jextract/jni: Propagate CompletableFuture cancellation to Swift tasks #947

Problem

Cancelling a future returned by a JNI async call currently changes only the Java future's state. The underlying Swift task keeps running. The generated thenApply result-conversion stage also prevents cancellation of the returned future from reaching the source future.

Example

public func waitForWork() async throws -> Int64 {
  try await Task.sleep(for: .seconds(60))
  return 1
}

A Java caller can cancel the returned future:

var future = MySwiftLibrary.waitForWork();
future.cancel(false);

The Java future is cancelled, but the Swift task continues running. Cancellation of the returned future should also be forwarded to the Swift task, so cancellation-aware operations such as Task.sleep can observe it.

Swift cancellation is cooperative: forwarding cancellation does not interrupt an operating-system thread or wait for the task to finish.

Reference workaround implementation: qoncept#10