Docserver: Add support for viewing docs with a codereview patch applied

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

+# Copyright 2013 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.
+
+from compiled_file_system import CompiledFileSystem
+from file_system import FileNotFoundError
+
+class ChainedCompiledFileSystem(object):
+  ''' A CompiledFileSystem implementation that fetches data from a chain of
+  CompiledFileSystems that have different file systems and separate cache
+  namespaces.
+
+  The rules for the compiled file system chain are:
+    - Versions are fetched from the first compiled file system's underlying
+      file system.
+    - Each compiled file system is read in the reverse order (the last one is
+      read first). If the version matches, return the data. Otherwise, read
+      from the previous compiled file system until the first one is read.
+
+  It is used to chain compiled file systems whose underlying file systems are
+  slightly different. This makes it possible to reuse cached compiled data in
+  one of them without recompiling everything that is shared by them.
+  '''
+  class Factory(CompiledFileSystem.Factory):
+    def __init__(self,
+                 factory_chain):
+      self._factory_chain = factory_chain
+
+    def Create(self, populate_function, cls, category=None):
+      return ChainedCompiledFileSystem(
+          [factory.Create(populate_function, cls, category)
+           for factory in self._factory_chain])
+
+  def __init__(self, compiled_fs_chain):
+    assert len(compiled_fs_chain) > 0
+    self._compiled_fs_chain = compiled_fs_chain
+
+  def GetFromFile(self, path, binary=False):
+    # It's possible that a new file is added in the first compiled file system
+    # and it doesn't exist in other compiled file systems.
+    try:
+      version = self._compiled_fs_chain[0]._file_system.Stat(path).version

[not at google - send to devlin, 2013/05/12 05:13:42]

p.s. however this code ends up turning out, it's quite subtle, it needs some
comments.

+      for compiled_fs in reversed(self._compiled_fs_chain):
+        cache_entry = compiled_fs._GetCacheEntryFromFile(path, binary)
+        if cache_entry.version == version:
+          return cache_entry.cache_data
+    except FileNotFoundError:
+      pass
+    # Try first operation again to generate the correct stack trace
+    return self._compiled_fs_chain[0].GetFromFile(path, binary)
+
+  def GetFromFileListing(self, path):
+    if not path.endswith('/'):
+      path += '/'
+    try:
+      version = self._compiled_fs_chain[0]._file_system.Stat(path).version
+      for compiled_fs in reversed(self._compiled_fs_chain):
+        cache_entry = compiled_fs._GetCacheEntryFromFileListing(path)
+        if cache_entry.version == version:
+          return cache_entry.cache_data
+    except FileNotFoundError:
+      pass
+    # Try first operation again to generate the correct stack trace
+    return self._compiled_fs_chain[0].GetFromFileListing(path)