Enumerating NSAttributedString Attributes on Multiple Threads causes EXC_BAD_ACCESS

Originator:oliver.drobnik
Number:rdar://15139980 Date Originated:Oct 3, 2013
Status:Open Resolved:
Product:iOS SDK Product Version:7.0 (11A465)
Classification:Serious Bug Reproducible:Always
 
Summary:
If you enumerate attributes on two different NSAttributedString instances on two background threads an EXC_BAD_ACCESS is caused by -attribute:atIndex:effectiveRange: method.

Steps to Reproduce:
Launch the provided sample app.

Expected Results:
There should be no crash.

Actual Results:
The app crashes showing two crashed threads at DTCoreTextLayoutFrame.m:949.

I'm enclosing a screen shot of the crash.

If you enable the the synchronization at line 939 then the crash no longer occurs.

Version:
7.0 (11A465)

Notes:
This bug was also present in iOS 6.

Configuration:
iOS 7 Simulator

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