[Swift] Fail to load executable when extending DispatchQueue with protocol

Originator:zach
Number:rdar://26850367 Date Originated:16-Jun-2016 04:38 PM
Status:Closed Resolved:true
Product:Developer Tools Product Version:Xcode Version 8.0 beta (8S128d)
Classification:Other Bug Reproducible:Always
 
Summary:
In adapting a GCD-oriented framework for Swift 3, it becomes possible to extend `DispatchQueue` to conform to a protocol, whereas this was not possible when `dispatch_queue_t` was an opaque class type.

Unfortunately, actually doing the conformance causes a runtime dylib linking failure.

Steps to Reproduce:
1. Create a framework using the "Framework & Library -> Cocoa Touch Framework" template, with the "Include Unit Tests" box checked.
2. Add a protocol with methods to the framework as a public member.
3. Import `Dispatch` and make `DispatchQueue` implement the protocol.
4. Attempt to run the tests.

Expected Results:
Empty test bundle having linked the framework runs successfully.

Actual Results:
Tests fail without any notes in the debug log or Test logs (see attached). The Tests item in the Report navigator reads: "Test target DeferredTests encountered an error (Early unexpected exit, operation never finished bootstrapping - no restart will be attempted)".

If code coverage is enabled, Xcode presents the rather cryptic dialog "Code Coverage Data Generation Failed - Unable to retrieve the profile data files from 'My Mac'."

Version:
Xcode Version 8.0 beta (8S128d)
OS X El Capitan 10.11.6 Beta (15G12a)

Notes:
In the system console log, the following helpful bits are left behind:
6/16/16 4:33:33.744 PM xctest[26546]: The bundle “DeferredTests” couldn’t be loaded. Try reinstalling the bundle.
6/16/16 4:33:33.744 PM xctest[26546]: (dlopen(/.../DerivedData/Deferred/Build/Intermediates/CodeCoverage/Products/Debug/DeferredTests.xctest/Contents/MacOS/DeferredTests, 265): Symbol not found: _OBJC_CLASS_$_OS_dispatch_queue
  Referenced from: /.../DerivedData/Deferred/Build/Intermediates/CodeCoverage/Products/Debug/Deferred.framework/Versions/A/Deferred
  Expected in: /usr/lib/libSystem.B.dylib
 in /.../DerivedData/Deferred/Build/Intermediates/CodeCoverage/Products/Debug/Deferred.framework/Versions/A/Deferred)




Configuration:


Attachments:
'MiniDeferred.zip' and 'Session-DeferredTests-2016-06-16_163317-OJyju9.log' were successfully uploaded.

Comments

https://github.com/apple/swift/commit/7d1a2e8339c0d83a1143698e0f90ad020dfeb1b2 notes that only @objc members of extensions are forbidden based on the objc_runtime_visible rules, so this would appear to be a legitimate bug. (Fingers crossed.)

By zach at June 19, 2016, 5:26 a.m. (reply...)

I performed some further investigation. The load failure doesn't occur on a 10.12 target; it's trivially reproducible in an app target running on 10.11 from Swift 3 with the 10.12 SDK.

The protocol conformance witness table hard-emits a reference to the OS_dispatch_queue class, which was only available at runtime on previous OS'. All other normal uses of the class route through objc_lookUpClass based on the objc_runtime_visible Clang attribute.

By zach at June 19, 2016, 5:25 a.m. (reply...)

Sample project: https://bucket.waldowski.me/26850367-MiniDeferred

By zach at June 16, 2016, 11:39 p.m. (reply...)

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