Merge pull request #944 from nspcc-dev/notifications-2.x

Notifications 2.x

config/protocol.unit_testnet.yml

@@ -49,9 +49,10 @@ ApplicationConfiguration:
   AttemptConnPeers: 5
   MinPeers: 1
   RPC:
+    Address: 127.0.0.1
     Enabled: true
     EnableCORSWorkaround: false
-    Port: 20332
+    Port: 0 # let the system choose port dynamically
   Prometheus:
     Enabled: false #since it's not useful for unit tests.
     Port: 2112

pkg/consensus/consensus.go

@@ -41,9 +41,6 @@ type Service interface {
 	OnTransaction(tx *transaction.Transaction)
 	// GetPayload returns Payload with specified hash if it is present in the local cache.
 	GetPayload(h util.Uint256) *Payload
-	// OnNewBlock notifies consensus service that there is a new block in
-	// the chain (without explicitly passing it to the service).
-	OnNewBlock()
 }
 
 type service struct {
@@ -61,7 +58,7 @@ type service struct {
 	transactions chan *transaction.Transaction
 	// blockEvents is used to pass a new block event to the consensus
 	// process.
-	blockEvents  chan struct{}
+	blockEvents  chan *coreb.Block
 	lastProposal []util.Uint256
 	wallet       *wallet.Wallet
 }
@@ -73,9 +70,6 @@ type Config struct {
 	// Broadcast is a callback which is called to notify server
 	// about new consensus payload to sent.
 	Broadcast func(p *Payload)
-	// RelayBlock is a callback that is called to notify server
-	// about the new block that needs to be broadcasted.
-	RelayBlock func(b *coreb.Block)
 	// Chain is a core.Blockchainer instance.
 	Chain core.Blockchainer
 	// RequestTx is a callback to which will be called
@@ -106,7 +100,7 @@ func NewService(cfg Config) (Service, error) {
 		messages: make(chan Payload, 100),
 
 		transactions: make(chan *transaction.Transaction, 100),
-		blockEvents:  make(chan struct{}, 1),
+		blockEvents:  make(chan *coreb.Block, 1),
 	}
 
 	if cfg.Wallet == nil {
@@ -163,7 +157,7 @@ var (
 
 func (s *service) Start() {
 	s.dbft.Start()
-
+	s.Chain.SubscribeForBlocks(s.blockEvents)
 	go s.eventLoop()
 }
 
@@ -203,11 +197,14 @@ func (s *service) eventLoop() {
 			s.dbft.OnReceive(&msg)
 		case tx := <-s.transactions:
 			s.dbft.OnTransaction(tx)
-		case <-s.blockEvents:
+		case b := <-s.blockEvents:
-			s.log.Debug("new block in the chain",
+			// We also receive our own blocks here, so check for index.
-				zap.Uint32("dbft index", s.dbft.BlockIndex),
+			if b.Index >= s.dbft.BlockIndex {
-				zap.Uint32("chain index", s.Chain.BlockHeight()))
+				s.log.Debug("new block in the chain",
-			s.dbft.InitializeConsensus(0)
+					zap.Uint32("dbft index", s.dbft.BlockIndex),
+					zap.Uint32("chain index", s.Chain.BlockHeight()))
+				s.dbft.InitializeConsensus(0)
+			}
 		}
 	}
 }
@@ -287,20 +284,6 @@ func (s *service) OnTransaction(tx *transaction.Transaction) {
 	}
 }
 
-// OnNewBlock notifies consensus process that there is a new block in the chain
-// and dbft should probably be reinitialized.
-func (s *service) OnNewBlock() {
-	if s.dbft != nil {
-		// If there is something in the queue already, the second
-		// consecutive event doesn't make much sense (reinitializing
-		// dbft twice doesn't improve it in any way).
-		select {
-		case s.blockEvents <- struct{}{}:
-		default:
-		}
-	}
-}
-
 // GetPayload returns payload stored in cache.
 func (s *service) GetPayload(h util.Uint256) *Payload {
 	p := s.cache.Get(h)
@@ -366,8 +349,6 @@ func (s *service) processBlock(b block.Block) {
 		if _, errget := s.Chain.GetBlock(bb.Hash()); errget != nil {
 			s.log.Warn("error on add block", zap.Error(err))
 		}
-	} else {
-		s.Config.RelayBlock(bb)
 	}
 }
 

pkg/core/blockchain.go

@@ -63,7 +63,9 @@ var (
 	persistInterval   = 1 * time.Second
 )
 
