[swift-corelibs-dev] Setting property "maxConcurrentOperationCount = 1" of NSOperation fails to execute operations using OpenSource Foundation.

[David P Grove]

I think the intuition about the dispatch overlay being the problem on Linux
is right on.  We may have to hack around the problem on the Foundation side
until an improved overlay is available to use.

--dave




From:	Philippe Hausler via swift-corelibs-dev
            <swift-corelibs-dev at swift.org>
To:	Mamatha Busi <mamabusi at in.ibm.com>
Cc:	swift-corelibs-dev at swift.org
Date:	05/06/2016 09:43 AM
Subject:	Re: [swift-corelibs-dev] Setting property
            "maxConcurrentOperationCount = 1" of NSOperation fails to
            execute operations using OpenSource Foundation.
Sent by:	swift-corelibs-dev-bounces at swift.org



I have a feeling this is associated with the changes for IUO types that
recently landed. I am very worried that this actually could happen on
Darwin targets as well. Perhaps it is the swift overlay for dispatch that
is correcting the failure on Darwin and the dispatch on Linux is missing
that annotation. Worth looking into.

Sent from my iPhone

On May 6, 2016, at 12:16 AM, Mamatha Busi <mamabusi at in.ibm.com> wrote:


      @ Philippe: Your right. Converting the 'attr' to an optional did do
      the job of creating a serial queue successfully. Thanks for that. I
      will create a PR for this.

      But this makes me think as to why until now, this was not caught by
      the compiler itself?

      Regards
      Mamatha

       ----- Original message -----
       From: phausler at apple.com
       To: Mamatha Busi/India/IBM at IBMIN
       Cc: swift-corelibs-dev <swift-corelibs-dev at swift.org>
       Subject: Re: [swift-corelibs-dev] Setting property
       "maxConcurrentOperationCount = 1" of NSOperation fails to execute
       operations using OpenSource Foundation.
       Date: Thu, May 5, 2016 10:54 PM

       Hmm that seems unfortunate. I wonder if the serial creation is due
       to an unwrapped optional? var attr: dispatch_queue_attr_t? instead
       might do the trick… or alternatively we could just let the
       underlying queue be concurrent all the time and enforce the max ops
       via making the semaphore always instantiated (in the case of max ops
       being 1) and initializing it to 1 to gate the operations.

             On May 5, 2016, at 1:15 AM, Mamatha Busi via
             swift-corelibs-dev <swift-corelibs-dev at swift.org> wrote:

             Hello



             Code snippet:


             ———————————————————————


                  let operation1 : NSBlockOperation = NSBlockOperation
             (block: {


                         sleep(1)


                         print("Opertion1")


                     })


                     let operation2 : NSBlockOperation = NSBlockOperation
             (block: {


                         sleep(1)


                         print("Opertion2”)


                     })





                     var operations = [NSOperation]()


                     operations.append(operation1)


                     operations.append(operation2)





                     let queue = NSOperationQueue()


                     queue.maxConcurrentOperationCount = 1


                     queue.addOperations(operations, waitUntilFinished:
             true)





             ————————————————————————





             The above code snippet of adding operations to an operation
             queue and executing with the property
             ‘maxConcurrentOperationCount = 1’ fails while executing the
             above with the OpenSource Foundation and libDispatch of MAC
             inside Xcode.


             The error I am seeing is:


             fatal error: unexpectedly found nil while unwrapping an
             Optional value


             Stack trace points to: attr = DISPATCH_QUEUE_SERIAL


             which implies that the libDispatch macro is coming as nil
             during the creation of the serial queue using libDispatch in
             the file NSOperationQueue


             The same test-case passes on OSx.


             When I do not restrict the serial operation i.e. I remove ‘
             queue.maxConcurrentOperationCount = 1’ , test case executes
             successfully.


             Setting it to a different value other than 1 also causes no
             problems.


             Am I doing something wrong with the API. If I set the property
             ‘ queue.maxConcurrentOperationCount = 1’ after adding
             operations to the queue, then the problem does not occur. But
             then this restricts me to control the operation execution to
             be serial. So, I expect the above snippet to work on
             OpenSource as well. Any thoughts on this?



             Thank you.

             Regards
             Mamatha


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