parlia urgent: BEP-131 bug. Cause new validators on blockNumber%Epoch = 11 are slashed often.

[Jolly23]

Please review and merge this PR ASAP. It continually makes mistake for all validators.

Description

Inside parlia snapshot, BEP-131 validator set is updated at number%s.config.Epoch == uint64(len(snap.Validators)/2).

Simplify it is blockNumber%200 = 10

parlia.SignRecently() is the first function be called when miner.worker getting NewChainHead signal, which controls the starting of the packing tasks.

            case head := <-w.chainHeadCh:
			if !w.isRunning() {
				continue
			}
			clearPending(head.Block.NumberU64())
			timestamp = time.Now().Unix()
			if p, ok := w.engine.(*parlia.Parlia); ok {
				signedRecent, err := p.SignRecently(w.chain, head.Block.Header())
				if err != nil {
					log.Info("Not allowed to propose block", "err", err)
					continue
				}
				if signedRecent {
					log.Info("Signed recently, must wait")
					continue
				}
			}
			commit(true, commitInterruptNewHead)

But there is bug inside the parlia.SignRecently() causes the newValidatorSet is not updated on blockNumber%200 = 10

func (p *Parlia) SignRecently(chain consensus.ChainReader, parent *types.Header) (bool, error) {
	snap, err := p.snapshot(chain, parent.Number.Uint64(), parent.ParentHash, nil)

Obviously we cannot find a block by mismatched blockNumber and blockHash. In the line above they are parent.Number.Uint64() and parent.ParentHash.

However the parent.ParentHash hits a cache of parlia.Snapshot. The cached snapshot cannot snap.apply the latest chain head.

Thus the very import part ChangeValidatorSet doesn't run on a RecentSigned validator and new imported validator of current Epoch.

Rationale

BNB48 Club reported some related cases.

Example

recent slash list, blockNumber%Epoch = 11 occur frequently

The slashed validator

This was a new imported validator on Epoch of 22243800, it was a candidate in last Epoch. He was in-turn at 22243811.

But he didn't loaded the newValidatorSet at 22243810 so he missed 22243811.

Changes

func (p *Parlia) SignRecently(chain consensus.ChainReader, parent *types.Block) (bool, error) {
	snap, err := p.snapshot(chain, parent.NumberU64(), parent.Hash(), nil)

[brilliant-lx]

Nice catch, thank you for your contribution.

[Jolly23]

related issue #1133

The base branch was changed.