From 3d704a3a3a7ad8427cb35c2325a481680917314a Mon Sep 17 00:00:00 2001
From: Evgenii Stratonikov <evgeniy@nspcc.ru>
Date: Tue, 14 Jan 2020 14:34:09 +0300
Subject: [PATCH] consensus: try to use previous proposal after ChangeView

When system and network pressure is high it can be beneficial
to use transactions which and were already proposed.
The assumption is that they will be in other node's memory pool
with more probability.
---
 pkg/consensus/consensus.go      | 23 ++++++++++++++--
 pkg/consensus/consensus_test.go | 48 +++++++++++++++++++++++++++++++++
 2 files changed, 69 insertions(+), 2 deletions(-)

diff --git a/pkg/consensus/consensus.go b/pkg/consensus/consensus.go
index 38326766e..6aebda543 100644
--- a/pkg/consensus/consensus.go
+++ b/pkg/consensus/consensus.go
@@ -55,6 +55,7 @@ type service struct {
 	// everything in single thread.
 	messages     chan Payload
 	transactions chan *transaction.Transaction
+	lastProposal []util.Uint256
 }
 
 // Config is a configuration for consensus services.
@@ -205,7 +206,9 @@ func (s *service) OnPayload(cp *Payload) {
 	// we use switch here because other payloads could be possibly added in future
 	switch cp.Type() {
 	case payload.PrepareRequestType:
-		s.txx.Add(&cp.GetPrepareRequest().(*prepareRequest).minerTx)
+		req := cp.GetPrepareRequest().(*prepareRequest)
+		s.txx.Add(&req.minerTx)
+		s.lastProposal = req.transactionHashes
 	}
 
 	s.messages <- *cp
@@ -328,7 +331,23 @@ func (s *service) getBlock(h util.Uint256) block.Block {
 
 func (s *service) getVerifiedTx(count int) []block.Transaction {
 	pool := s.Config.Chain.GetMemPool()
-	txx := pool.GetVerifiedTransactions()
+
+	var txx []*transaction.Transaction
+
+	if s.dbft.ViewNumber > 0 {
+		txx = make([]*transaction.Transaction, 0, len(s.lastProposal))
+		for i := range s.lastProposal {
+			if tx, ok := pool.TryGetValue(s.lastProposal[i]); ok {
+				txx = append(txx, tx)
+			}
+		}
+
+		if len(txx) < len(s.lastProposal)/2 {
+			txx = pool.GetVerifiedTransactions()
+		}
+	} else {
+		txx = pool.GetVerifiedTransactions()
+	}
 
 	res := make([]block.Transaction, len(txx)+1)
 	for i := range txx {
diff --git a/pkg/consensus/consensus_test.go b/pkg/consensus/consensus_test.go
index 66909fcf2..1cd450f77 100644
--- a/pkg/consensus/consensus_test.go
+++ b/pkg/consensus/consensus_test.go
@@ -9,6 +9,7 @@ import (
 	"github.com/CityOfZion/neo-go/pkg/core/transaction"
 	"github.com/CityOfZion/neo-go/pkg/util"
 	"github.com/nspcc-dev/dbft/block"
+	"github.com/nspcc-dev/dbft/payload"
 	"github.com/stretchr/testify/require"
 	"go.uber.org/zap/zaptest"
 )
@@ -28,6 +29,53 @@ func TestNewService(t *testing.T) {
 	require.Equal(t, tx, txx[1])
 }
 
+func TestService_GetVerified(t *testing.T) {
+	srv := newTestService(t)
+	txs := []*transaction.Transaction{
+		newMinerTx(1),
+		newMinerTx(2),
+		newMinerTx(3),
+		newMinerTx(4),
+	}
+	pool := srv.Chain.GetMemPool()
+	item := core.NewPoolItem(txs[3], new(feer))
+
+	require.True(t, pool.TryAdd(txs[3].Hash(), item))
+
+	hashes := []util.Uint256{txs[0].Hash(), txs[1].Hash(), txs[2].Hash()}
+
+	p := new(Payload)
+	p.SetType(payload.PrepareRequestType)
+	p.SetPayload(&prepareRequest{transactionHashes: hashes})
+	p.SetValidatorIndex(1)
+
+	priv, _ := getTestValidator(1)
+	require.NoError(t, p.Sign(priv))
+
+	srv.OnPayload(p)
+	require.Equal(t, hashes, srv.lastProposal)
+
+	srv.dbft.ViewNumber = 1
+
+	t.Run("new transactions will be proposed in case of failure", func(t *testing.T) {
+		txx := srv.getVerifiedTx(10)
+		require.Equal(t, 2, len(txx), "there is only 1 tx in mempool")
+		require.Equal(t, txs[3], txx[1])
+	})
+
+	t.Run("more than half of the last proposal will be reused", func(t *testing.T) {
+		for _, tx := range txs[:2] {
+			item := core.NewPoolItem(tx, new(feer))
+			require.True(t, pool.TryAdd(tx.Hash(), item))
+		}
+
+		txx := srv.getVerifiedTx(10)
+		require.Contains(t, txx, txs[0])
+		require.Contains(t, txx, txs[1])
+		require.NotContains(t, txx, txs[2])
+	})
+}
+
 func TestService_ValidatePayload(t *testing.T) {
 	srv := newTestService(t)
 	priv, _ := getTestValidator(1)