types: remove VerifyFutureCommit (#4961)

It was previously used by the now removed `lite` package.

types/validator_set.go

@@ -714,82 +714,6 @@ func (vals *ValidatorSet) VerifyCommit(chainID string, blockID BlockID,
 	return ErrNotEnoughVotingPowerSigned{Got: talliedVotingPower, Needed: votingPowerNeeded}
 }
 
-// VerifyFutureCommit will check to see if the set would be valid with a different
-// validator set.
-//
-// vals is the old validator set that we know.  Over 2/3 of the power in old
-// signed this block.
-//
-// In Tendermint, 1/3 of the voting power can halt or fork the chain, but 1/3
-// can't make arbitrary state transitions.  You still need > 2/3 Byzantine to
-// make arbitrary state transitions.
-//
-// To preserve this property in the light client, we also require > 2/3 of the
-// old vals to sign the future commit at H, that way we preserve the property
-// that if they weren't being truthful about the validator set at H (block hash
-// -> vals hash) or about the app state (block hash -> app hash) we can slash
-// > 2/3.  Otherwise, the light client isn't providing the same security
-// guarantees.
-//
-// Even if we added a slashing condition that if you sign a block header with
-// the wrong validator set, then we would only need > 1/3 of signatures from
-// the old vals on the new commit, it wouldn't be sufficient because the new
-// vals can be arbitrary and commit some arbitrary app hash.
-//
-// newSet is the validator set that signed this block.  Only votes from new are
-// sufficient for 2/3 majority in the new set as well, for it to be a valid
-// commit.
-//
-// NOTE: This doesn't check whether the commit is a future commit, because the
-// current height isn't part of the ValidatorSet.  Caller must check that the
-// commit height is greater than the height for this validator set.
-func (vals *ValidatorSet) VerifyFutureCommit(newSet *ValidatorSet, chainID string,
-	blockID BlockID, height int64, commit *Commit) error {
-	oldVals := vals
-
-	// Commit must be a valid commit for newSet.
-	err := newSet.VerifyCommit(chainID, blockID, height, commit)
-	if err != nil {
-		return err
-	}
-
-	// Check old voting power.
-	oldVotingPower := int64(0)
-	seen := map[int32]bool{}
-
-	for idx, commitSig := range commit.Signatures {
-		if commitSig.Absent() {
-			continue // OK, some signatures can be absent.
-		}
-
-		// See if this validator is in oldVals.
-		oldIdx, val := oldVals.GetByAddress(commitSig.ValidatorAddress)
-		if val == nil || seen[oldIdx] {
-			continue // missing or double vote...
-		}
-		seen[oldIdx] = true
-
-		// Validate signature.
-		voteSignBytes := commit.VoteSignBytes(chainID, int32(idx))
-		if !val.PubKey.VerifyBytes(voteSignBytes, commitSig.Signature) {
-			return fmt.Errorf("wrong signature (#%d): %X", idx, commitSig.Signature)
-		}
-		// Good!
-		if blockID.Equals(commitSig.BlockID(commit.BlockID)) {
-			oldVotingPower += val.VotingPower
-		}
-		// else {
-		// It's OK that the BlockID doesn't match.  We include stray
-		// signatures (~votes for nil) to measure validator availability.
-		// }
-	}
-
-	if got, needed := oldVotingPower, oldVals.TotalVotingPower()*2/3; got <= needed {
-		return ErrNotEnoughVotingPowerSigned{Got: got, Needed: needed}
-	}
-	return nil
-}
-
 // VerifyCommitTrusting verifies that trustLevel of the validator set signed
 // this commit.
 //