[SR-1859] DispatchQueue setTarget fails with EXC_BAD_INSTRUCTION

[swift-ci]

Previous ID	SR-1859
Radar	None
Original Reporter	Andreas Grosam (JIRA User)
Type	Bug
Status	Closed
Resolution	Won't Do

Additional Detail from JIRA

Votes	0
Component/s	libdispatch, Standard Library
Labels	Bug, RunTimeCrash
Assignee	Andreas Grosam (JIRA)
Priority	Medium

md5: 731bcc2142f4e03e3ff0ffc5490a8cc6

Issue Description:

With Swift 3, setting the target queue of an existing dispatch queue fails in the Simulator for iOS and tvOS. It does not fail on macOS. (watchOS not tested, and not tested on actual devices).

This did not fail with Swift 2.2.

The code below shows the issue:

func testSetTargetQueue() {
    let queue = DispatchQueue(label: "queue", attributes: DispatchQueueAttributes.serial)
    let syncQueue = DispatchQueue(label: "sync_queue", attributes: DispatchQueueAttributes.serial) 
    queue.setTarget(queue: syncQueue)   // EXC_BAD_INSTRUCTION.  Error: "Cannot change the target of this queue after it has been activated"     
}

[belkadan]

cc mww (JIRA User)

[swift-ci]

Comment by Matt Wright (JIRA)

This is an intended behaviour change, DispatchQueue() accepts a ```queue:``` parameter for setting the target queue. After which the target queue hierarchy is immutable.

[belkadan]

Is there any point where it makes sense to call setTarget then? Should it just be hidden?

[swift-ci]

Comment by Matt Wright (JIRA)

If you create a DispatchQueue with the .initiallyInactive flag then it's valid to call setTarget on it, it's similarly safe for sources pre-activate()/resume().

[swift-ci]

Comment by Pierre Habouzit (JIRA)

FWIW DispatchQueue.init() uses dispatch_queue_create_with_target() from the C interface, which documents this behavior extensively (in <dispatch/queue.h>). New APIs in Dispatch already enforce this behavior, and the Swift3 overlay is such a New API Surface.

[swift-ci]

Comment by Matt Wright (JIRA)

Closing given that this is both intended behaviour, and there is still a reason for DIspatchObject.setTarget(queue🙂 to exist.

[swift-ci]

Comment by Andreas Grosam (JIRA)

The new API and restrictions makes sense to me. Thanks for clearing this issue.