[teamhimeh]

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.

For example:

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

With this change, cancelling its generated Java future also cancels the Swift task, so the suspension observes cancellation:

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

Swift cancellation remains cooperative. This does not interrupt an operating-system thread or wait for the task to finish.

Changes

• Add a SwiftCompletableFuture runtime implementation that owns a retained Swift task handle and cancels/releases it safely on completion or cancellation.
• Bind each generated JNI async task to its native future, including tasks that complete before the handle is installed.
• Convert results through a cancellation-forwarding mapping stage, preserving success, failure, and conversion exceptions while keeping the generated public return type as CompletableFuture.
• Update JNI signatures and generator expectations for the concrete native future type. Keep legacyFuture behavior unchanged.
• Document cooperative cancellation and the boundary between generated result conversion and future stages added by callers.
• Add generator regressions, Java future tests, and JNI runtime tests that observe Swift cancellation for scalar and Optional results and exercise immediate task completion.

Validation

• Confirmed the new generator regressions fail before the fix; the legacy-mode regression passes before and after.
• swift test --disable-experimental-prebuilts --disable-sandbox: full Swift test suite passed.
• ./gradlew :SwiftKitCore:test :Samples:SwiftJavaExtractJNISampleApp:test --no-daemon: 22 runtime-library tests and 289 JNI sample tests passed, with no failures or skips.
• swift-format lint --strict on changed Swift files and git diff --check passed.

Tested with Apple Swift 6.4 and JetBrains Runtime 25 on macOS.