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
5from compiled_file_system import CompiledFileSystem
6from docs_server_utils import StringIdentity
7from file_system import FileNotFoundError
8from future import Future
9from path_util import ToDirectory
10
11
12class ChainedCompiledFileSystem(object):
13  '''A CompiledFileSystem implementation that fetches data from a chain of
14  possible FileSystems. The chain consists of some number of FileSystems which
15  may have cached data for their CompiledFileSystem instances (injected on
16  Factory construction) + the main FileSystem (injected at Creation time).
17
18  The expected configuration is that the main FileSystem is a PatchedFileSystem
19  and the chain the FileSystem which it patches, but with file systems
20  constructed via the HostFileSystemIterator the main FileSystems could be
21  anything.
22
23  This slightly unusual configuration is primarily needed to avoid re-compiling
24  data for PatchedFileSystems, which are very similar to the FileSystem which
25  they patch. Re-compiling data is expensive and a waste of memory resources.
26  ChainedCompiledFileSystem shares the data.
27  '''
28  class Factory(CompiledFileSystem.Factory):
29    def __init__(self, file_system_chain, object_store):
30      self._file_system_chain = file_system_chain
31      self._object_store = object_store
32
33    def Create(self, file_system, populate_function, cls, category=None):
34      return ChainedCompiledFileSystem(
35          # Chain of CompiledFileSystem instances.
36          tuple(CompiledFileSystem.Factory(self._object_store).Create(
37                    fs, populate_function, cls, category=category)
38                for fs in [file_system] + self._file_system_chain),
39          # Identity, as computed by all file systems.
40          StringIdentity(*(fs.GetIdentity() for fs in self._file_system_chain)))
41
42  def __init__(self, compiled_fs_chain, identity):
43    '''|compiled_fs_chain| is a list of tuples (compiled_fs, file_system).
44    '''
45    assert len(compiled_fs_chain) > 0
46    self._compiled_fs_chain = compiled_fs_chain
47    self._identity = identity
48
49  def GetFromFile(self, path, skip_not_found=False):
50    return self._GetImpl(
51        path,
52        lambda cfs: cfs.GetFromFile(path, skip_not_found=skip_not_found),
53        lambda cfs: cfs._GetFileVersionFromCache(path))
54
55  def GetFromFileListing(self, path):
56    path = ToDirectory(path)
57    return self._GetImpl(
58        path,
59        lambda compiled_fs: compiled_fs.GetFromFileListing(path),
60        lambda compiled_fs: compiled_fs._GetFileListingVersionFromCache(path))
61
62  def _GetImpl(self, path, reader, version_getter):
63    # Strategy: Get the current version of |path| in main FileSystem, then run
64    # through |_compiled_fs_chain| in *reverse* to find the "oldest" FileSystem
65    # with an up-to-date version of that file.
66    #
67    # Obviously, if files have been added in the main FileSystem then none of
68    # the older FileSystems will be able to find it.
69    read_and_version_futures = [(reader(fs), version_getter(fs), fs)
70                                for fs in self._compiled_fs_chain]
71
72    def resolve():
73      try:
74        # The first file system contains both files of a newer version and
75        # files shared with other compiled file systems. We are going to try
76        # each compiled file system in the reverse order and return the data
77        # when version matches. Data cached in other compiled file system will
78        # be reused whenever possible so that we don't need to recompile things
79        # that are not changed across these file systems.
80        first_version = read_and_version_futures[0][1].Get()
81        for (read_future,
82             version_future,
83             compiled_fs) in reversed(read_and_version_futures):
84          if version_future.Get() == first_version:
85            return read_future.Get()
86      except FileNotFoundError:
87        pass
88      # Try an arbitrary operation again to generate a realistic stack trace.
89      return read_and_version_futures[0][0].Get()
90
91    return Future(callback=resolve)
92
93  def GetIdentity(self):
94    return self._identity
95