Introduce CommitSig alias for Vote in Commit (#3245)

* types: memoize height/round in commit instead of first vote

* types: commit.ValidateBasic in VerifyCommit

* types: new CommitSig alias for Vote

In preparation for reducing the redundancy in Commits, we introduce the
CommitSig as an alias for Vote. This is non-breaking on the protocol,
and minor breaking on the Go API, as Commit now contains a list of
CommitSig instead of Vote.

* remove dependence on ToVote

* update some comments

* fix tests

* fix tests

* fixes from review

types/block.go

@@ -477,39 +477,77 @@ func (h *Header) StringIndented(indent string) string {
 
 //-------------------------------------
 
+// CommitSig is a vote included in a Commit.
+// For now, it is identical to a vote,
+// but in the future it will contain fewer fields
+// to eliminate the redundancy in commits.
+// See https://github.com/tendermint/tendermint/issues/1648.
+type CommitSig Vote
+
+// String returns the underlying Vote.String()
+func (cs *CommitSig) String() string {
+	return cs.toVote().String()
+}
+
+// toVote converts the CommitSig to a vote.
+// Once CommitSig has fewer fields than vote,
+// converting to a Vote will require more information.
+func (cs *CommitSig) toVote() *Vote {
+	if cs == nil {
+		return nil
+	}
+	v := Vote(*cs)
+	return &v
+}
+
 // Commit contains the evidence that a block was committed by a set of validators.
 // NOTE: Commit is empty for height 1, but never nil.
 type Commit struct {
 	// NOTE: The Precommits are in order of address to preserve the bonded ValidatorSet order.
 	// Any peer with a block can gossip precommits by index with a peer without recalculating the
 	// active ValidatorSet.
-	BlockID    BlockID `json:"block_id"`
-	Precommits []*Vote `json:"precommits"`
+	BlockID    BlockID      `json:"block_id"`
+	Precommits []*CommitSig `json:"precommits"`
 
 	// Volatile
-	firstPrecommit *Vote
-	hash           cmn.HexBytes
-	bitArray       *cmn.BitArray
+	height   int64
+	round    int
+	hash     cmn.HexBytes
+	bitArray *cmn.BitArray
 }
 
-// FirstPrecommit returns the first non-nil precommit in the commit.
-// If all precommits are nil, it returns an empty precommit with height 0.
-func (commit *Commit) FirstPrecommit() *Vote {
+// VoteSignBytes constructs the SignBytes for the given CommitSig.
+// The only unique part of the SignBytes is the Timestamp - all other fields
+// signed over are otherwise the same for all validators.
+func (commit *Commit) VoteSignBytes(chainID string, cs *CommitSig) []byte {
+	return cs.toVote().SignBytes(chainID)
+}
+
+// memoizeHeightRound memoizes the height and round of the commit using
+// the first non-nil vote.
+func (commit *Commit) memoizeHeightRound() {
 	if len(commit.Precommits) == 0 {
-		return nil
+		return
 	}
-	if commit.firstPrecommit != nil {
-		return commit.firstPrecommit
+	if commit.height > 0 {
+		return
 	}
 	for _, precommit := range commit.Precommits {
 		if precommit != nil {
-			commit.firstPrecommit = precommit
-			return precommit
+			commit.height = precommit.Height
+			commit.round = precommit.Round
+			return
 		}
 	}
-	return &Vote{
-		Type: PrecommitType,
-	}
+}
+
+// ToVote converts a CommitSig to a Vote.
+// If the CommitSig is nil, the Vote will be nil.
+// When CommitSig is reduced to contain fewer fields,
+// this will need access to the ValidatorSet to properly
+// reconstruct the vote.
+func (commit *Commit) ToVote(cs *CommitSig) *Vote {
+	return cs.toVote()
 }
 
 // Height returns the height of the commit
@@ -517,7 +555,8 @@ func (commit *Commit) Height() int64 {
 	if len(commit.Precommits) == 0 {
 		return 0
 	}
-	return commit.FirstPrecommit().Height
+	commit.memoizeHeightRound()
+	return commit.height
 }
 
 // Round returns the round of the commit
@@ -525,7 +564,8 @@ func (commit *Commit) Round() int {
 	if len(commit.Precommits) == 0 {
 		return 0
 	}
-	return commit.FirstPrecommit().Round
+	commit.memoizeHeightRound()
+	return commit.round
 }
 
 // Type returns the vote type of the commit, which is always VoteTypePrecommit
@@ -554,12 +594,13 @@ func (commit *Commit) BitArray() *cmn.BitArray {
 	return commit.bitArray
 }
 
-// GetByIndex returns the vote corresponding to a given validator index
+// GetByIndex returns the vote corresponding to a given validator index.
+// Implements VoteSetReader.
 func (commit *Commit) GetByIndex(index int) *Vote {
-	return commit.Precommits[index]
+	return commit.Precommits[index].toVote()
 }
 
-// IsCommit returns true if there is at least one vote
+// IsCommit returns true if there is at least one vote.
 func (commit *Commit) IsCommit() bool {
 	return len(commit.Precommits) != 0
 }