types: remove extra validation in VerifyCommit

plus make sure LastCommit is always non-nil

state/validation.go

@@ -86,12 +86,9 @@ func validateBlock(evidencePool EvidencePool, stateDB dbm.DB, state State, block
 			return errors.New("block at height 1 can't have LastCommit signatures")
 		}
 	} else {
-		if len(block.LastCommit.Signatures) != state.LastValidators.Size() {
-			return types.NewErrInvalidCommitSignatures(state.LastValidators.Size(), len(block.LastCommit.Signatures))
-		}
-		err := state.LastValidators.VerifyCommit(
-			state.ChainID, state.LastBlockID, block.Height-1, block.LastCommit)
-		if err != nil {
+		// LastCommit.Signatures length is checked in VerifyCommit.
+		if err := state.LastValidators.VerifyCommit(
+			state.ChainID, state.LastBlockID, block.Height-1, block.LastCommit); err != nil {
 			return err
 		}
 	}

types/block.go

@@ -60,10 +60,10 @@ func (b *Block) ValidateBasic() error {
 	}
 
 	// Validate the last commit and its hash.
+	if b.LastCommit == nil {
+		return errors.New("nil LastCommit")
+	}
 	if b.Header.Height > 1 {
-		if b.LastCommit == nil {
-			return errors.New("nil LastCommit")
-		}
 		if err := b.LastCommit.ValidateBasic(); err != nil {
 			return fmt.Errorf("wrong LastCommit: %v", err)
 		}

types/validator_set.go

@@ -642,8 +642,13 @@ func (vals *ValidatorSet) VerifyCommit(chainID string, blockID BlockID,
 		return NewErrInvalidCommitSignatures(vals.Size(), len(commit.Signatures))
 	}
 
-	if err := commit.ValidateBasic(); err != nil {
-		return err
+	// Validate Height and BlockID.
+	if height != commit.Height {
+		return NewErrInvalidCommitHeight(height, commit.Height)
+	}
+	if !blockID.Equals(commit.BlockID) {
+		return fmt.Errorf("invalid commit -- wrong block ID: want %v, got %v",
+			blockID, commit.BlockID)
 	}
 	if height != commit.Height {
 		return NewErrInvalidCommitHeight(height, commit.Height)
@@ -670,12 +675,12 @@ func (vals *ValidatorSet) VerifyCommit(chainID string, blockID BlockID,
 			return fmt.Errorf("wrong signature (#%d): %X", idx, commitSig.Signature)
 		}
 		// Good!
-		if blockID.Equals(commitSig.BlockID(commit.BlockID)) {
+		if commitSig.ForBlock() {
 			talliedVotingPower += val.VotingPower
 		}
 		// else {
-		// It's OK that the BlockID doesn't match.  We include stray
-		// signatures (~votes for nil) to measure validator availability.
+		// It's OK. We include stray signatures (~votes for nil) to measure
+		// validator availability.
 		// }
 
 		// return as soon as +2/3 of the signatures are verified