-// Blockchain represents the blockchain.
+// Blockchain represents the blockchain. It maintans internal state representing
+// the state of the ledger that can be accessed in various ways and changed by
+// adding new blocks or headers.
 type Blockchain struct {
 	config config.ProtocolConfiguration
 
@@ -122,12 +124,27 @@ type Blockchain struct {
 	log *zap.Logger
 
 	lastBatch *storage.MemBatch
+
+	// Notification subsystem.
+	events  chan bcEvent
+	subCh   chan interface{}
+	unsubCh chan interface{}
+}
+
+// bcEvent is an internal event generated by the Blockchain and then
+// broadcasted to other parties. It joins the new block and associated
+// invocation logs, all the other events visible from outside can be produced
+// from this combination.
+type bcEvent struct {
+	block          *block.Block
+	appExecResults []*state.AppExecResult
 }
 
 type headersOpFunc func(headerList *HeaderHashList)
 
 // NewBlockchain returns a new blockchain object the will use the
-// given Store as its underlying storage.
+// given Store as its underlying storage. For it to work correctly you need
+// to spawn a goroutine for its Run method after this initialization.
 func NewBlockchain(s storage.Store, cfg config.ProtocolConfiguration, log *zap.Logger) (*Blockchain, error) {
 	if log == nil {
 		return nil, errors.New("empty logger")
@@ -163,6 +180,9 @@ func NewBlockchain(s storage.Store, cfg config.ProtocolConfiguration, log *zap.L
 		memPool:       mempool.NewMemPool(cfg.MemPoolSize),
 		keyCache:      make(map[util.Uint160]map[string]*keys.PublicKey),
 		log:           log,
+		events:        make(chan bcEvent),
+		subCh:         make(chan interface{}),
+		unsubCh:       make(chan interface{}),
 
 		generationAmount:  genAmount,
 		decrementInterval: decrementInterval,
@@ -264,7 +284,8 @@ func (bc *Blockchain) init() error {
 	return nil
 }
 
-// Run runs chain loop.
+// Run runs chain loop, it needs to be run as goroutine and executing it is
+// critical for correct Blockchain operation.
 func (bc *Blockchain) Run() {
 	persistTimer := time.NewTimer(persistInterval)
 	defer func() {
@@ -277,6 +298,7 @@ func (bc *Blockchain) Run() {
 		}
 		close(bc.runToExitCh)
 	}()
+	go bc.notificationDispatcher()
 	for {
 		select {
 		case <-bc.stopCh:
@@ -296,6 +318,82 @@ func (bc *Blockchain) Run() {
 	}
 }
 
