Not detected call to `Files.readString(path)`

[pmendelski]

Expected Behavior

public static void main(String[] args) {
    BlockHound.install();
    Mono.fromCallable(() -> {
        try {
            return Files.readString(testFilePath);
        } catch (IOException e) {
            throw new RuntimeException("Could not read file: " + testFilePath, e);
        }
    })
            .subscribeOn(Schedulers.parallel())
            .block();
    // Expected blocking call error
}

Actual Behavior

BlockHound does not detect call Files.readString(path).

Steps to Reproduce

I created small project to test this issue.

Your Environment

• Reactor version(s) used: 3.4.5
• JVM version: OpenJDK 64-Bit Server VM AdoptOpenJDK (build 11.0.7+10, mixed mode)
• OS and version: Linux 5.8.0-49-generic 20.04.1-Ubuntu x86_64

[bsideup]

Hey @pmendelski,

Mono.fromCallable implements a special optimization that makes it execute on the same thread when block() is used.
Hint: try adding System.out.println(Thread.currentThread()) to your fromCallable's lambda.

You can verify it by changing block() to toFuture().get() that doesn't use the optimization.

/cc @simonbasle

[pmendelski]

Thanks for such a fast answer.

I added logs and tried toFuture().get() but it seems to not solve the problem.
In logs I can see that the code is executed on the parallel scheduler and BlockHound did not detect any of the calls.

public static void main(String[] args) throws ExecutionException, InterruptedException {
    BlockHound.install();

    Mono.fromCallable(() -> {
        System.out.println("block() - " + Thread.currentThread().getName());
        return readFile(testFilePath);
    })
            .subscribeOn(Schedulers.parallel())
            .block();

    Mono.fromCallable(() -> {
        System.out.println("toFuture() - " + Thread.currentThread().getName());
        return readFile(testFilePath);
    })
            .subscribeOn(Schedulers.parallel())
            .toFuture()
            .get();

    Mono.delay(Duration.ofMillis(10))
            .doOnNext(it -> {
                System.out.println("doOnNext() - " + Thread.currentThread().getName());
                readFile(testFilePath);
            })
            .block();
}

static String readFile(Path path) {
    try {
        return Files.readString(path);
    } catch (IOException e) {
        throw new RuntimeException("Could not read file: " + path, e);
    }
}

// Logs:
// block() - parallel-1
// toFuture() - parallel-2
// doOnNext() - parallel-3