Docserver: Persist stat cache for versioned file systems

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

Index: chrome/common/extensions/docs/server2/caching_file_system.py
diff --git a/chrome/common/extensions/docs/server2/caching_file_system.py b/chrome/common/extensions/docs/server2/caching_file_system.py
index b0573716f1fbc2fa56cc68e163bd5d627a9f2856..bacb9ed8ab111ad72af2d87f85399a426d3d01e5 100644
--- a/chrome/common/extensions/docs/server2/caching_file_system.py
+++ b/chrome/common/extensions/docs/server2/caching_file_system.py
@@ -14,16 +14,19 @@ from third_party.json_schema_compiler.memoize import memoize
 class CachingFileSystem(FileSystem):
   '''FileSystem which implements a caching layer on top of |file_system|. It's
   smart, using Stat() to decided whether to skip Read()ing from |file_system|,
-  and only Stat()ing directories never files.
+  and only Stat()ing directories never files. If |fail_on_miss| is True, then
+  stat cache misses are treated like FileNotFound, rather than falling back onto
+  the underlying filesystem.
   '''
-  def __init__(self, file_system, object_store_creator):
+  def __init__(self, file_system, object_store_creator, fail_on_miss=False):
     self._file_system = file_system
+    self._fail_on_miss = fail_on_miss
     def create_object_store(category, **optargs):
       return object_store_creator.Create(
           CachingFileSystem,
           category='%s/%s' % (file_system.GetIdentity(), category),
           **optargs)
-    self._stat_cache = create_object_store('stat')
+    self._stat_cache = create_object_store('stat', start_empty=False)
     # The read caches can start populated (start_empty=False) because file
     # updates are picked up by the stat, so it doesn't need the force-refresh
     # which starting empty is designed for. Without this optimisation, cron
@@ -56,10 +59,16 @@ class CachingFileSystem(FileSystem):
                                 (path, dir_path, dir_stat.child_versions))
       return StatInfo(file_version)
 
+    def raise_not_found(path):
+      raise FileNotFoundError('Got 404 when stat\'ing  %s' % path)

[not at google - send to devlin, 2014/10/20 21:06:57]

It might complicate things for callers - but can we use a different exception
type here, like CacheMissError? Callers can always turn that into
FileNotFoundErrors if they really want.

[Ken Rockot(use gerrit already), 2014/10/22 03:19:54]

Will do, but it will take longer to sort that out. Lots of callers do this
special case handling for FileNotFoundError.

+
     dir_stat = self._stat_cache.Get(dir_path).Get()
     if dir_stat is not None:
       return Future(callback=lambda: make_stat_info(dir_stat))
 
+    if self._fail_on_miss:
+      return Future(callback=lambda: raise_not_found(dir_path))
+
     def next(dir_stat):
       assert dir_stat is not None  # should have raised a FileNotFoundError
       # We only ever need to cache the dir stat.
@@ -169,12 +178,6 @@ class CachingFileSystem(FileSystem):
       return dirs, files
     return self._file_system.Walk(root, depth=depth, file_lister=file_lister)
 
-  def GetCommitID(self):
-    return self._file_system.GetCommitID()
-
-  def GetPreviousCommitID(self):
-    return self._file_system.GetPreviousCommitID()
-
   def GetIdentity(self):
     return self._file_system.GetIdentity()