cc @Daenyth

LGTM, but a test would be really good for this.

We discussed how to write a test on Discord.
https://discord.com/channels/632277896739946517/632278585700384799/956566207211905115

The tl;dr is create an uncancelable "never" CompletableFuture that will not complete, and always returns false when attempts to be canceled. The correct implementation should get backpressured on cancellation, whereas the broken one should terminate.

There was some discussion on Discord about how to test this - I can't promise to do that in the next couple of days, so feel free to fork this PR with a test.

Edit: what @armanbilge said

@armanbilge Can you please handle this? Thanks.

Yup I'm working on #2902, I can do this next.

Closing in favor of #2907.