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Liveness Tracking

At the beginning of each block, we update the ValidatorSigningInfo for each validator and check if they've crossed below the liveness threshold over a sliding window. This sliding window is defined by SignedBlocksWindow and the index in this window is determined by IndexOffset found in the validator's ValidatorSigningInfo. For each block processed, the IndexOffset is incremented regardless if the validator signed or not. Once the index is determined, the MissedBlocksBitArray and MissedBlocksCounter are updated accordingly.

Finally, in order to determine if a validator crosses below the liveness threshold, we fetch the maximum number of blocks missed, maxMissed, which is SignedBlocksWindow - (MinSignedPerWindow * SignedBlocksWindow) and the minimum height at which we can determine liveness, minHeight. If the current block is greater than minHeight and the validator's MissedBlocksCounter is greater than maxMissed, they will be slashed by SlashFractionDowntime, will be jailed for DowntimeJailDuration, and have the following values reset: MissedBlocksBitArray, MissedBlocksCounter, and IndexOffset.

Note: Liveness slashes do NOT lead to a tombstombing.

height := block.Height

for vote in block.LastCommitInfo.Votes {
signInfo := GetValidatorSigningInfo(vote.Validator.Address)

// This is a relative index, so we counts blocks the validator SHOULD have
// signed. We use the 0-value default signing info if not present, except for
// start height.
index := signInfo.IndexOffset % SignedBlocksWindow()

// Update MissedBlocksBitArray and MissedBlocksCounter. The MissedBlocksCounter
// just tracks the sum of MissedBlocksBitArray. That way we avoid needing to
// read/write the whole array each time.
missedPrevious := GetValidatorMissedBlockBitArray(vote.Validator.Address, index)
missed := !signed

switch {
case !missedPrevious && missed:
// array index has changed from not missed to missed, increment counter
SetValidatorMissedBlockBitArray(vote.Validator.Address, index, true)

case missedPrevious && !missed:
// array index has changed from missed to not missed, decrement counter
SetValidatorMissedBlockBitArray(vote.Validator.Address, index, false)

// array index at this index has not changed; no need to update counter

if missed {
// emit events...

minHeight := signInfo.StartHeight + SignedBlocksWindow()
maxMissed := SignedBlocksWindow() - MinSignedPerWindow()

// If we are past the minimum height and the validator has missed too many
// jail and slash them.
if height > minHeight && signInfo.MissedBlocksCounter > maxMissed {
validator := ValidatorByConsAddr(vote.Validator.Address)

// emit events...

// We need to retrieve the stake distribution which signed the block, so we
// subtract ValidatorUpdateDelay from the block height, and subtract an
// additional 1 since this is the LastCommit.
// Note, that this CAN result in a negative "distributionHeight" up to
// -ValidatorUpdateDelay-1, i.e. at the end of the pre-genesis block (none) = at the beginning of the genesis block.
// That's fine since this is just used to filter unbonding delegations & redelegations.
distributionHeight := height - sdk.ValidatorUpdateDelay - 1

Slash(vote.Validator.Address, distributionHeight, vote.Validator.Power, SlashFractionDowntime())

signInfo.JailedUntil = block.Time.Add(DowntimeJailDuration())

// We need to reset the counter & array so that the validator won't be
// immediately slashed for downtime upon rebonding.
signInfo.MissedBlocksCounter = 0
signInfo.IndexOffset = 0

SetValidatorSigningInfo(vote.Validator.Address, signInfo)