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[SR-8878] Using Mirror to access a weak reference results in object being retained indefinitely #51384

swift-ci opened this issue Sep 28, 2018 · 5 comments


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swift-ci commented Sep 28, 2018

Previous ID SR-8878
Radar rdar://problem/44872927
Original Reporter abrindamour (JIRA User)
Type Bug
Status Resolved
Resolution Done

Attachment: Download


Xcode Version 10.0 (10A255)

Does NOT happen in Xcode 9.4 (9F1027a)

Additional Detail from JIRA
Votes 0
Component/s Standard Library
Labels Bug, 4.2Regression, Leak
Assignee @mikeash
Priority Medium

md5: f2e7d2b8f34cc831eb5f63c6de45d673

is duplicated by:

  • SR-9282 Weak variables in structs leak on String(describing:) or print
  • SR-9400 Mirror make memory leak if children access to IBOutlet

Issue Description:

Please see attached example file "Bug.swift". This file creates an object "WeakHolderObject" which has a weak reference to another object, "AnObject" via the field "weakObject". A Mirror is then used to access the "weakObject" field. All of this is done in an autoreleasepool so that all memory should be released when the pool ends. However, when run in Xcode 10, "AnObject" is not deallocated and is retained indefinitely. Removing the Mirror operation OR running in Xcode 9.4 prevents this problem from occurring.

An additional trickier effect of this can be seen in "Bug2.swift" which is very similar, but we add a 2nd field "anotherField" to "WeakHolderObject" which is declared after the first field, and try to access that with a mirror instead. This still causes "AnObject" to not be deallocated. However, if you reorder the field declarations so that "anotherField" is declared first, then the problem does not occur.

This issue causes "Heisenbugs" in which just looking at an object's state can cause memory retention, which then can affect program correctness. In my company's application, we use Mirror to create a state dump when a user reports an issue. As a result of this, the act of reporting a bug can cause more bugs!

Steps to Reproduce
Run the attached examples "Bug.swift" and "Bug2.swift" in Xcode10.

Expected Results
Both examples should print "Released"

Actual Results
Both examples instead print "Retained". As noted above, running these examples in Xcode 9.4 produce expected results.

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Collaborator Author

swift-ci commented Sep 28, 2018

Comment by Andrew Brindamour (JIRA)

I filed this with Apple Bugs first as #44872927 because I didn't think to file it here first. Hopefully I got the rdar:// URL correct...

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belkadan commented Oct 1, 2018

Noted in that Radar that you also filed here. Thanks!

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mikeash commented Oct 1, 2018

Works for me! Thanks for the report.

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mikeash commented Oct 9, 2018

And here's the PR with my fix: #19795

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mikeash commented Oct 10, 2018

Merged the fix.

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