isLowPowerModeEnabled causes a deadlock on iOS 15
| Originator: | ncreated | ||
| Number: | rdar://FB9741207 | Date Originated: | Nov 5, 2020 |
| Status: | Open | Resolved: | |
| Product: | iOS | Product Version: | 15 |
| Classification: | Reproducible: | Very rare |
We work on a SDK and our users report unfrequent crashes (on iOS15, 15.1 and 15.2) in the code that can be simplified to:
```
observer = NotificationCenter.default
.addObserver(
forName: .NSProcessInfoPowerStateDidChange,
object: nil,
queue: .main
) { notification in
guard let processInfo = notification.object as? ProcessInfo else {
return
}
let lmpe = processInfo.isLowPowerModeEnabled // CRASH on _os_unfair_lock_recursive_abort in libsystem_platform.dylib
print(lmpe)
}
```
In some circumstances, getting `.isLowPowerModeEnabled` from `ProcessInfo` **within the `. NSProcessInfoPowerStateDidChange` notification callback** causes a dead lock in `libsystem_platform.dylib` and leads to `_os_unfair_lock_recursive_abort` crash.
Unfortunately, I didn’t succeed in reproducing this issue locally and capturing Apple’s crash report, so I can only rely on backtraces reported by our users (below). Some of them include `WebCore` symbols, which could be a potential external factor causing this crash. I also found this recent change to Low Power Mode management in WebKit (dated Dec 2020), but I’m not sure if this is relevant: https://trac.webkit.org/changeset/270406/webkit.
```
Crashed: com.apple.main-thread
0 libsystem_platform.dylib 0x60c0 _os_unfair_lock_recursive_abort + 36
1 libsystem_platform.dylib 0xa10 _os_unfair_lock_lock_slow + 304
2 Foundation 0x29730 -[NSProcessInfo(NSProcessInfoHardwareState) isLowPowerModeEnabled] + 68
3 Datadog 0x2a680 closure #1 in LowPowerModeMonitor.init(initialProcessInfo:notificationCenter:) + 140 (MobileDevice.swift:140)
4 Datadog 0x2a720 thunk for @escaping @callee_guaranteed (@in_guaranteed Notification) -> () + 4339345184 (<compiler-generated>:4339345184)
5 Foundation 0x355b0 -[__NSObserver _doit:] + 348
6 CoreFoundation 0x2aee8 __CFNOTIFICATIONCENTER_IS_CALLING_OUT_TO_AN_OBSERVER__ + 28
7 CoreFoundation 0xc6b9c ___CFXRegistrationPost_block_invoke + 52
8 CoreFoundation 0x99f54 _CFXRegistrationPost + 456
9 CoreFoundation 0x40d54 _CFXNotificationPost + 716
10 Foundation 0x1b028 -[NSNotificationCenter postNotificationName:object:userInfo:] + 96
11 Foundation 0x939d4 NSProcessInfoNotifyPowerState + 188
12 Foundation 0x29768 -[NSProcessInfo(NSProcessInfoHardwareState) isLowPowerModeEnabled] + 124
13 WebCore 0x9118 <redacted> + 96
14 WebCore 0xa8516c WebCore::LowPowerModeNotifier::LowPowerModeNotifier(WTF::Function<void (bool)>&&) + 52
15 WebCore 0x19aac4c WebCore::Page::Page(WebCore::PageConfiguration&&) + 1848
16 WebKitLegacy 0x3e34 -[WebView(WebPrivate) _commonInitializationWithFrameName:groupName:] + 3060
17 WebKitLegacy 0x3214 -[WebView(WebPrivate) _initWithFrame:frameName:groupName:] + 116
