chained_compiled_file_system.py revision cedac228d2dd51db4b79ea1e72c7f249408ee061 
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
9
10
11class ChainedCompiledFileSystem(object):
12  '''A CompiledFileSystem implementation that fetches data from a chain of
13  possible FileSystems. The chain consists of some number of FileSystems which
14  may have cached data for their CompiledFileSystem instances (injected on
15  Factory construction) + the main FileSystem (injected at Creation time).
16
17  The expected configuration is that the main FileSystem is a PatchedFileSystem
18  and the chain the FileSystem which it patches, but with file systems
19  constructed via the HostFileSystemIterator the main FileSystems could be
20  anything.
21
22  This slightly unusual configuration is primarily needed to avoid re-compiling
23  data for PatchedFileSystems, which are very similar to the FileSystem which
24  they patch. Re-compiling data is expensive and a waste of memory resources.
25  ChainedCompiledFileSystem shares the data.
26  '''
27  class Factory(CompiledFileSystem.Factory):
28    def __init__(self, file_system_chain, object_store):
29      self._file_system_chain = file_system_chain
30      self._object_store = object_store
31
32    def Create(self, file_system, populate_function, cls, category=None):
33      return ChainedCompiledFileSystem(
34          # Chain of CompiledFileSystem instances.
35          tuple(CompiledFileSystem.Factory(self._object_store).Create(
36                    fs, populate_function, cls, category=category)
37                for fs in [file_system] + self._file_system_chain),
38          # Identity, as computed by all file systems.
39          StringIdentity(*(fs.GetIdentity() for fs in self._file_system_chain)))
40
41  def __init__(self, compiled_fs_chain, identity):
42    '''|compiled_fs_chain| is a list of tuples (compiled_fs, file_system).
43    '''
44    assert len(compiled_fs_chain) > 0
45    self._compiled_fs_chain = compiled_fs_chain
46    self._identity = identity
47
48  def GetFromFile(self, path):
49    return self._GetImpl(
50        path,
51        lambda compiled_fs: compiled_fs.GetFromFile(path),
52        lambda compiled_fs: compiled_fs.GetFileVersion(path))
53
54  def GetFromFileListing(self, path):
55    if not path.endswith('/'):
56      path += '/'
57    return self._GetImpl(
58        path,
59        lambda compiled_fs: compiled_fs.GetFromFileListing(path),
60        lambda compiled_fs: compiled_fs.GetFileListingVersion(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_futures = [(reader(compiled_fs), compiled_fs)
70                    for compiled_fs in self._compiled_fs_chain]
71
72    def resolve():
73      try:
74        first_compiled_fs = self._compiled_fs_chain[0]
75        # The first file system contains both files of a newer version and
76        # files shared with other compiled file systems. We are going to try
77        # each compiled file system in the reverse order and return the data
78        # when version matches. Data cached in other compiled file system will
79        # be reused whenever possible so that we don't need to recompile things
80        # that are not changed across these file systems.
81        first_version = version_getter(first_compiled_fs)
82        for read_future, compiled_fs in reversed(read_futures):
83          if version_getter(compiled_fs) == first_version:
84            return read_future.Get()
85      except FileNotFoundError:
86        pass
87      # Try an arbitrary operation again to generate a realistic stack trace.
88      return read_futures[0][0].Get()
89
90    return Future(callback=resolve)
91
92  def GetIdentity(self):
93    return self._identity
94