Devserver: allow SubversionFileSystem to be pinned to a specific rev on construction

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

Index: chrome/common/extensions/docs/server2/app_yaml_helper.py
diff --git a/chrome/common/extensions/docs/server2/app_yaml_helper.py b/chrome/common/extensions/docs/server2/app_yaml_helper.py
new file mode 100644
index 0000000000000000000000000000000000000000..6dcc15664574ba09304f60b3c704928747bf430f
--- /dev/null
+++ b/chrome/common/extensions/docs/server2/app_yaml_helper.py
@@ -0,0 +1,131 @@
+# 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.
+
+import logging
+
+_APP_YAML_CONTAINER = '''
+application: chrome-apps-doc
+version: %s
+runtime: python27
+api_version: 1
+threadsafe: false
+'''
+
+class AppYamlHelper(object):
+  '''Parses the app.yaml file, and is able to step back in the host file
+  system's revision history to find when it changed to some given version.
+  '''
+  class Delegate(object):
+    def GetHostFileSystemForRevision(self, revision):
+      '''Revision may not be None.
+      '''
+      raise NotImplementedError()
+
+  def __init__(self,
+               app_yaml_path,
+               file_system_at_head,
+               delegate,
+               object_store_creator):
+    self._app_yaml_path = app_yaml_path
+    self._file_system_at_head = file_system_at_head
+    self._delegate = delegate
+    self._store = object_store_creator.Create(
+        AppYamlHelper,
+        category=file_system_at_head.GetIdentity(),
+        start_empty=False)
+
+  @staticmethod
+  def ExtractVersion(app_yaml, key='version'):
+    '''Extracts the 'version' key from the contents of an app.yaml file.
+    Allow overriding the key to parse e.g. the cron file ('target').
+    '''
+    # We could properly parse this using a yaml library but Python doesn't have
+    # one built in so whatevs.
+    key_colon = '%s:' % key
+    versions = [line.strip()[len(key_colon):].strip()
+                for line in app_yaml.split('\n')
+                if line.strip().startswith(key_colon)]
+    if not versions:
+      raise ValueError('No versions found for %s in %s' % (
+          key, app_yaml))
+    if len(set(versions)) > 1:
+      raise ValueError('Inconsistent versions found for %s in %s: %s' % (
+          key, app_yaml, versions))
+    return versions[0]
+
+  @staticmethod
+  def IsGreater(lhs, rhs):
+    '''Return whether the app.yaml version |lhs| > |rhs|. This is tricky
+    because versions are typically not numbers but rather 2-0-9, 2-0-12,
+    2-1-0, etc - and 2-1-0 > 2-0-10 > 2-0-9.
+    '''
+    lhs_parts = lhs.replace('-', '.').split('.')
+    rhs_parts = rhs.replace('-', '.').split('.')
+    while lhs_parts and rhs_parts:
+      lhs_msb = int(lhs_parts.pop(0))
+      rhs_msb = int(rhs_parts.pop(0))
+      if lhs_msb != rhs_msb:
+        return lhs_msb > rhs_msb
+    return len(lhs) > len(rhs)
+
+  @staticmethod
+  def GenerateAppYaml(version):
+    '''Probably only useful for tests.
+    '''
+    return _APP_YAML_CONTAINER % version
+
+  def IsUpToDate(self, app_version):
+    '''Returns True if the |app_version| is up to date with respect to the one
+    checked into the host file system.
+    '''
+    checked_in_app_version = AppYamlHelper.ExtractVersion(
+        self._file_system_at_head.ReadSingle(self._app_yaml_path))
+    if app_version == checked_in_app_version:
+      return True
+    if AppYamlHelper.IsGreater(app_version, checked_in_app_version):
+      logging.warning(
+          'Server is too new! Checked in %s < currently running %s' % (
+              checked_in_app_version, app_version))
+      return True
+    return False
+
+  def GetFirstRevisionGreaterThan(self, app_version):
+    '''Finds the first revision that the version in app.yaml was greater than
+    |app_version|.
+
+    WARNING: if there is no such revision (e.g. the app is up to date, or
+    *oops* the app is even newer) then this will throw a ValueError. Use
+    IsUpToDate to validate the input before calling this method.
+    '''
+    stored = self._store.Get(app_version).Get()
+    if stored is None:
+      stored = self._GetFirstRevisionGreaterThanImpl(app_version)
+      assert stored is not None
+      self._store.Set(app_version, stored)
+    return stored
+
+  def _GetFirstRevisionGreaterThanImpl(self, app_version):
+    def get_app_yaml_revision(file_system):
+      return int(file_system.Stat(self._app_yaml_path).version)
+
+    def has_greater_app_version(file_system):
+      app_version_in_file_system = AppYamlHelper.ExtractVersion(
+          file_system.ReadSingle(self._app_yaml_path))
+      return AppYamlHelper.IsGreater(app_version_in_file_system, app_version)
+
+    found = None
+    next_file_system = self._file_system_at_head
+
+    while has_greater_app_version(next_file_system):
+      found = get_app_yaml_revision(next_file_system)
+      # Back up a revision then find when app.yaml was last updated before then.
+      if found == 0:
+        logging.warning('All revisions are greater than %s' % app_version)
+        return 0
+      next_file_system = self._delegate.GetHostFileSystemForRevision(
+          found - 1)
+
+    if found is None:
+      raise ValueError('All revisions are less than %s' % app_version)
+    return found