[Swift] Calls to "po <# something #>" in the debugger cause a deadlock in String.init<T>(_ instance: T)

Originator:eliacereda
Number:rdar://23875049 Date Originated:13 December 2015
Status:Open Resolved:
Product:Developer Tools Product Version:
Classification:Deadlock Reproducible:Always
 
Summary:
After a call to "po <# something #>" in the debugger, subsequent calls to String.init<T>(_ instance: T) and related methods will deadlock.

This seems to happen only on actual iOS devices (i.e. not the simulator). I've created a proof of concept and attached it to the bug report. The stack trace of the deadlock is also attached.

Steps to Reproduce:
Run the attached application, tap on the "Test deadlock" button. 

Expected Results:
The call to "po" should not cause deadlocks.

Actual Results:
The application hangs. Pausing the debugger now will reveal that the process is stuck on a __psynch_mutexwait call inside String.init<T>(_ instance: T).

If the breakpoint on nop() is disabled the program doesn't hang.

Version:
Xcode 7.2 (7C68)

Notes:
I'm filing a bug here instead of the opensource bug tracker because this bug only happens on iOS devices and I don't know of a way to reproduce it with opensource tools (unless there's a way to run the Swift REPL on device?)

Configuration:
Happens on iPhone 6 - iOS 9.0.2 (13A452)
Doesn't happen on iPhone Simulator

Attachments:
'StringInterpolationDeadlock.zip' (http://cl.ly/e4SI) and 'stacktrace.png' (http://cl.ly/e4TA) were successfully uploaded.

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