tortoise: reuse decoded votes (#5009)

related: #3006

it also reduced 300MB of used memory on my machine.

ballot signs opinion hash, and provides sidecar to decode that opinion hash. if node decoded one sidecar it can store it in memory
and reuse for all following ballots with the same opinion hash.

tortoise/state.go

@@ -94,7 +94,7 @@ func (s *state) layer(lid types.LayerID) *layerInfo {
 	layer, exist := s.layers[lid]
 	if !exist {
 		layersNumber.Inc()
-		layer = &layerInfo{lid: lid}
+		layer = &layerInfo{lid: lid, opinions: map[types.Hash32]votes{}}
 		s.layers[lid] = layer
 	}
 	return layer
@@ -179,6 +179,10 @@ type layerInfo struct {
 	verifying      verifyingInfo
 	coinflip       sign
 
+	// unique opinions recorded from the ballots in this layer.
+	// ballot votes an opinion and encodes sidecar
+	opinions map[types.Hash32]votes
+
 	opinion types.Hash32
 	// a pointer to the value stored on the previous layerInfo object
 	// it is stored as a pointer so that when previous layerInfo is evicted

tortoise/tortoise.go

@@ -60,6 +60,7 @@ func newTurtle(logger *zap.Logger, config Config) *turtle {
 	t.layers[genesis] = &layerInfo{
 		lid:            genesis,
 		hareTerminated: true,
+		opinions:       map[types.Hash32]votes{},
 	}
 	t.verifying = newVerifying(config, t.state)
 	t.full = newFullTortoise(config, t.state)
@@ -748,11 +749,23 @@ func (t *turtle) decodeBallot(ballot *types.BallotTortoiseData) (*ballotInfo, ty
 		zap.Uint32("lid", ballot.Layer.Uint32()),
 	)
 
-	votes, min, err := decodeVotes(t.evicted, binfo.layer, base, ballot.Opinion.Votes)
-	if err != nil {
-		return nil, 0, err
+	layer := t.layer(binfo.layer)
+
+	existing, exists := layer.opinions[ballot.Opinion.Hash]
+	var min types.LayerID
+	if exists {
+		binfo.votes = existing
+	} else {
+		var (
+			votes votes
+			err   error
+		)
+		votes, min, err = decodeVotes(t.evicted, binfo.layer, base, ballot.Opinion.Votes)
+		if err != nil {
+			return nil, 0, err
+		}
+		binfo.votes = votes
 	}
-	binfo.votes = votes
 	t.logger.Debug("decoded exceptions",
 		zap.Stringer("block", binfo.id),
 		zap.Uint32("lid", binfo.layer.Uint32()),
@@ -771,15 +784,22 @@ func (t *turtle) storeBallot(ballot *ballotInfo, min types.LayerID) error {
 	}
 
 	t.state.addBallot(ballot)
-	for current := ballot.votes.tail; current != nil && !current.lid.Before(min); current = current.prev {
-		for i, block := range current.supported {
-			existing := t.getBlock(block.header())
-			if existing != nil {
-				current.supported[i] = existing
-			} else {
-				t.addBlock(block)
+	layer := t.layer(ballot.layer)
+	existing, exists := layer.opinions[ballot.opinion()]
+	if exists {
+		ballot.votes = existing
+	} else {
+		for current := ballot.votes.tail; current != nil && !current.lid.Before(min); current = current.prev {
+			for i, block := range current.supported {
+				existing := t.getBlock(block.header())
+				if existing != nil {
+					current.supported[i] = existing
+				} else {
+					t.addBlock(block)
+				}
 			}
 		}
+		layer.opinions[ballot.opinion()] = ballot.votes
 	}
 	if !ballot.layer.After(t.processed) {
 		if err := t.countBallot(ballot); err != nil {