Docserver: Update Future.Then() to be more Promise-like

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

 # Copyright (c) 2012 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.
 
 import posixpath
 import sys
 
 from file_system import FileSystem, StatInfo, FileNotFoundError
 from future import Future
 from path_util import IsDirectory
@@ skipping 45 matching lines @@
       file_version = dir_stat.child_versions.get(file_path)
       if file_version is None:
         raise FileNotFoundError('No stat found for %s in %s (found %s)' %
                                 (path, dir_path, dir_stat.child_versions))
       return StatInfo(file_version)
 
     dir_stat = self._stat_object_store.Get(dir_path).Get()
     if dir_stat is not None:
       return Future(value=make_stat_info(dir_stat))
 
-    dir_stat_future = self._MemoizedStatAsyncFromFileSystem(dir_path)
-    def resolve():
-      dir_stat = dir_stat_future.Get()
+    def next(dir_stat):
       assert dir_stat is not None  # should have raised a FileNotFoundError
       # We only ever need to cache the dir stat.
       self._stat_object_store.Set(dir_path, dir_stat)
       return make_stat_info(dir_stat)
-    return Future(callback=resolve)
+    return self._MemoizedStatAsyncFromFileSystem(dir_path).Then(next)
 
   @memoize
   def _MemoizedStatAsyncFromFileSystem(self, dir_path):
     '''This is a simple wrapper to memoize Futures to directory stats, since
     StatAsync makes heavy use of it. Only cache directories so that the
     memoized cache doesn't blow up.
     '''
     assert IsDirectory(dir_path)
     return self._file_system.StatAsync(dir_path)
 
   def Read(self, paths, skip_not_found=False):
     '''Reads a list of files. If a file is in memcache and it is not out of
     date, it is returned. Otherwise, the file is retrieved from the file system.
     '''
     cached_read_values = self._read_object_store.GetMulti(paths).Get()
     cached_stat_values = self._stat_object_store.GetMulti(paths).Get()
 
     # Populate a map of paths to Futures to their stat. They may have already
     # been cached in which case their Future will already have been constructed
     # with a value.
     stat_futures = {}
 
-    def swallow_file_not_found_error(future):
-      def resolve():
-        try: return future.Get()
-        except FileNotFoundError: return Nnone
-      return Future(callback=resolve)
+    def handle(error):
+      if isinstance(error, FileNotFoundError):
+        return None
+      raise error
 
     for path in paths:
       stat_value = cached_stat_values.get(path)
       if stat_value is None:
         stat_future = self.StatAsync(path)
         if skip_not_found:
-          stat_future = swallow_file_not_found_error(stat_future)
+          stat_future = stat_future.Then(lambda x: x, handle)
       else:
         stat_future = Future(value=stat_value)
       stat_futures[path] = stat_future
 
     # Filter only the cached data which is fresh by comparing to the latest
     # stat. The cached read data includes the cached version. Remove it for
     # the result returned to callers.
     fresh_data = dict(
         (path, data) for path, (data, version) in cached_read_values.iteritems()
         if stat_futures[path].Get().version == version)
 
     if len(fresh_data) == len(paths):
       # Everything was cached and up-to-date.
       return Future(value=fresh_data)
 
-    # Read in the values that were uncached or old.
-    read_futures = self._file_system.Read(
-        set(paths) - set(fresh_data.iterkeys()),
-        skip_not_found=skip_not_found)
-    def resolve():
-      new_results = read_futures.Get()
+    def next(new_results):
       # Update the cache. This is a path -> (data, version) mapping.
       self._read_object_store.SetMulti(
           dict((path, (new_result, stat_futures[path].Get().version))
                for path, new_result in new_results.iteritems()))
       new_results.update(fresh_data)
       return new_results
-    return Future(callback=resolve)
+    # Read in the values that were uncached or old.
+    return self._file_system.Read(set(paths) - set(fresh_data.iterkeys()),
+                                  skip_not_found=skip_not_found).Then(next)
 
   def GetIdentity(self):
     return self._file_system.GetIdentity()
 
   def __repr__(self):
     return '%s of <%s>' % (type(self).__name__, repr(self._file_system))