18 UIFoundation 0xdd028 -[NSHTMLReader _loadUsingWebKit] + 832
19 Foundation 0x3de0c __NSThreadPerformPerform + 232
20 CoreFoundation 0xbb030 __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__ + 28
21 CoreFoundation 0xcbcf0 __CFRunLoopDoSource0 + 208
22 CoreFoundation 0x5ff8 __CFRunLoopDoSources0 + 268
23 CoreFoundation 0xb804 __CFRunLoopRun + 820
24 CoreFoundation 0x1f3c8 CFRunLoopRunSpecific + 600
25 GraphicsServices 0x138c GSEventRunModal + 164
26 UIKitCore 0x51b060 -[UIApplication _run] + 1100
27 UIKitCore 0x298b8c UIApplicationMain + 2124
```
```
Crashed: com.apple.main-thread
0 libsystem_platform.dylib 0x8e4c _os_unfair_lock_recursive_abort + 36
1 libsystem_platform.dylib 0x17e4 _os_unfair_lock_lock_slow + 324
2 Foundation 0x27b08 -[NSProcessInfo(NSProcessInfoHardwareState) isLowPowerModeEnabled] + 64
3 Datadog 0x2a680 closure #1 in LowPowerModeMonitor.init(initialProcessInfo:notificationCenter:) + 140 (MobileDevice.swift:140)
4 Datadog 0x2a720 thunk for @escaping @callee_guaranteed (@in_guaranteed Notification) -> () + 4374603552 (<compiler-generated>:4374603552)
5 Foundation 0x335f8 -[__NSObserver _doit:] + 316
6 CoreFoundation 0x28c5c __CFNOTIFICATIONCENTER_IS_CALLING_OUT_TO_AN_OBSERVER__ + 20
7 CoreFoundation 0xbd564 ___CFXRegistrationPost_block_invoke + 48
8 CoreFoundation 0x92b44 _CFXRegistrationPost + 416
9 CoreFoundation 0x3d764 _CFXNotificationPost + 696
10 Foundation 0x19c98 -[NSNotificationCenter postNotificationName:object:userInfo:] + 92
11 Foundation 0x8d998 NSProcessInfoNotifyPowerState + 184
12 Foundation 0x27b40 -[NSProcessInfo(NSProcessInfoHardwareState) isLowPowerModeEnabled] + 120
13 Datadog 0x2a680 closure #1 in LowPowerModeMonitor.init(initialProcessInfo:notificationCenter:) + 140 (MobileDevice.swift:140)
14 Datadog 0x2a720 thunk for @escaping @callee_guaranteed (@in_guaranteed Notification) -> () + 4374603552 (<compiler-generated>:4374603552)
15 Foundation 0x335f8 -[__NSObserver _doit:] + 316
16 CoreFoundation 0x28c5c __CFNOTIFICATIONCENTER_IS_CALLING_OUT_TO_AN_OBSERVER__ + 20
17 CoreFoundation 0xbd564 ___CFXRegistrationPost_block_invoke + 48
18 Foundation 0x4e21c __NSBLOCKOPERATION_IS_CALLING_OUT_TO_A_BLOCK__ + 16
19 Foundation 0x5f6f4 -[NSBlockOperation main] + 100
20 Foundation 0x39ddc __NSOPERATION_IS_INVOKING_MAIN__ + 20
21 Foundation 0x49f1c -[NSOperation start] + 780
22 Foundation 0x4d314 __NSOPERATIONQUEUE_IS_STARTING_AN_OPERATION__ + 20
23 Foundation 0x5aa2c __NSOQSchedule_f + 180
24 libdispatch.dylib 0x632ec _dispatch_call_block_and_release + 24
25 libdispatch.dylib 0x642f0 _dispatch_client_callout + 16
26 libdispatch.dylib 0x109a0 _dispatch_main_queue_callback_4CF$VARIANT$mp + 936
27 CoreFoundation 0x4d7f8 __CFRUNLOOP_IS_SERVICING_THE_MAIN_DISPATCH_QUEUE__ + 12
28 CoreFoundation 0xb0f8 __CFRunLoopRun + 2528
29 CoreFoundation 0x1dd8c CFRunLoopRunSpecific + 572
30 GraphicsServices 0x19a0 GSEventRunModal + 160
31 UIKitCore 0x4edfa8 -[UIApplication _run] + 1080
