[SR-1769] qos_class_self() doesn't interoperate with DispatchQoS

[glessard]

Previous ID	SR-1769
Radar	rdar://problem/26879347
Original Reporter	@glessard
Type	Bug
Status	Resolved
Resolution	Done

Environment

Xcode 8 (beta, 8S128d) with Swift 3.0 preview 1

Additional Detail from JIRA

Votes	0
Component/s	libdispatch
Labels	Bug
Assignee	mww (JIRA)
Priority	Medium

md5: 4e42e84469231d8d359f27b0d0008de7

Issue Description:

A frequently-used function alongside libdispatch, qos_class_self() did not get updated with the Dispatch module. An equivalent that returns a DispatchQoS instance would be most welcome.

extension DispatchQoS {
  static func current() -> DispatchQoS // equivalent to qos_class_self()
  static var main: DispatchQoS // equivalent to qos_class_main()
  init(qos: qos_class_t) // perhaps with a relativePriority?
}

[swift-ci]

Comment by Matt Wright (JIRA)

Addressed in 8ac413a0b5a9f927f44438df3376a7757f607953.

[glessard]

It looks like the solution here was to add a failable initializer to DispatchQoS.QoSClass. This adds a level of optionality to use cases of qos_class_self() that formerly did not require it. In particular, this hurts cases where qos_class_self() was used as the default parameter for a function call:

func work(qos: qos_class_t = qos_class_self()) {}

This worked just fine (similarly to the #line macros for the assert functions.) However now one must handle optionality, and directly using the return value of qos_class_self(), it doesn't seem warranted. (Am I wrong about this?)

It can be worked around with the ?? operator, but it's awfully heavy:

func work(qos: DispatchQoS = DispatchQoS(qosClass: DispatchQoS.QoSClass(rawValue: qos_class_self()) ?? .unspecified, relativePriority: 0))

Thanks!

[swift-ci]

Comment by Daniel A. Steffen (JIRA)

be careful, qos_class_self() can return 0 but that does not mean the same thing as passing .unspecified for e.g. a workitem or a queue attribute

qos_class_self() returning 0 means the calling thread was opted out of QoS by calling legacy thread configuration API and as a result e.g. may have very high priority. (above the one reachable by QoS user-interacive)

.unspecified on workitems/queues means no priority information was specified and more or less that the "ambient" QoS should be used (the one on the queue submitted to or on the target queue etc)

[glessard]

The failable initializer takes the zero value just fine. Is there a valid return value from qos_class_self() that could result in the failable initializer returning nil?

(if not, a "better" workaround is to simply force-unwrap:

  func work3(qos: DispatchQoS = DispatchQoS(qosClass: DispatchQoS.QoSClass(rawValue: qos_class_self())!, relativePriority: 0)) {}

but that is still a lot heavier than the original)