`RequestContextAwareCompletableFuture` callback not invoked when context mismatches.

Let’s say we have the following code:

ServiceRequestContext ctx1 = ServiceRequestContext.of(...);
ServiceRequestContext ctx2 = ServiceRequestContext.of(...);
try (SafeCloseable sc1 = ctx1.push()) {
    CompletableFuture f = ctx2.makeContextAware(new CompletableFuture());
    AtomicBoolean callbackCalled = new AtomicBoolean();
    f.whenComplete((res, cause) -> callbackCalled.set(true));

    f.complete(null);    

    // The callback will never be invoked and
    // the following assertion will fail.
    assert callbackCalled.get(); // <-- Failure!
}

It seems like everything is working as expected. The callback (BiConsumer) was not invoked because it was decorated by ctx2.makeContextAware(BiConsumer) and the callback is invoked only when ctx2.pushIfAbsent() succeeds (see), to protect our users from the accidental context conflict like the above example.

The problem is that a user does not have a way to know this happened. It is really confusing because the callback will never be invoked. It is neither possible to handle the IllegalStateException raised by pushIfAbsent():

f.whenComplete((res, cause) -> callbackCalled.set(true))
 .exceptionally(cause -> {
     // We never reach here, either, because this callback
     // will be decorated again and fail at `pushIfAbsent()` again.
     ...
 });

What would be the best way to let a user know about this problem?

[trustin]

By the way, once we introduce RequestContext.parent() we might not need to copy the attributes. When not found, could just check the parent.

[minwoox]

How about logging at WARN level instead of raising an exception for the future callbacks? If a user wraps something explicitly via ctx.makeContextAware(), we could raise an IllegalStateException as before, though, i.e.

Had a chat with @trustin and decided to leave warning logs multiple times to let the users know something is going wrong. We will still throw an exception because otherwise some critical resources can be shared such as access token, etc.