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

   1 # Copyright 2013 The Chromium Authors. All rights reserved.
   2 # Use of this source code is governed by a BSD-style license that can be
   3 # found in the LICENSE file.
   4
   5 from compiled_file_system import CompiledFileSystem
   6 from file_system import FileNotFoundError
   7 from future import Gettable, Future
   8
   9
  10 class ChainedCompiledFileSystem(object):
  11   '''A CompiledFileSystem implementation that fetches data from a chain of
  12   possible FileSystems. The chain consists of some number of FileSystems which
  13   may have cached data for their CompiledFileSystem instances (injected on
  14   Factory construction) + the main FileSystem (injected at Creation time).
  15
  16   The expected configuration is that the main FileSystem is a PatchedFileSystem
  17   and the chain the FileSystem which it patches, but with file systems
  18   constructed via the HostFileSystemIterator the main FileSystems could be
  19   anything.
  20
  21   This slightly unusual configuration is primarily needed to avoid re-compiling
  22   data for PatchedFileSystems, which are very similar to the FileSystem which
  23   they patch. Re-compiling data is expensive and a waste of memory resources.
  24   ChainedCompiledFileSystem shares the data.
  25   '''
  26   class Factory(CompiledFileSystem.Factory):
  27     def __init__(self, file_system_chain, object_store):
  28       self._file_system_chain = file_system_chain
  29       self._object_store = object_store
  30
  31     def Create(self, file_system, populate_function, cls, category=None):
  32       return ChainedCompiledFileSystem(tuple(
  33           CompiledFileSystem.Factory(self._object_store).Create(
  34               fs, populate_function, cls, category=category)
  35           for fs in [file_system] + self._file_system_chain))
  36
  37   def __init__(self, compiled_fs_chain):
  38     '''|compiled_fs_chain| is a list of tuples (compiled_fs, file_system).
  39     '''
  40     assert len(compiled_fs_chain) > 0
  41     self._compiled_fs_chain = compiled_fs_chain
  42
  43   def GetFromFile(self, path):
  44     return self._GetImpl(
  45         path,
  46         lambda compiled_fs: compiled_fs.GetFromFile(path),
  47         lambda compiled_fs: compiled_fs.GetFileVersion(path))
  48
  49   def GetFromFileListing(self, path):
  50     if not path.endswith('/'):
  51       path += '/'
  52     return self._GetImpl(
  53         path,
  54         lambda compiled_fs: compiled_fs.GetFromFileListing(path),
  55         lambda compiled_fs: compiled_fs.GetFileListingVersion(path))
  56
  57   def _GetImpl(self, path, reader, version_getter):
  58     # Strategy: Get the current version of |path| in main FileSystem, then run
  59     # through |_compiled_fs_chain| in *reverse* to find the "oldest" FileSystem
  60     # with an up-to-date version of that file.
  61     #
  62     # Obviously, if files have been added in the main FileSystem then none of
  63     # the older FileSystems will be able to find it.
  64     read_futures = [(reader(compiled_fs), compiled_fs)
  65                     for compiled_fs in self._compiled_fs_chain]
  66
  67     def resolve():
  68       try:
  69         first_compiled_fs = self._compiled_fs_chain[0]
  70         # The first file system contains both files of a newer version and
  71         # files shared with other compiled file systems. We are going to try
  72         # each compiled file system in the reverse order and return the data
  73         # when version matches. Data cached in other compiled file system will
  74         # be reused whenever possible so that we don't need to recompile things
  75         # that are not changed across these file systems.
  76         first_version = version_getter(first_compiled_fs)
  77         for read_future, compiled_fs in reversed(read_futures):
  78           if version_getter(compiled_fs) == first_version:
  79             return read_future.Get()
  80       except FileNotFoundError:
  81         pass
  82       # Try an arbitrary operation again to generate a realistic stack trace.
  83       return read_futures[0][0].Get()
  84
  85     return Future(delegate=Gettable(resolve))