Adds a ready event to BackgroundConsumer to wait on start. (#7499)

* Adds a ready event to BackgroundConsumer to allow waiting for the background thread to start

Author: crwilcox

google/api_core/bidi.py

@@ -505,8 +505,9 @@ def _on_call_done(self, future):
  # when the RPC has terminated.
  self.resume()
 
- def _thread_main(self):
+ def _thread_main(self, ready):
  try:
+ ready.set()
  self._bidi_rpc.add_done_callback(self._on_call_done)
  self._bidi_rpc.open()
 
@@ -555,11 +556,19 @@ def _thread_main(self):
  def start(self):
  """Start the background thread and begin consuming the thread."""
  with self._operational_lock:
+ ready = threading.Event()
  thread = threading.Thread(
- name=_BIDIRECTIONAL_CONSUMER_NAME, target=self._thread_main
+ name=_BIDIRECTIONAL_CONSUMER_NAME,
+ target=self._thread_main,
+ args=(ready,)
  )
  thread.daemon = True
  thread.start()
+ # Other parts of the code rely on `thread.is_alive` which
+ # isn't sufficient to know if a thread is active, just that it may
+ # soon be active. This can cause races. Further protect
+ # against races by using a ready event and wait on it to be set.
+ ready.wait()
  self._thread = thread
  _LOGGER.debug("Started helper thread %s", thread.name)