules in further
+// responses to fuse:
+//
+//  *  The ID of "foo" is base + FooInodeOffset.
+//  *  The ID of "dir" is base + DirInodeOffset.
+//  *  The ID of "dir/bar" is base + BarInodeOffset.
+//
+// If this method has never been called, the file system behaves as if it were
+// called with base set to fuse.RootInodeID + 1.
+//
+// REQUIRES: base > fuse.RootInodeID
+func (fs *CachingFS) RenumberInodes(base fuse.InodeID)
+
+// Cause further queries for the attributes of inodes to use the supplied time
+// as the inode's mtime.
+func (fs *CachingFS) SetMtime(mtime time.Time)