Add URLSessionDownloadTask.cancel(byProducingResumeData:)

[joshavant]

This is an attempt at resolving the implementation for URLSessionDownloadTask.cancel(byProducingResumeData:).

This function seems to represent an edge case in portability. After communicating with @phausler, it appears the Objective-C implementation of this relies on an Apple-maintained XPC process to manage the bookmarking of partially downloaded data. Of course, this behavior is not portable to the framework project.

One approach that was considered was serializing the URLSessionDownloadTask instance and returning that to the caller as the opaque blob given to the completion handler. However, the Objective-C implementation of URLSessionTask does not implement any NSCoding flavor publicly, so conformance would need to be implemented in this framework with internal or greater access control. But, unfortunately, that functionality is not currently supported in Swift.

The approach suggested by @phausler was simply to... hardcode calling the completion block. This is what was done in this PR.

The only thing worth highlighting about the approach is that a token Data instance is passed to the completion block to prevent unexpected failures that may be caused from passing nil instead.

Open Issues:

• downloadTask(withResumeData:) (and related) would remain broken by this approach. What should be done with these functions?
• Unit test(s)

[pushkarnk]

@swift-ci please test

[millenomi]

This would be prudent if data was usually expected, but the contract of this method goes:

If the download is resumable, the completion handler is provided with a resumeData object. Your app can later pass this object to a session’s downloadTask(withResumeData:) or downloadTask(withResumeData:completionHandler:) method to create a new task that resumes the download where it left off.

Since we do not yet support resuming on Linux, no downloads are resumable, and so we can safely pass nil here. Let's not confuse downstream clients into thinking the download is resumable when we can't resume it.

[joshavant]

Great point. PR updated, accordingly.

[millenomi]

I think I'll request a change. This breaks the contract.

[pushkarnk]

@swift-ci please test

[millenomi]

@swift-ci please test

[joshavant]

🎉

What's the process to merge this in, now that everything is passing?

[millenomi]

Me hitting a button.