Touch Bar Crashers

Since the MacBook Pro with Touch Bar was released, I’ve noticed a small but steady influx of new crash reports against my apps. Typically, the crashes in something CALayer related, and the correlated crash log often descends from “-[DFRElement dealloc]”:

Thread 0 Crashed:: Dispatch queue:
0   libobjc.A.dylib               	0x00007fffea5211cf objc_class::demangledName(bool) + 33
1      	0x00007fffd58b6c20 ___forwarding___ + 144
2      	0x00007fffd58b6b08 _CF_forwarding_prep_0 + 120
3          	0x00007fffdb7012a8 -[CALayer actionForKey:] + 95
4          	0x00007fffdb6fbccf CA::Layer::mark_visible(CA::Transaction*, bool) + 345
5          	0x00007fffdb6fbe8c CA::Layer::set_visible(unsigned int) + 306
6          	0x00007fffdb6eef21 CA::Context::invalidate() + 75
7	0x00007fffe180c7bd -[DFRElement dealloc] + 113

That DFR prefix rang a bell for me, because it’s the same prefix as some of Apple’s private frameworks that support the Touch Bar simulator in Xcode, which I also use in my free, standalone Touch Bar simulator, Touché.

User comments associated with these crashes often mentioned connecting or disconnecting an external monitor, or with waking their MacBook from sleep.

I speculated that the crashes might represent some subtle bug that is only likely to happen when the Touch Bar infrastructure on the Mac is forced to stop and start again. I would be surprised if, when the computer goes to sleep, the physical Touch Bar on the MacBook didn’t also stop streaming content from the computer. When the computer wakes, it would make sense for it to trigger a resumption of that streaming.

Hmm, if only I had a Touch Bar Mac, so I could try to reproduce these crashes. I would open my apps, then I would close the laptop lid, and open it again, say 100,000 times or so, until I reproduce the problem.

Since I had already written a Touch Bar simulator app, which itself has the ability to stop and start streaming of the Touch Bar content, I speculated that I might be able to provoke the same crashes from within my app. Indeed, some of my users who reported these crashes were not using Touch Bar Macs, but were running Touché.

I rigged up a special version of my app that would simply show and hide the Touch Bar simulator window repeatedly, perhaps thousands of times per second. Maybe that would trigger the crashing bug that only rarely affects my customers.

It crashes my apps all right, usually in less than a second. The funny thing is it seemed to only crash my apps. None of Apple’s apps seemed to be dying. Other 3rd party apps on my Mac were also holding up fine.

Finally I came across another app that also crashed, and came up with a theory: only apps that are built with a suitably older SDK are crashing. For reasons I won’t go into, most of my apps are still built on a Mac OS X 10.6 Mac running Xcode 6. Sure enough, as I went out scouting for apps that I suspected were of a similar vintage, I was able to crash them in “less than a second” as well.

If you’re on a Touch Bar Mac, or running Touché, and seeing crashes in apps that culminate in CALayer, I think there’s a very good chance it represents a bug in Apple’s Touch Bar support on the Mac. I’ve filed a bug with a reprodeucable test case: Radar #29537507.