+// notificationDispatcher manages subscription to events and broadcasts new events.
+func (bc *Blockchain) notificationDispatcher() {
+	var (
+		// These are just sets of subscribers, though modelled as maps
+		// for ease of management (not a lot of subscriptions is really
+		// expected, but maps are convenient for adding/deleting elements).
+		blockFeed        = make(map[chan<- *block.Block]bool)
+		txFeed           = make(map[chan<- *transaction.Transaction]bool)
+		notificationFeed = make(map[chan<- *state.NotificationEvent]bool)
+		executionFeed    = make(map[chan<- *state.AppExecResult]bool)
+	)
+	for {
+		select {
+		case <-bc.stopCh:
+			return
+		case sub := <-bc.subCh:
+			switch ch := sub.(type) {
+			case chan<- *block.Block:
+				blockFeed[ch] = true
+			case chan<- *transaction.Transaction:
+				txFeed[ch] = true
+			case chan<- *state.NotificationEvent:
+				notificationFeed[ch] = true
+			case chan<- *state.AppExecResult:
+				executionFeed[ch] = true
+			default:
+				panic(fmt.Sprintf("bad subscription: %T", sub))
+			}
+		case unsub := <-bc.unsubCh:
+			switch ch := unsub.(type) {
+			case chan<- *block.Block:
+				delete(blockFeed, ch)
+			case chan<- *transaction.Transaction:
+				delete(txFeed, ch)
+			case chan<- *state.NotificationEvent:
+				delete(notificationFeed, ch)
+			case chan<- *state.AppExecResult:
+				delete(executionFeed, ch)
+			default:
+				panic(fmt.Sprintf("bad unsubscription: %T", unsub))
+			}
+		case event := <-bc.events:
+			// We don't want to waste time looping through transactions when there are no
+			// subscribers.
+			if len(txFeed) != 0 || len(notificationFeed) != 0 || len(executionFeed) != 0 {
+				var aerIdx int
+				for _, tx := range event.block.Transactions {
+					if tx.Type == transaction.InvocationType {
+						aer := event.appExecResults[aerIdx]
+						if !aer.TxHash.Equals(tx.Hash()) {
+							panic("inconsistent application execution results")
+						}
+						aerIdx++
+						for ch := range executionFeed {
+							ch <- aer
+						}
+						if aer.VMState == "HALT" {
+							for i := range aer.Events {
+								for ch := range notificationFeed {
+									ch <- &aer.Events[i]
+								}
+							}
+						}
+					}
+					for ch := range txFeed {
+						ch <- tx
+					}
+				}
+			}
+			for ch := range blockFeed {
+				ch <- event.block
+			}
+		}
+	}
+}
+
 // Close stops Blockchain's internal loop, syncs changes to persistent storage
 // and closes it. The Blockchain is no longer functional after the call to Close.
 func (bc *Blockchain) Close() {
@@ -459,6 +557,7 @@ func (bc *Blockchain) getSystemFeeAmount(h util.Uint256) uint32 {
 // and all tests are in place, we can make a more optimized and cleaner implementation.
 func (bc *Blockchain) storeBlock(block *block.Block) error {
 	cache := dao.NewCached(bc.dao)
+	appExecResults := make([]*state.AppExecResult, 0, len(block.Transactions))
 	fee := bc.getSystemFeeAmount(block.PrevHash)
 	for _, tx := range block.Transactions {
 		fee += uint32(bc.SystemFee(tx).IntegralValue())
@@ -712,27 +811,36 @@ func (bc *Blockchain) storeBlock(block *block.Block) error {
 				Stack:       v.Estack().ToContractParameters(),
 				Events:      systemInterop.notifications,
 			}
+			appExecResults = append(appExecResults, aer)
 			err = cache.PutAppExecResult(aer)
 			if err != nil {
 				return errors.Wrap(err, "failed to Store notifications")
 			}
 		}
 	}
-	bc.lock.Lock()
-	defer bc.lock.Unlock()
 
 	if bc.config.SaveStorageBatch {
 		bc.lastBatch = cache.DAO.GetBatch()
 	}
 
+	bc.lock.Lock()
 	_, err := cache.Persist()
 	if err != nil {
+		bc.lock.Unlock()
 		return err
 	}
 	bc.topBlock.Store(block)
 	atomic.StoreUint32(&bc.blockHeight, block.Index)
-	updateBlockHeightMetric(block.Index)
 	bc.memPool.RemoveStale(bc.isTxStillRelevant)
+	bc.lock.Unlock()
+
+	updateBlockHeightMetric(block.Index)
+	// Genesis block is stored when Blockchain is not yet running, so there
+	// is no one to read this event. And it doesn't make much sense as event
+	// anyway.
+	if block.Index != 0 {
+		bc.events <- bcEvent{block, appExecResults}
+	}
 	return nil
 }
 
@@ -1179,6 +1287,68 @@ func (bc *Blockchain) GetConfig() config.ProtocolConfiguration {
 	return bc.config
 }
 
