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[SR-5439] [Beta3] Runtime exclusivity failure for KVO context pointers #48013

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zwaldowski opened this issue Jul 12, 2017 · 3 comments
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[SR-5439] [Beta3] Runtime exclusivity failure for KVO context pointers #48013

zwaldowski opened this issue Jul 12, 2017 · 3 comments
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@zwaldowski
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bug A deviation from expected or documented behavior. Also: expected but undesirable behavior. compiler The Swift compiler in itself

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@zwaldowski
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Previous ID SR-5439
Radar rdar://problem/33265254
Original Reporter @zwaldowski
Type Bug
Status Resolved
Resolution Done
Environment

Xcode version 9.0 beta 3 (9M174d) / swiftlang 900.0.52

Additional Detail from JIRA
Votes 0
Component/s Compiler
Labels Bug
Assignee @zwaldowski
Priority Medium

md5: b013264626250a5532385bbccf726cb5

Issue Description:

Up until Swift 4, "Using Swift with Cocoa and Objective-C" recommended "creat[ing] a global context variable" to take the address of to pass to the context parameter of `NSObject.addObserver(_:forKeyPath:options:context🙂`.

As of Beta 3, this practice fails runtime exclusivity checks, producing console logs and warnings in Xcode like:

Simultaneous accesses to 0x10416e278, but modification requires exclusive access.
Previous access (a modification) started at REDACTED`StatusViewController.addViewControllerObservers(_:) + 709 (0x1026521d5).
Current access (a modification) started at:
0    libswiftCore.dylib                 0x000000010d2ec7a0 swift_beginAccess + 505
1    REDACTED                           0x0000000102655ed0 StatusViewController.observeValue(forKeyPath:of:change:context:) + 198
2    REDACTED                           0x0000000102659f30 @objc StatusViewController.observeValue(forKeyPath:of:change:context:) + 648
3    Foundation                         0x0000000106d9ef2b NSKeyValueNotifyObserver + 349
4    Foundation                         0x0000000106d7f469 -[NSObject(NSKeyValueObserverRegistration) _addObserver:forProperty:options:context:] + 255
5    Foundation                         0x0000000106d7e3cb -[NSObject(NSKeyValueObserverRegistration) addObserver:forKeyPath:options:context:] + 103
6    REDACTED                           0x0000000102651f10 StatusViewController.addViewControllerObservers(_:) + 776
7    REDACTED                           0x0000000102651e20 StatusViewController.contentViewController.didset + 199
8    REDACTED                           0x0000000102651900 StatusViewController.contentViewController.setter + 313
9    REDACTED                           0x000000010265a1d0 StatusViewController.showContentViewController(_:) + 250
10   REDACTED                           0x000000010265a370 @objc StatusViewController.showContentViewController(_:) + 60
11   REDACTED                           ...
16   Deferred                           0x0000000106ccd6a0 closure #​1 in Deferred.notify(flags:upon:execute:) + 366
17   Deferred                           0x0000000106cd0150 partial apply for closure #​1 in Deferred.notify(flags:upon:execute:) + 79
18   Deferred                           0x0000000106ca8b90 thunk for @callee_owned () -> () + 41
19   libdispatch.dylib                  0x000000010e0466d6 _dispatch_block_async_invoke2 + 106
20   libdispatch.dylib                  0x000000010e039798 _dispatch_client_callout + 8
21   libdispatch.dylib                  0x000000010e044ec8 _dispatch_main_queue_callback_4CF + 639
22   CoreFoundation                     0x0000000108632de0 __CFRUNLOOP_IS_SERVICING_THE_MAIN_DISPATCH_QUEUE__ + 9
23   CoreFoundation                     0x00000001085f6e30 __CFRunLoopRun + 2162
24   CoreFoundation                     0x00000001085f6a20 CFRunLoopRunSpecific + 409
25   GraphicsServices                   0x000000010f4f99a4 GSEventRunModal + 62
26   UIKit                              0x000000010ae5cc13 UIApplicationMain + 159
27   REDACTED                           0x00000001023fd7b0 main + 55
28   libdyld.dylib                      0x000000010e0bab94 start + 1

While the Swift 4 materials recommend use of the new `KeyPath`-based overlay, this leaves 3.2 users in a lurch.

Workarounds:

  • Replacing the context with an `unsafeBitCast` of a `Selector`

  • Replacing the context with an `unsafeBitCast` of a `Foo.self` metatype.

  • Removing the context (only applicable to 1-ary observations not involved in a class tree.)
@jckarter
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@swift-ci create

@belkadan
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Another workaround: avoid using the initial observing option.

@jckarter
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Looks like @rjmccall came up with a fix in #11150

@swift-ci swift-ci transferred this issue from apple/swift-issues Apr 25, 2022
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@zwaldowski zwaldowski

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bug A deviation from expected or documented behavior. Also: expected but undesirable behavior. compiler The Swift compiler in itself
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@jckarter @zwaldowski @belkadan