Merge pull request swiftlang#1592 from joshavant/add-URLSessionDownloadTask.cancelbyProducingResumeData

Add URLSessionDownloadTask.cancel(byProducingResumeData:)

Author: millenomi

Docs/Status.md

@@ -81,7 +81,7 @@ There is no _Complete_ status for test coverage because there are always additio
     | `URLSessionTask`             | Mostly Complete | Incomplete    | `cancel()`, `createTransferState(url:)` with streams, and others remain unimplemented                              |
     | `URLSessionDataTask`         | Complete        | Incomplete    |                                                                                                                    |
     | `URLSessionUploadTask`       | Complete        | None          |                                                                                                                    |
-    | `URLSessionDownloadTask`     | Incomplete      | Incomplete    | `cancel(byProducingResumeData:)` remains unimplemented                                                             |
+    | `URLSessionDownloadTask`     | Incomplete      | Incomplete    |                                                                                                                  |
     | `URLSessionStreamTask`       | Unimplemented   | None          |                                                                                                                    |
     | `TaskRegistry`               | N/A             | N/A           | For internal use only                                                                                              |
     | `TransferState`              | N/A             | N/A           | For internal use only                                                                                              |

Foundation/URLSession/URLSessionTask.swift

@@ -428,7 +428,18 @@ open class URLSessionDownloadTask : URLSessionTask {
      * If resume data cannot be created, the completion handler will be
      * called with nil resumeData.
      */
-    open func cancel(byProducingResumeData completionHandler: @escaping (Data?) -> Void) { NSUnimplemented() }
+    open func cancel(byProducingResumeData completionHandler: @escaping (Data?) -> Void) {
+        super.cancel()
+        
+        /*
+         * In Objective-C, this method relies on an Apple-maintained XPC process
+         * to manage the bookmarking of partially downloaded data. Therefore, the
+         * original behavior cannot be directly ported, here.
+         *
+         * Instead, we just call the completionHandler directly.
+         */
+        completionHandler(nil)
+    }
 }
 
 /*