+// SubscribeForBlocks adds given channel to new block event broadcasting, so when
+// there is a new block added to the chain you'll receive it via this channel.
+// Make sure it's read from regularly as not reading these events might affect
+// other Blockchain functions.
+func (bc *Blockchain) SubscribeForBlocks(ch chan<- *block.Block) {
+	bc.subCh <- ch
+}
+
+// SubscribeForTransactions adds given channel to new transaction event
+// broadcasting, so when there is a new transaction added to the chain (in a
+// block) you'll receive it via this channel. Make sure it's read from regularly
+// as not reading these events might affect other Blockchain functions.
+func (bc *Blockchain) SubscribeForTransactions(ch chan<- *transaction.Transaction) {
+	bc.subCh <- ch
+}
+
+// SubscribeForNotifications adds given channel to new notifications event
+// broadcasting, so when an in-block transaction execution generates a
+// notification you'll receive it via this channel. Only notifications from
+// successful transactions are broadcasted, if you're interested in failed
+// transactions use SubscribeForExecutions instead. Make sure this channel is
+// read from regularly as not reading these events might affect other Blockchain
+// functions.
+func (bc *Blockchain) SubscribeForNotifications(ch chan<- *state.NotificationEvent) {
+	bc.subCh <- ch
+}
+
+// SubscribeForExecutions adds given channel to new transaction execution event
+// broadcasting, so when an in-block transaction execution happens you'll receive
+// the result of it via this channel. Make sure it's read from regularly as not
+// reading these events might affect other Blockchain functions.
+func (bc *Blockchain) SubscribeForExecutions(ch chan<- *state.AppExecResult) {
+	bc.subCh <- ch
+}
+
+// UnsubscribeFromBlocks unsubscribes given channel from new block notifications,
+// you can close it afterwards. Passing non-subscribed channel is a no-op.
+func (bc *Blockchain) UnsubscribeFromBlocks(ch chan<- *block.Block) {
+	bc.unsubCh <- ch
+}
+
+// UnsubscribeFromTransactions unsubscribes given channel from new transaction
+// notifications, you can close it afterwards. Passing non-subscribed channel is
+// a no-op.
+func (bc *Blockchain) UnsubscribeFromTransactions(ch chan<- *transaction.Transaction) {
+	bc.unsubCh <- ch
+}
+
+// UnsubscribeFromNotifications unsubscribes given channel from new
+// execution-generated notifications, you can close it afterwards. Passing
+// non-subscribed channel is a no-op.
+func (bc *Blockchain) UnsubscribeFromNotifications(ch chan<- *state.NotificationEvent) {
+	bc.unsubCh <- ch
+}
+
+// UnsubscribeFromExecutions unsubscribes given channel from new execution
+// notifications, you can close it afterwards. Passing non-subscribed channel is
+// a no-op.
+func (bc *Blockchain) UnsubscribeFromExecutions(ch chan<- *state.AppExecResult) {
+	bc.unsubCh <- ch
+}
+
 // CalculateClaimable calculates the amount of GAS which can be claimed for a transaction with value.
 // First return value is GAS generated between startHeight and endHeight.
 // Second return value is GAS returned from accumulated SystemFees between startHeight and endHeight.

pkg/core/blockchain_test.go

@@ -2,13 +2,19 @@ package core
 
 import (
 	"testing"
+	"time"
 
