The recent changes related to adding support for working beatmap load
cancellation exposed a flaw in the beatmap difficulty cache. With the
way the difficulty computation logic was written, any error in the
calculation process (including beatmap load timeout, or cancellation)
would result in a 0.00 star rating being permanently cached in memory
for the given beatmap.
To resolve, change the difficulty cache's return type to nullable.
In failure scenarios, `null` is returned, rather than
`default(StarDifficulty)` as done previously.
`DifficultyCacheLookup`s were storing raw `Mod` instances into their
`OrderedMods` field. This could cause the cache lookups to wrongly
succeed in cases of mods with settings. The particular case that
triggered this fix was Difficulty Adjust.
Because the difficulty cache is backed by a dictionary, there are two
stages to the lookup; first `GetHashCode()` is used to find the
appropriate hash bucket to look in, and then items from that hash bucket
are compared against the key being searched for via the implementation
of `Equals()`.
As it turns out, the first hashing step ended up being the saving grace
in most cases, as the hash computation included the values of the mod
settings. But the Difficulty Adjust failure case was triggered by the
quirk that `GetHashCode(0) == GetHashCode(null) == 0`.
In such a case, the `Equals()` fallback was used. But as it turns out,
because the `Mod` instance stored to lookups was not cloned and
therefore potentially externally mutable, it could be polluted after
being stored to the dictionary, and therefore breaking the equality
check. Even though all of the setting values were compared, the hash
bucket didn't match the actual contents of the lookup anymore (because
they were mutated externally, e.g. by the user changing the mod setting
values in the mod settings overlay).
To resolve, clone out the mod structure before creating all difficulty
lookups.
The previous code would run a calcaulation for the beatmap's own ruleset
if the current one failed. While this does make sense, with the current
way we use this component (and the implementation flow) it is quite unsafe.
The to the call on `.Result` in the `catch` block, this would 100%
deadlock due to the thread concurrency of the `ThreadedTaskScheduler`
being 1. Even if the nested run could be run inline (it should be), the
task scheduler won't even get to the point of checking whether this is
feasible due to it being saturated by the already running task.
I'm not sure if we still need this fallback lookup logic. After removing
it, it's feasible that 0 stars will be returned during the scenario that
previously caused a deadlock, but I don't necessarily think this is
incorrect. There may be another reason for this needing to exist which
I'm not aware of (diffcalc?) but if that's the case we may want to move
the try-catch handling to the point of usage.
To reproduce the deadlock scenario with 100% success (the repro
instructions in the linked issue aren't that simple and require some
patience and good timing), the main portion of the lookup can be changed
to randomly trigger a nested lookup:
```
if (RNG.NextSingle() > 0.5f)
return GetAsync(new
DifficultyCacheLookup(key.Beatmap, key.Beatmap.Ruleset,
key.OrderedMods)).Result;
else
return new StarDifficulty(attributes);
```
After switching beatmap once or twice, pausing debug and viewing the
state of threads should show exactly what is going on.
