Context lost when using await but works with subscribe

[jdussouillez]

Description

I have a method using an Uni and returning a Multi. But the twist is: I need some data when the Multi is completed, and this data is accessible in the Uni, before I create the Multi. So I'm using a context to store this data and get it back later.

private Multi<String> fetch() {
    return step1()
        .call(step1Result -> step2()
            // Store "step2" result in the context
            .withContext((s2, ctx) -> s2.invoke(step2Result -> ctx.put("s2", step2Result)))
        )
        .chain(step1Result -> step3())
        .onItem()
        .transformToMulti(step3Result -> step4(step3Result))
        .withContext((multi, ctx) -> multi
            .onCompletion()
            .call(() -> {
                // I need "step2" result here to do some stuff. When using `subscribe` I have "s2" value in context. When using `await` I don't have it
                return Uni.createFrom().voidItem();
            })
        );
}

Full code here

Actual behavior

When I'm using a subscribe then everything works fine:

var results = fetch()
    .subscribe()
    .asStream()
    .toList();
// "s2" value was in the context when the Multi completed: OK

var results = fetch()
    .collect()
    .asList()
    .subscribe()
    .asCompletionStage()
    .join();
// "s2" value was in the context when the Multi completed: OK

But when I'm using await then my context is empty!

var results = fetch()
    .collect()
    .asList()
    .await()
    .indefinitely();
// "s2" value wasn't in the context when the Multi completed: Failure

Expected behavior

I guess the context propagation should work the same way, whether we use subscribe or await, right?

Analysis

From the Uni::subscribe Javadoc:

Unlike await(), this method configures non-blocking retrieval of the item and failure.

But I don't get why this would have an impact on the context when my multi completes.

Reproducer

https://github.com/jdussouillez/mutiny-context-from-uni-to-multi/tree/master

[jponge]

You should use awaitUsing(context) to pass an initial context, even Context.empty() will be fine.

What you are observing might be a side effect of what happens when operators check for subscribers context. I'll have a look to see if this is a bug or not 😉

[jponge]

There's a minor bug here, will be tracked in #1457

[jdussouillez]

Oh OK thanks for the bug report!

I confirm the workaround works: awaitUsing with an empty context works like a charm.

I updated the reproducer project: jdussouillez/mutiny-context-from-uni-to-multi@eabedf4