Cancelling a task cancels a future the task was waiting for. How does it work?










2














I have an awaitable object implementing a request/reply transaction. If the transaction times out, it will be retried few times before giving up and raising an exception.



Now assume it always times out, because that is the case I have problem with.



When a task starts this operation and then gets cancelled, the retries will continue. This is not what I want. I want to cancel the operation entirely.



I prepared a MCVE and noticed, that a future that the task is waiting for gets cancelled when the task is cancelled. This suits me fine, it could be a base for a solution, but I do not understand why that future gets cancelled and if I can rely on it.



import asyncio

RETRIES = 2
TIMEOUT = 1.0

class ClientRPC:
def __init__(self):
self._reply = None
self._retries = RETRIES

def __await__(self):
self.start()
return self._reply.__await__()

def start(self):
loop = asyncio.get_event_loop()
if self._reply is None:
self._reply = loop.create_future()
loop.call_later(TIMEOUT, self.handle_timeout)
# send a request
print("REQUEST")

def handle_timeout(self):
print("TIMEOUT")
print("future", repr(self._reply._state))
if self._retries > 0:
self._retries -= 1
self.start()
else:
self._reply.set_exception(RuntimeError("Timeout!"))

def handle_reply(self, reply):
# unused in this example
pass

async def client():
transaction = ClientRPC()
try:
reply = await transaction
except asyncio.CancelledError:
print("--CANCELLED--")

async def test():
loop = asyncio.get_event_loop()
task = loop.create_task(client())
await asyncio.sleep(1.5)
task.cancel()
await asyncio.sleep(3)

asyncio.run(test()) # python 3.7+


Output (traceback omitted):




REQUEST
TIMEOUT
future 'PENDING'
REQUEST
--CANCELLED--
TIMEOUT
future 'CANCELLED' <-- why?
REQUEST
TIMEOUT
future 'CANCELLED'
Exception in callback ClientRPC.handle_timeout()
handle:
asyncio.base_futures.InvalidStateError: invalid state









share|improve this question


























    2














    I have an awaitable object implementing a request/reply transaction. If the transaction times out, it will be retried few times before giving up and raising an exception.



    Now assume it always times out, because that is the case I have problem with.



    When a task starts this operation and then gets cancelled, the retries will continue. This is not what I want. I want to cancel the operation entirely.



    I prepared a MCVE and noticed, that a future that the task is waiting for gets cancelled when the task is cancelled. This suits me fine, it could be a base for a solution, but I do not understand why that future gets cancelled and if I can rely on it.



    import asyncio

    RETRIES = 2
    TIMEOUT = 1.0

    class ClientRPC:
    def __init__(self):
    self._reply = None
    self._retries = RETRIES

    def __await__(self):
    self.start()
    return self._reply.__await__()

    def start(self):
    loop = asyncio.get_event_loop()
    if self._reply is None:
    self._reply = loop.create_future()
    loop.call_later(TIMEOUT, self.handle_timeout)
    # send a request
    print("REQUEST")

    def handle_timeout(self):
    print("TIMEOUT")
    print("future", repr(self._reply._state))
    if self._retries > 0:
    self._retries -= 1
    self.start()
    else:
    self._reply.set_exception(RuntimeError("Timeout!"))

    def handle_reply(self, reply):
    # unused in this example
    pass

    async def client():
    transaction = ClientRPC()
    try:
    reply = await transaction
    except asyncio.CancelledError:
    print("--CANCELLED--")

    async def test():
    loop = asyncio.get_event_loop()
    task = loop.create_task(client())
    await asyncio.sleep(1.5)
    task.cancel()
    await asyncio.sleep(3)

    asyncio.run(test()) # python 3.7+


    Output (traceback omitted):




    REQUEST
    TIMEOUT
    future 'PENDING'
    REQUEST
    --CANCELLED--
    TIMEOUT
    future 'CANCELLED' <-- why?
    REQUEST
    TIMEOUT
    future 'CANCELLED'
    Exception in callback ClientRPC.handle_timeout()
    handle:
    asyncio.base_futures.InvalidStateError: invalid state









    share|improve this question
























      2












      2








      2


      1





      I have an awaitable object implementing a request/reply transaction. If the transaction times out, it will be retried few times before giving up and raising an exception.



      Now assume it always times out, because that is the case I have problem with.



      When a task starts this operation and then gets cancelled, the retries will continue. This is not what I want. I want to cancel the operation entirely.



