crashing RubyMotion with libdispatch and blocks

app_delegate.rb

class Foo
  def self.bar(&block)
    block.call(42)
  end
end

class AppDelegate
  def application(application, didFinishLaunchingWithOptions:launchOptions)
    Dispatch::Queue.concurrent(:low).async do
      Foo.bar{|res| p "the answer is #{res}"}
    end
    true 
  end
end

crashlog.txt

Crashed Thread:  2  Dispatch queue: com.apple.root.low-priority

Exception Type:  EXC_BAD_ACCESS (SIGBUS)
Exception Codes: KERN_PROTECTION_FAILURE at 0x0000000000000001

VM Regions Near 0x1:
--> __PAGEZERO             0000000000000000-0000000000001000 [    4K] ---/--- SM=NUL  /Users/USER/Library/Application Support/iPhone Simulator/*/sdrubios.app/sdrubios
    __TEXT                 0000000000001000-00000000001e2000 [ 1924K] r-x/r-x SM=COW  /Users/USER/Library/Application Support/iPhone Simulator/*/sdrubios.app/sdrubios

Application Specific Information:
iPhone Simulator 345, iPhone OS 6.0 (iPhone/10A5316k)
 

Thread 0:: Dispatch queue: com.apple.main-thread
0   libsystem_kernel.dylib        	0x96eecc22 mach_msg_trap + 10
1   libsystem_kernel.dylib        	0x96eec1f6 mach_msg + 70
2   liblaunch.dylib               	0x992c2a6d 0x992bb000 + 31341
3   liblaunch.dylib               	0x992bfaf3 bootstrap_look_up3 + 83
4   liblaunch.dylib               	0x992bfc5a bootstrap_look_up + 80
5   BackBoardServices             	0x03cd4aad _BKSServerPortHelper + 145
6   BackBoardServices             	0x03cd4a17 BKSHIDServicesServerPort + 48
7   BackBoardServices             	0x03ce24db -[BKSAccelerometer _checkIn] + 654
8   BackBoardServices             	0x03ce1fc3 -[BKSAccelerometer setAccelerometerEventsEnabled:] + 100
9   SpringBoardServices           	0x03cb6934 -[SBSAccelerometer setAccelerometerEventsEnabled:] + 50
10  UIKit                         	0x01446a8d -[UIMotionEvent _willResume] + 50
11  Foundation                    	0x00be1f89 __57-[NSNotificationCenter addObserver:selector:name:object:]_block_invoke_0 + 40
12  CoreFoundation                	0x009440c5 ___CFXNotificationPost_block_invoke_0 + 85
13  CoreFoundation                	0x0089deba _CFXNotificationPost + 2138
14  Foundation                    	0x00b16a72 -[NSNotificationCenter postNotificationName:object:userInfo:] + 98
15  UIKit                         	0x012a2f40 -[UIApplication _stopDeactivatingForReason:] + 346
16  UIKit                         	0x012a4b5d -[UIApplication _runWithURL:payload:launchOrientation:statusBarStyle:statusBarHidden:] + 1236
17  UIKit                         	0x012b52be -[UIApplication handleEvent:withNewEvent:] + 1131
18  UIKit                         	0x012b5f9f -[UIApplication sendEvent:] + 85
19  UIKit                         	0x012a83fd _UIApplicationHandleEvent + 9602
20  GraphicsServices              	0x03d13f39 _PurpleEventCallback + 339
21  GraphicsServices              	0x03d13c10 PurpleEventCallback + 46
22  CoreFoundation                	0x0085eda5 __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE1_PERFORM_FUNCTION__ + 53
23  CoreFoundation                	0x0085eb12 __CFRunLoopDoSource1 + 146
24  CoreFoundation                	0x0088fb46 __CFRunLoopRun + 2118
25  CoreFoundation                	0x0088eed4 CFRunLoopRunSpecific + 276
26  CoreFoundation                	0x0088edab CFRunLoopRunInMode + 123
27  UIKit                         	0x012a428f -[UIApplication _run] + 774
28  UIKit                         	0x012a5e71 UIApplicationMain + 1211
29  sdrubios                      	0x000034be main + 750
30  sdrubios                      	0x00003185 start + 53

Thread 1:: Dispatch queue: com.apple.libdispatch-manager
0   libsystem_kernel.dylib        	0x96eef926 kevent64 + 10
1   libdispatch.dylib             	0x0390da3a _dispatch_mgr_invoke + 858
2   libdispatch.dylib             	0x0390d6e0 _dispatch_mgr_thread + 61

Thread 2 Crashed:: Dispatch queue: com.apple.root.low-priority
0   sdrubios                      	0x0015e8ad rb_vm_add_block_lvar_use + 45
1   sdrubios                      	0x000d0ee2 rb_proc_alloc_with_block + 82
2   sdrubios                      	0x00169157 rb_vm_current_block_object + 119
3   sdrubios                      	0x0005d292 rb_scope__bar__ + 18
4   sdrubios                      	0x00157b99 rb_vm_dispatch + 3945
5   sdrubios                      	0x0005d25a vm_dispatch + 506
6   sdrubios                      	0x0005e2d2 rb_scope__application:didFinishLaunchingWithOptions:__block__ + 274 (app_delegate.rb:32)
7   sdrubios                      	0x0015b99f rb_vm_block_eval + 1535
8   sdrubios                      	0x0014bbd1 rb_block_release_eval + 33
9   sdrubios                      	0x00168ad8 rb_rescue2 + 24
10  sdrubios                      	0x000c5b50 rb_rescue + 64
11  sdrubios                      	0x0014bb77 rb_block_dispatcher + 55
12  libdispatch.dylib             	0x03919304 _dispatch_client_callout + 14
13  libdispatch.dylib             	0x0390b239 _dispatch_root_queue_drain + 286
14  libdispatch.dylib             	0x0390b02f _dispatch_worker_thread3 + 20
15  libsystem_c.dylib             	0x9c27db24 _pthread_wqthread + 346
16  libsystem_c.dylib             	0x9c27f6fe start_wqthread + 30

Note that

class Foo
  def self.bar(&block)
    yield(42)
  end
end

Will also make the app crash.

a work around should be:

class Foo
  def self.bar(&block)
    block.send(:retain)
    block.call(42)              # yield(42) should work too
  end
end

you should probably open a ticket, there were a similar issue with Symbols

Thanks Sean, a ticket was opened and this gist was provided. Laurent should see your message.
The problem seems quite obvious once reduced, a bit less when someone reported a problem with BubbleWrap ;)

I haven't heard anything about this bug which really isn't a good sign.

Yes, this has gotten me even when running the test suite - I had to tighten up the waits for the reactor specs to avoid it - since I knew it was a problem with RM and not my code.