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Flipboard / NSUserDefaultsSimulatorPerformanceBoost

Speed up reads from NSUserDefaults in the simulator
http://engineering.flipboard.com/2015/03/nsuserdefaults-performance/
BSD 3-Clause "New" or "Revised" License
40 stars 5 forks source link

dispatch_barrier_async can lead to inconsistent values when using mutable objects #2

Open timonus opened 9 years ago

timonus commented 9 years ago

dispatch_barrier_async works great for this project, except for the following situation:

  1. NSMutableDictionary *registeredDefaults = [[NSMutableDictionary alloc] init];
  2. [[NSUserDefaults standardUserDefaults] registerDefaults:registeredDefaults];
  3. [registeredDefaults setObject:@"FOO" forKey:@"BAR"];
  4. NSObject *value = [[NSUserDefaults standardUserDefaults] objectForKey:@"BAR"];
    • With sim performance ON, value may be equal to nil or equal to @"FOO" depending on if line 3 executed before the internal barrier_async block executed.
    • With sim performance OFF, value is always equal to nil. Mutations after line 2 are not acknowledged.

The code above is incorrect anyways (you wouldn't expect it to work) -- but we still need to honor the behavior of NSUserDefaults. You could emulate this bug using -setObject:forKey: with a mutable object.

timonus commented 9 years ago

Attempted to resolve in https://github.com/Flipboard/NSUserDefaultsSimulatorPerformanceBoost/pull/1, but that leads to deadlocking. We need recursive locking.