      I prepared a MCVE and noticed, that a future that the task is waiting for gets cancelled when the task is cancelled. This suits me fine, it could be a base for a solution, but I do not understand why that future gets cancelled and if I can rely on it.



      import asyncio

      RETRIES = 2
      TIMEOUT = 1.0

      class ClientRPC:
      def __init__(self):
      self._reply = None
      self._retries = RETRIES

      def __await__(self):
      self.start()
      return self._reply.__await__()

      def start(self):
      loop = asyncio.get_event_loop()
      if self._reply is None:
      self._reply = loop.create_future()
      loop.call_later(TIMEOUT, self.handle_timeout)
      # send a request
      print("REQUEST")

      def handle_timeout(self):
      print("TIMEOUT")
      print("future", repr(self._reply._state))
      if self._retries > 0:
      self._retries -= 1
      self.start()
      else:
      self._reply.set_exception(RuntimeError("Timeout!"))

      def handle_reply(self, reply):
      # unused in this example
      pass

      async def client():
      transaction = ClientRPC()
      try:
      reply = await transaction
      except asyncio.CancelledError:
      print("--CANCELLED--")

      async def test():
      loop = asyncio.get_event_loop()
      task = loop.create_task(client())
      await asyncio.sleep(1.5)
      task.cancel()
      await asyncio.sleep(3)

      asyncio.run(test()) # python 3.7+


      Output (traceback omitted):




      REQUEST
      TIMEOUT
      future 'PENDING'
      REQUEST
      --CANCELLED--
      TIMEOUT
      future 'CANCELLED' <-- why?
      REQUEST
      TIMEOUT
      future 'CANCELLED'
      Exception in callback ClientRPC.handle_timeout()
      handle:
      asyncio.base_futures.InvalidStateError: invalid state









      share|improve this question













      I have an awaitable object implementing a request/reply transaction. If the transaction times out, it will be retried few times before giving up and raising an exception.



      Now assume it always times out, because that is the case I have problem with.



      When a task starts this operation and then gets cancelled, the retries will continue. This is not what I want. I want to cancel the operation entirely.



      I prepared a MCVE and noticed, that a future that the task is waiting for gets cancelled when the task is cancelled. This suits me fine, it could be a base for a solution, but I do not understand why that future gets cancelled and if I can rely on it.



      import asyncio

      RETRIES = 2
      TIMEOUT = 1.0

      class ClientRPC:
      def __init__(self):
      self._reply = None
      self._retries = RETRIES

      def __await__(self):
      self.start()
      return self._reply.__await__()

      def start(self):
      loop = asyncio.get_event_loop()
      if self._reply is None:
      self._reply = loop.create_future()
      loop.call_later(TIMEOUT, self.handle_timeout)
      # send a request
      print("REQUEST")

      def handle_timeout(self):
      print("TIMEOUT")
      print("future", repr(self._reply._state))
      if self._retries > 0:
      self._retries -= 1
      self.start()
      else:
      self._reply.set_exception(RuntimeError("Timeout!"))

      def handle_reply(self, reply):
      # unused in this example
      pass

      async def client():
      transaction = ClientRPC()
      try:
      reply = await transaction
      except asyncio.CancelledError:
      print("--CANCELLED--")

      async def test():
      loop = asyncio.get_event_loop()
      task = loop.create_task(client())
      await asyncio.sleep(1.5)
      task.cancel()
      await asyncio.sleep(3)

      asyncio.run(test()) # python 3.7+


      Output (traceback omitted):




      REQUEST
      TIMEOUT
      future 'PENDING'
      REQUEST
      --CANCELLED--
      TIMEOUT
      future 'CANCELLED' <-- why?
      REQUEST
      TIMEOUT
      future 'CANCELLED'
      Exception in callback ClientRPC.handle_timeout()
      handle:
      asyncio.base_futures.InvalidStateError: invalid state






      python python-asyncio






      share|improve this question













      share|improve this question











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      share|improve this question










      asked Nov 13 '18 at 8:51









      VPfBVPfB

      4,24211129




      4,24211129






















          1 Answer
          1






          active

          oldest

          votes


















          2















          I prepared a MCVE and noticed, that a future that the task is waiting for gets cancelled when the task is cancelled. This suits me fine, it could be a base for a solution, but I do not understand why that future gets cancelled and if I can rely on it.




          Yes, if the task awaits a future, that future will be cancelled. That future can be another task, so the cancellation will spread to the bottom-most future that is awaited. The implementation makes sure of that, but the documentation doesn't make it explicit.



          I would go on to rely on this behavior, for two reasons:



          • it is impossible to change it at this point without a major breakage in backward compatibility. Developers have already rejected smaller changes because they would break existing code.


          • there is no other way to implement this that wouldn't lead to resource leaks. If the task you are cancelling is awaiting a future, what do you do except cancel it? If you just let it run in the background, you are potentially keeping it around forever, because the future might never exit on its own. If this were "fixed" by just dropping it from the scheduler (again, without cancellation), the future would never get a chance to clean up the resources it acquired, which would certainly cause a resource leak.


          Thus it is safe to rely on canceling being propagated downward, with the exception of futures shielded with asyncio.shield(), which is reserved for futures that are meant to remain running in the background and have their own lifetime management.






          share|improve this answer






















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            1 Answer
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            1 Answer
            1






            active

            oldest

            votes









            active

            oldest

            votes






            active

            oldest

            votes









            2















            I prepared a MCVE and noticed, that a future that the task is waiting for gets cancelled when the task is cancelled. This suits me fine, it could be a base for a solution, but I do not understand why that future gets cancelled and if I can rely on it.




