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

[Ian Kelly]

On Thu, Sep 25, 2014 at 3:46 AM, Marko Rauhamaa <marko at pacujo.net> wrote:
> 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.

Easily solved using a re-entrant lock. Alternatively, one could use
synchronization to make sure the future can't complete until all
callbacks have been added. That would have the benefit of keeping the
callback processing on the same thread as the future, as the OP
wanted.