ThreadPoolExecutor - callback guaranteed to run in same thread as the submitted function?

[Marko Rauhamaa]

Ian Kelly <ian.g.kelly at gmail.com>:

> The documentation for Future.add_done_callback says: "If the future
> has already completed or been cancelled, fn will be called
> immediately."

That sounds really bad.

There should be a guarantee that the callback is not called from the
same thread before returning from Future.add_done_callback. Otherwise,
the caller may have a really difficult time handling preemption and
avoiding deadlocks at the same time.

Example (pseudocode):

   def callback(self):
      with self.lock:
          ...

   def xyz(self, f):
      with self.lock:
          ...
          f.add_done_callback(self.callback)

The code will deadlock if the callback is invoked immediately.


Marko