Specifically, `Cancel()` calls were not thread safe. Due to a series of
events, `ListPollingComponent` could call `Cancel` from a non-update
thread, leading to a race condition where both a `Success` and `Fail`
event can be fired.
This is intended to be the simplest fix possible, locking and guarding
specifically on the callbacks. Further work could be done in the future
to improve the flow surrounding `pendingFailure`, potentially reducing
redundant work and cleaning up the code, but that's not happening here.
Closes https://github.com/ppy/osu/issues/13632.
Until now, API requests sent to dummy API were just lost in the void. In most cases this somehow worked as expected, but any logic which is waiting on a request to finish will potentially never get a response.
Going forward, I'm not 100% sure that every `Wait` on a web response will have local timeout logic (I think there is a certain amount of assumption that this is being managed for us by `APIAccess`), so I've made this change to better handle such cases going forward. Now, rather than nothing happening, requests will trigger a failure via the existing exception logic rather than silently pretending the request never arrived.