def GetArtifactKnowledgeBase(client_obj, allow_uninitialized=False):
  """This generates an artifact knowledge base from a GRR client.

  Args:
    client_obj: A GRRClient object which is opened for reading.
    allow_uninitialized: If True we accept an uninitialized knowledge_base.

  Returns:
    A KnowledgeBase semantic value.

  Raises:
    ArtifactProcessingError: If called when the knowledge base has not been
    initialized.
    KnowledgeBaseUninitializedError: If we failed to initialize the knowledge
    base.

  This is needed so that the artifact library has a standardized
  interface to the data that is actually stored in the GRRClient object in
  the GRR datastore.

  We expect that the client KNOWLEDGE_BASE is already filled out through the,
  KnowledgeBaseInitialization flow, but attempt to make some intelligent
  guesses if things failed.
  """
  client_schema = client_obj.Schema
  kb = client_obj.Get(client_schema.KNOWLEDGE_BASE)
  if not allow_uninitialized:
    if not kb:
      raise artifact_utils.KnowledgeBaseUninitializedError(
          "KnowledgeBase empty for %s." % client_obj.urn)
    if not kb.os:
      raise artifact_utils.KnowledgeBaseAttributesMissingError(
          "KnowledgeBase missing OS for %s. Knowledgebase content: %s" %
          (client_obj.urn, kb))
  if not kb:
    kb = client_schema.KNOWLEDGE_BASE()
    SetCoreGRRKnowledgeBaseValues(kb, client_obj)

  if kb.os == "Windows":
    # Add fallback values.
    if not kb.environ_allusersappdata and kb.environ_allusersprofile:
      # Guess if we don't have it already.
      if kb.os_major_version >= 6:
        kb.environ_allusersappdata = u"c:\\programdata"
        kb.environ_allusersprofile = u"c:\\programdata"
      else:
        kb.environ_allusersappdata = (u"c:\\documents and settings\\All Users\\"
                                      "Application Data")
        kb.environ_allusersprofile = u"c:\\documents and settings\\All Users"

  return kb