Mono.toFuture(): invocation of cancel after completion.

[anuchandy]

We have a code like below which does the following:

1. Create a local file
2. Download contents to this file
3. If the subscription is canceled or there is an error we remove the possibly partially written file

final String fileName = "myLocalFile.txt";
Mono<String> fileDownloadMono = Mono.using(() -> new PrintWriter(fileName, "UTF-8"),
        new Function<PrintWriter, Mono<String>>() {
            @Override
            public Mono<String> apply(PrintWriter printWriter) {
                // Download from network.
                System.out.println("Downloading from network");
                String downloadedContent = "hello";
                // write to local file
                System.out.println("Writing to local file");
                printWriter.write(downloadedContent);
                return Mono.just(fileName);
            }
        },
        printWriter -> {
            printWriter.close();
        })
        .doFinally(signalType -> {
            if (signalType == SignalType.ON_ERROR
                || signalType == SignalType.CANCEL) {
                System.out.println("Unable to complete download/write to file, deleting unfinished file.");
                (new File(fileName)).delete();
            }
        });

This works fine. The issue we're facing is when we convert this Mono to a Future like below:

fileDownloadMono.toFuture();

toFuture() operator cancels the Mono as soon as the Mono completes, which result in the removal of successfully completed file.

Code in toFuture() operator that cancel the Mono is here.

[simonbasle]

Yeah it doesn't sound like this is needed at all for a Mono publisher. Note that it would be necessary if the MonoToCompletableFuture (which is a Subscriber) was subscribed to eg. a Flux...

What I can do to guide any potential future use of this subscriber with anything else than a Mono is to add a constructor parameter that drives whether or not to cancel in onNext. (and of course Mono.toFuture() would prevent such cancellation).

[anuchandy]

thank you @simonbasle!