[swift-users] Crash with swift_weakLoadStrong in the stack

[Jon Shier]

These crashes are uploaded from device, namely an iPad 4 (running the app in compatibility mode) uploaded through Crashlytics and then downloaded from there. I haven’t been able to reproduce the crash and so I haven’t seen a raw dump. Here’s the full crash stack, redacted, from an iPad 4 running iOS 10.1.1 (this is an iOS 10+ app).

#0. Crashed: com.apple.main-thread
0  App					0x665ac Controller.handleSomeNotification(SomeNotification) -> () (Controller.swift:92)
1  libswiftCore.dylib			0x131854f swift_unknownWeakLoadStrong + 10
2  App					0x65cfc Controller.handleFinishSomeNotification(Notification) -> () (Controller.swift)
3  App					0x65dd8 @objc Controller.handleSomeOtherNotification(Notification) -> () + 437720
4  CoreFoundation			0x1bafa761 __CFNOTIFICATIONCENTER_IS_CALLING_OUT_TO_AN_OBSERVER__ + 10
5  CoreFoundation			0x1bafa09d _CFXRegistrationPost + 386
6  CoreFoundation			0x1baf9e81 ___CFXNotificationPost_block_invoke + 40
7  CoreFoundation			0x1bb5581d -[_CFXNotificationRegistrar find:object:observer:enumerator:] + 1188
8  CoreFoundation			0x1ba5a09d _CFXNotificationPost + 540
9  App					0x15b004 specialized specialized NotificationCenter.post<A> (A, forName : NSNotification.Name) -> () (Notifications.swift)
10 App					0x8d6f4 SomeListener.(post(SomeNotification : SomeNotification) -> ()).(closure #1) (SomeNotificationHandler.swift)
11 App					0x96318 partial apply for SomeListener.(post(SomeNotification : SomeNotification) -> ()).(closure #1) (SomeNotificationHandler.swift)
12 libdispatch.dylib			0x1b1f5097 _dispatch_call_block_and_release + 10
13 libdispatch.dylib			0x1b1f5083 _dispatch_client_callout + 22
14 libdispatch.dylib			0x1b1f95fd _dispatch_main_queue_callback_4CF + 890
15 CoreFoundation			0x1bb0aa17 __CFRUNLOOP_IS_SERVICING_THE_MAIN_DISPATCH_QUEUE__ + 8
16 CoreFoundation			0x1bb08cff __CFRunLoopRun + 1422
17 CoreFoundation			0x1ba58073 CFRunLoopRunSpecific + 486
18 CoreFoundation			0x1ba57e81 CFRunLoopRunInMode + 104
19 GraphicsServices		0x1d204bfd GSEventRunModal + 156
20 UIKit					0x20c0c82f -[UIApplication _run] + 574
21 UIKit					0x20c06f61 UIApplicationMain + 150
22 App					0x4bc38 main (AppDelegate.swift:12)
23 libdispatch.dylib			0x1b22250b (Missing)

Essentially I have a listener waiting on a background queue for a push notification to come in, which then repackages the payload into a Notification using a generic convenience method I wrote, and posted onto the main queue. Controller is listening for this two separate notifications, one of which is the SomeNotification. But the @objc method in there is the selector for the other notification, SomeOther. There should be no path between 3 and 2 in the stack, and there are no weak references I can see, except perhaps the implicit ones from NotificationCenter. Badly resymbolicated log? Is that even possible?


Jon

> On Feb 13, 2017, at 6:52 PM, Greg Parker <gparker at apple.com> wrote:
> 
>> 
>> On Feb 13, 2017, at 12:18 PM, Jon Shier via swift-users <swift-users at swift.org <mailto:swift-users at swift.org>> wrote:
>> 
>> Swift Users:
>> 	I’m currently seeing a crash in my iOS app that has no apparent cause, but a bit of Swift runtime machinery in the stack has me confused.
>> 
>> #0. Crashed: com.apple.main-thread
>> 0  App                        0x665ac Controller.handleOtherNotification(Notification) -> () (Controller.swift:92)
>> 1  libswiftCore.dylib         0x131854f swift_unknownWeakLoadStrong + 10
>> 2  App                        0x65cfc Controller.handleFinish(Notification) -> () (Controller.swift)
>> 3  App                        0x65dd8 @objc Controller.handleNotification(Notification) -> () + 437720
>> 
>> Controller.swift: 92 is a call to a custom UIView subclass method that takes an optional date contained extracted from the notification. Any idea what the core callout would be due to? There are no weak or unknown values being used here. Once the notification observers are called it’s all internal to Controller.
>> 
>> One thing I just noticed is that the line at 3 is the selector for a different notification, which should lead down the path see from 2 onward. It’s redacted and so not easy to see, but in Controller, there’s no path that leads from handleNotification to handleFinish.
> 
> That backtrace does look strange. Even if there were some surprising call to swift_unknownWeakLoadStrong() in handleFinish(), there's no way that swift_unknownWeakLoadStrong() would call handleOtherNotification().
> 
> (swift_unknownWeakLoadStrong + 10 is the instruction after a call, assuming you're on 64-bit iOS simulator, but that call is to swift::isNativeSwiftWeakReference() which itself doesn't call anything.)
> 
> Also, in frame 3, the byte offset from the start of handleNotification() is larger than the address itself. And all of these addresses look too small if you're on 64-bit.
> 
> Where did this backtrace come from? Do you have a crash log as generated by the OS?
> 
> 
> -- 
> Greg Parker     gparker at apple.com <mailto:gparker at apple.com>     Runtime Wrangler

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