Docserver: Persist stat cache for versioned file systems

chrome/common/extensions/docs/server2/host_file_system_provider.py

 # Copyright 2013 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.
 
 from caching_file_system import CachingFileSystem
 from gitiles_file_system import GitilesFileSystem
 from local_file_system import LocalFileSystem
 from offline_file_system import OfflineFileSystem
 from third_party.json_schema_compiler.memoize import memoize
 
 
 class HostFileSystemProvider(object):
   '''Provides host file systems ("host" meaning the file system that hosts the
   server's source code and templates) tracking master, or any branch.
 
   File system instances are memoized to maintain the in-memory caches across
   multiple callers.
   '''
   def __init__(self,
                object_store_creator,
                pinned_commit=None,
                default_master_instance=None,
                offline=False,
-               constructor_for_test=None):
+               constructor_for_test=None,
+               cache_only=False):
     '''
     |object_store_creator|
       Provides caches for file systems that need one.
     |pinned_commit|
       If not None, the commit at which a 'master' file system will be created.
       If None, 'master' file systems will use HEAD.
     |default_master_instance|
       If not None, 'master' file systems provided by this class without a
       specific commit will return |default_master_instance| instead.
     |offline|
       If True all provided file systems will be wrapped in an OfflineFileSystem.
     |constructor_for_test|
       Provides a custom constructor rather than creating GitilesFileSystems.
+    |cache_only|
+      If True, all provided file systems will be cache-only, meaning that cache
+      misses will result in errors rather than cache updates.
     '''
     self._object_store_creator = object_store_creator
     self._pinned_commit = pinned_commit
     self._default_master_instance = default_master_instance
     self._offline = offline
     self._constructor_for_test = constructor_for_test
+    self._cache_only = cache_only
 
   @memoize
   def GetMaster(self, commit=None):
     '''Gets a file system tracking 'master'. Use this method rather than
     GetBranch('master') because the behaviour is subtly different; 'master' can
     be pinned to a specific commit (|pinned_commit| in constructor) and can have
     have its default instance overridden (|default_master_instance| in the
     constructor).
 
     |commit| if non-None determines a specific commit to pin the host file
     system at, though it will be ignored if it's newer than |pinned_commit|.
     If None then |commit| will track |pinned_commit| if is has been
     set, or just HEAD (which might change during server runtime!).
     '''
     if commit is None:
       if self._default_master_instance is not None:
         return self._default_master_instance
       return self._Create('master', commit=self._pinned_commit)
-    if self._pinned_commit is not None:
-      # XXX(ahernandez): THIS IS WRONG. Should be
-      # commit = Oldest(commit, self._pinned_commit).
-      commit = min(commit, self._pinned_commit)
     return self._Create('master', commit=commit)
 
   @memoize
   def GetBranch(self, branch):
     '''Gets a file system tracking |branch|, for example '1150' - anything other
     than 'master', which must be constructed via the GetMaster() method.
 
     Note: Unlike GetMaster this function doesn't take a |commit| argument
     since we assume that branches hardly ever change, while master frequently
     changes.
     '''
     assert isinstance(branch, basestring), 'Branch %s must be a string' % branch
     assert branch != 'master', (
         'Cannot specify branch=\'master\', use GetMaster()')
     return self._Create(branch)
 
   def _Create(self, branch, commit=None):
     '''Creates Gitiles file systems (or if in a test, potentially whatever
     |self._constructor_for_test specifies). Wraps the resulting file system in
     an Offline file system if the offline flag is set, and finally wraps it in
     a Caching file system.
     '''
+    empty_stat_cache = True
     if self._constructor_for_test is not None:
       file_system = self._constructor_for_test(branch=branch, commit=commit)
     else:
       file_system = GitilesFileSystem.Create(branch=branch, commit=commit)
+      # GitilesFileSystem supports the notion of unstable identity, which means
+      # its CachingFileSystem can use a persistent stat info cache without
+      # breaking everything.
+      empty_stat_cache = False
     if self._offline:
       file_system = OfflineFileSystem(file_system)
-    return CachingFileSystem(file_system, self._object_store_creator)
+    return CachingFileSystem(file_system, self._object_store_creator,
+        fail_on_miss=self._cache_only, empty_stat_cache=empty_stat_cache)
 
   @staticmethod
   def ForLocal(object_store_creator, **optargs):
     '''Used in creating a server instance on localhost.
     '''
     return HostFileSystemProvider(
         object_store_creator,
         constructor_for_test=lambda **_: LocalFileSystem.Create(),
         **optargs)
 
   @staticmethod
   def ForTest(file_system, object_store_creator, **optargs):
     '''Used in creating a test server instance. The HostFileSystemProvider
     returned here will always return |file_system| when its Create() method is
     called.
     '''
     return HostFileSystemProvider(
         object_store_creator,
         constructor_for_test=lambda **_: file_system,
         **optargs)