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`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. |
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