D6376: Fix double delete crash during shutdown

["René J.V. Bertin"]

rjvbb added a comment.


  In https://phabricator.kde.org/D6376#119376, @cullmann wrote:
  
  > Next try ;=)
  >  Question is, can really such a race happen, that finishNotification triggers duplicate insertion?
  
  
  Do you see any other explanation why a crash could occur under qDeleteAll, one that is due to a bug in KNotifications and not in Phonon itself?
  
  I'd say the source for a duplicate insertion (and thus the location of the race condition) would be a concurrent execution of `m_reusablePhonons.takeFirst()`, where more than 1 thread gets hold of the same item.
  
  > At least with a hash set, this won't lead to later issues.
  
  Why not use a barrier or lock to ensure that the `takeFirst()` requests are executed one by one?
  
  FWIW, a race condition wouldn't only lead to duplicate entries in the list (something I'd hope Qt knows how to handle) but also to more than 1 thread using the same Phonon object at the same time. Who's to tell what state that object will be in?

REPOSITORY
  R289 KNotifications

REVISION DETAIL
  https://phabricator.kde.org/D6376

To: cullmann, #frameworks
Cc: mpyne, rjvbb
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