32 UIKitCore 0x28222c UIApplicationMain + 2060
```
```
Thread 0 [Crashed]:
0 libsystem_platform.dylib 0x1f15920c0 _os_unfair_lock_recursive_abort + 36
1 libsystem_platform.dylib 0x1f158ca10 _os_unfair_lock_lock_slow + 303
2 Foundation 0x183211730 -[NSProcessInfo(NSProcessInfoHardwareState) isLowPowerModeEnabled] + 67
3 Datadog 0x102bdfa28 closure #1 in LowPowerModeMonitor.init(MobileDevice.swift:140)
4 Datadog 0x102bdfac8 thunk for @escaping @callee_guaranteed (<compiler-generated>:0)
5 Foundation 0x18321d5b0 -[__NSObserver _doit:] + 347
6 CoreFoundation 0x1819e8ee8 _CFNOTIFICATIONCENTER_IS_CALLING_OUT_TO_AN_OBSERVER__ + 27
7 CoreFoundation 0x181a84b9c __CFXRegistrationPost_block_invoke + 51
8 CoreFoundation 0x181a57f54 _CFXRegistrationPost + 455
9 CoreFoundation 0x1819fed54 _CFXNotificationPost + 715
10 Foundation 0x183203028 -[NSNotificationCenter postNotificationName:object:userInfo:] + 95
11 Foundation 0x18327b9d4 NSProcessInfoNotifyPowerState + 187
12 Foundation 0x183211768 -[NSProcessInfo(NSProcessInfoHardwareState) isLowPowerModeEnabled] + 123
13 WebCore 0x190a48118 -[WebLowPowerModeObserver initWithNotifier:] + 95
14 WebCore 0x1914c416c WebCore::LowPowerModeNotifier::LowPowerModeNotifier(WTF::Function<void (bool)>&&) + 51
15 WebCore 0x1923e9c4c WebCore::Page::Page(WebCore::PageConfiguration&&) + 1847
16 WebKitLegacy 0x1a550de34 -[WebView(WebPrivate) _commonInitializationWithFrameName:groupName:] + 3059
17 WebKitLegacy 0x1a550d214 -[WebView(WebPrivate) _initWithFrame:frameName:groupName:] + 115
18 UIFoundation 0x18c7b9028 -[NSHTMLReader _loadUsingWebKit] + 831
19 UIFoundation 0x18c7ba15c -[NSHTMLReader attributedString] + 31
20 UIFoundation 0x18c7734e8 _NSReadAttributedStringFromURLOrData + 8419
21 UIFoundation 0x18c771378 -[NSAttributedString(NSAttributedStringUIFoundationAdditions) initWithData:options:documentAttributes:error:] + 155
```
All crashes occur on iOS 15 (so far we received reports for iOS 15, 15.1 and 15.2).
Similar feedback has already been reported in FB9661108.
---
**Please list the steps you took to reproduce the issue:**
With my best efforts, I couldn’t reproduce it locally. I only rely on crash reports reported by our customers.
**What did you expect to happen?**
Obtaining `ProcessInfo.processInfo.isLowPowerModeEnabled` should be thread safe no matter of surrounding code. Reading it from `.NSProcessInfoPowerStateDidChange` notification callback should not lead to crash.
**What actually happened?**
Sometimes, reading `.isLowPowerModeEnabled` from `ProcessInfo` **within the `. NSProcessInfoPowerStateDidChange` notification callback** causes a dead lock in `libsystem_platform.dylib` and leads to `_os_unfair_lock_recursive_abort` crash.
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The status of this issue was updated with: `
Recent Similar Reports:Less than 10 Resolution:Potential fix identified - For a future OS update `