[SR-3028] do-catch block break nested DispatchSource workitem

[swift-ci]

Previous ID	SR-3028
Radar	None
Original Reporter	michael-yuji (JIRA User)
Type	Bug
Status	Closed
Resolution	Invalid

Environment

OS X 10.12

Additional Detail from JIRA

Votes	0
Component/s	libdispatch
Labels	Bug
Assignee	None
Priority	Medium

md5: 13ab7af38c18a6a0a6f154f8ac1b81f0

Issue Description:

Basically, when use DispatchSource(tested Timer and FileSystem) within a do-catch block will break the GCD source.

Consider the code below, it works fine without do-catch block but stop working when the block is present. Since explicitly referencing source inside the eventhandler will solve the program despite nested in do-catch block.

My educational guess is that somehow the do-catch block is breaking the dispatch_retain / dispatch_release cycle.

// main.swift
import Dispatch
do { // remove the try-catch block the timer works fine
    let source = DispatchSource.makeTimerSource()
    source.setEventHandler {
        /* uncomment the next line, explicitly reference `source` will resolve the issue as well */
//     _ = source
        print("hello world")
    }
    source.scheduleRepeating(deadline: DispatchTime.now(), interval: DispatchTimerInterval.seconds(1))
    source.resume()
} catch {}
dispatchMain()

[belkadan]

Dispatch folks, does a DispatchSource need to be retained while it's active? If so, this pretty much behaves as expected, Yuuji; objects may be released as soon as nothing refers to them anymore. The fact that it works without a do-catch block is mostly a coincidence.

[swift-ci]

Comment by Pierre Habouzit (JIRA)

This is a behaves correctly, releasing the last reference on a source is an implicit cancelation. The best way to have it not happen is to indeed have a reference to the source from one of the handler.

calling `cancel` on the source will cause the handler to be destroyed and the retain cycle to be broken.

the C interface works exactly the same way.