api_utils.py 文件源码

def _run_until_rpc():
  """Eagerly evaluate tasklets until it is blocking on some RPC.

  Usually ndb eventloop el isn't run until some code calls future.get_result().

  When an async tasklet is called, the tasklet wrapper evaluates the tasklet
  code into a generator, enqueues a callback _help_tasklet_along onto
  the el.current queue, and returns a future.

  _help_tasklet_along, when called by the el, will
  get one yielded value from the generator. If the value if another future,
  set up a callback _on_future_complete to invoke _help_tasklet_along
  when the dependent future fulfills. If the value if a RPC, set up a
  callback _on_rpc_complete to invoke _help_tasklet_along when the RPC fulfills.
  Thus _help_tasklet_along drills down
  the chain of futures until some future is blocked by RPC. El runs
  all callbacks and constantly check pending RPC status.
  """
  el = eventloop.get_event_loop()
  while el.current:
    el.run0()