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stake-pool: Wait at least two epoch boundaries to set fee #3979
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2501babe
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Jan 26, 2023
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| pub fn update_epoch(&mut self) { | ||
| match self { | ||
| Self::None => {} | ||
| Self::One(_) => { | ||
| // The value has waited its last epoch | ||
| *self = Self::None; | ||
| } | ||
| // The value still has to wait one more epoch after this | ||
| Self::Two(v) => { | ||
| *self = Self::One(v.clone()); | ||
| } | ||
| } | ||
| } | ||
|
|
||
| /// Get the value if it's ready, which is only at `One` epoch remaining | ||
| pub fn get(&self) -> Option<&T> { | ||
| match self { | ||
| Self::None | Self::Two(_) => None, | ||
| Self::One(v) => Some(v), | ||
| } | ||
| } |
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I certainly had fun writing it 😄
2501babe
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Jan 26, 2023
HaoranYi
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Problem
If a malicious manager wants to hike a fee, they can still do so right at the end of an epoch and have it take into account immediately at the start of the next epoch, giving users no time to get their funds out if they want.
Solution
Force a wait of at least two epoch boundaries before applying a new fee. This solution needs to be backwards compatible with existing stake pools, which use
Option<Fee>. In Borsh, this is encoded as:This introduces a new enum
FutureEpoch, which encodes0asNone(same as option),1as "wait one epoch" (same as option), and then2as "wait two epochs" (the new state).The biggest issue with this approach is that it requires calling "update" in two different epochs before applying the fee, rather than having the fee apply at a particular future epoch. Meaning, you can do "set fee", wait two epochs, then call "update", and you'll only move forward one epoch. You must call "set fee", wait at least one epoch boundary, call "update", wait at least one more epoch boundary, and call "update" again. Thankfully, at worst, it just takes longer for the new fee to kick in, which is acceptable (to me).