            Yes, if the task awaits a future, that future will be cancelled. That future can be another task, so the cancellation will spread to the bottom-most future that is awaited. The implementation makes sure of that, but the documentation doesn't make it explicit.



            I would go on to rely on this behavior, for two reasons:



            • it is impossible to change it at this point without a major breakage in backward compatibility. Developers have already rejected smaller changes because they would break existing code.


            • there is no other way to implement this that wouldn't lead to resource leaks. If the task you are cancelling is awaiting a future, what do you do except cancel it? If you just let it run in the background, you are potentially keeping it around forever, because the future might never exit on its own. If this were "fixed" by just dropping it from the scheduler (again, without cancellation), the future would never get a chance to clean up the resources it acquired, which would certainly cause a resource leak.


            Thus it is safe to rely on canceling being propagated downward, with the exception of futures shielded with asyncio.shield(), which is reserved for futures that are meant to remain running in the background and have their own lifetime management.






            share|improve this answer



























              2















              I prepared a MCVE and noticed, that a future that the task is waiting for gets cancelled when the task is cancelled. This suits me fine, it could be a base for a solution, but I do not understand why that future gets cancelled and if I can rely on it.




              Yes, if the task awaits a future, that future will be cancelled. That future can be another task, so the cancellation will spread to the bottom-most future that is awaited. The implementation makes sure of that, but the documentation doesn't make it explicit.



              I would go on to rely on this behavior, for two reasons:



              • it is impossible to change it at this point without a major breakage in backward compatibility. Developers have already rejected smaller changes because they would break existing code.


              • there is no other way to implement this that wouldn't lead to resource leaks. If the task you are cancelling is awaiting a future, what do you do except cancel it? If you just let it run in the background, you are potentially keeping it around forever, because the future might never exit on its own. If this were "fixed" by just dropping it from the scheduler (again, without cancellation), the future would never get a chance to clean up the resources it acquired, which would certainly cause a resource leak.


              Thus it is safe to rely on canceling being propagated downward, with the exception of futures shielded with asyncio.shield(), which is reserved for futures that are meant to remain running in the background and have their own lifetime management.






              share|improve this answer

























                2












                2








                2







                I prepared a MCVE and noticed, that a future that the task is waiting for gets cancelled when the task is cancelled. This suits me fine, it could be a base for a solution, but I do not understand why that future gets cancelled and if I can rely on it.




                Yes, if the task awaits a future, that future will be cancelled. That future can be another task, so the cancellation will spread to the bottom-most future that is awaited. The implementation makes sure of that, but the documentation doesn't make it explicit.



                I would go on to rely on this behavior, for two reasons:



                • it is impossible to change it at this point without a major breakage in backward compatibility. Developers have already rejected smaller changes because they would break existing code.


                • there is no other way to implement this that wouldn't lead to resource leaks. If the task you are cancelling is awaiting a future, what do you do except cancel it? If you just let it run in the background, you are potentially keeping it around forever, because the future might never exit on its own. If this were "fixed" by just dropping it from the scheduler (again, without cancellation), the future would never get a chance to clean up the resources it acquired, which would certainly cause a resource leak.


                Thus it is safe to rely on canceling being propagated downward, with the exception of futures shielded with asyncio.shield(), which is reserved for futures that are meant to remain running in the background and have their own lifetime management.






                share|improve this answer















                I prepared a MCVE and noticed, that a future that the task is waiting for gets cancelled when the task is cancelled. This suits me fine, it could be a base for a solution, but I do not understand why that future gets cancelled and if I can rely on it.




                Yes, if the task awaits a future, that future will be cancelled. That future can be another task, so the cancellation will spread to the bottom-most future that is awaited. The implementation makes sure of that, but the documentation doesn't make it explicit.



                I would go on to rely on this behavior, for two reasons:



                • it is impossible to change it at this point without a major breakage in backward compatibility. Developers have already rejected smaller changes because they would break existing code.


                • there is no other way to implement this that wouldn't lead to resource leaks. If the task you are cancelling is awaiting a future, what do you do except cancel it? If you just let it run in the background, you are potentially keeping it around forever, because the future might never exit on its own. If this were "fixed" by just dropping it from the scheduler (again, without cancellation), the future would never get a chance to clean up the resources it acquired, which would certainly cause a resource leak.


                Thus it is safe to rely on canceling being propagated downward, with the exception of futures shielded with asyncio.shield(), which is reserved for futures that are meant to remain running in the background and have their own lifetime management.







                share|improve this answer














                share|improve this answer



                share|improve this answer








                edited Nov 13 '18 at 14:04

























                answered Nov 13 '18 at 12:41









                user4815162342user4815162342

                61k590141




                61k590141



























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