 	"github.com/nspcc-dev/neo-go/pkg/core/block"
+	"github.com/nspcc-dev/neo-go/pkg/core/state"
 	"github.com/nspcc-dev/neo-go/pkg/core/storage"
 	"github.com/nspcc-dev/neo-go/pkg/core/transaction"
 	"github.com/nspcc-dev/neo-go/pkg/crypto/hash"
+	"github.com/nspcc-dev/neo-go/pkg/encoding/address"
 	"github.com/nspcc-dev/neo-go/pkg/io"
 	"github.com/nspcc-dev/neo-go/pkg/util"
+	"github.com/nspcc-dev/neo-go/pkg/vm/emit"
+	"github.com/nspcc-dev/neo-go/pkg/vm/opcode"
+	"github.com/nspcc-dev/neo-go/pkg/wallet"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
@@ -229,3 +235,108 @@ func TestClose(t *testing.T) {
 	// This should never be executed.
 	assert.Nil(t, t)
 }
+
+func TestSubscriptions(t *testing.T) {
+	// We use buffering here as a substitute for reader goroutines, events
+	// get queued up and we read them one by one here.
+	const chBufSize = 16
+	blockCh := make(chan *block.Block, chBufSize)
+	txCh := make(chan *transaction.Transaction, chBufSize)
+	notificationCh := make(chan *state.NotificationEvent, chBufSize)
+	executionCh := make(chan *state.AppExecResult, chBufSize)
+
+	bc := newTestChain(t)
+	bc.SubscribeForBlocks(blockCh)
+	bc.SubscribeForTransactions(txCh)
+	bc.SubscribeForNotifications(notificationCh)
+	bc.SubscribeForExecutions(executionCh)
+
+	assert.Empty(t, notificationCh)
+	assert.Empty(t, executionCh)
+	assert.Empty(t, blockCh)
+	assert.Empty(t, txCh)
+
+	blocks, err := bc.genBlocks(1)
+	require.NoError(t, err)
+	assert.Eventually(t, func() bool { return len(blockCh) != 0 && len(txCh) != 0 }, time.Second, 10*time.Millisecond)
+	assert.Empty(t, notificationCh)
+	assert.Empty(t, executionCh)
+
+	b := <-blockCh
+	tx := <-txCh
+	assert.Equal(t, blocks[0], b)
+	assert.Equal(t, blocks[0].Transactions[0], tx)
+	assert.Empty(t, blockCh)
+	assert.Empty(t, txCh)
+
+	acc0, err := wallet.NewAccountFromWIF(privNetKeys[0])
+	require.NoError(t, err)
+	addr0, err := address.StringToUint160(acc0.Address)
+	require.NoError(t, err)
+
+	script := io.NewBufBinWriter()
+	emit.Bytes(script.BinWriter, []byte("yay!"))
+	emit.Syscall(script.BinWriter, "Neo.Runtime.Notify")
+	require.NoError(t, script.Err)
+	txGood1 := transaction.NewInvocationTX(script.Bytes(), 0)
+	txGood1.AddVerificationHash(addr0)
+	require.NoError(t, acc0.SignTx(txGood1))
+
+	// Reset() reuses the script buffer and we need to keep scripts.
+	script = io.NewBufBinWriter()
+	emit.Bytes(script.BinWriter, []byte("nay!"))
+	emit.Syscall(script.BinWriter, "Neo.Runtime.Notify")
+	emit.Opcode(script.BinWriter, opcode.THROW)
+	require.NoError(t, script.Err)
+	txBad := transaction.NewInvocationTX(script.Bytes(), 0)
+	txBad.AddVerificationHash(addr0)
+	require.NoError(t, acc0.SignTx(txBad))
+
+	script = io.NewBufBinWriter()
+	emit.Bytes(script.BinWriter, []byte("yay! yay! yay!"))
+	emit.Syscall(script.BinWriter, "Neo.Runtime.Notify")
+	require.NoError(t, script.Err)
+	txGood2 := transaction.NewInvocationTX(script.Bytes(), 0)
+	txGood2.AddVerificationHash(addr0)
+	require.NoError(t, acc0.SignTx(txGood2))
+
+	txMiner := newMinerTX()
+	invBlock := newBlock(bc.config, bc.BlockHeight()+1, bc.CurrentHeaderHash(), txMiner, txGood1, txBad, txGood2)
+	require.NoError(t, bc.AddBlock(invBlock))
+
+	require.Eventually(t, func() bool {
+		return len(blockCh) != 0 && len(txCh) != 0 &&
+			len(notificationCh) != 0 && len(executionCh) != 0
+	}, time.Second, 10*time.Millisecond)
+
+	b = <-blockCh
+	require.Equal(t, invBlock, b)
+	assert.Empty(t, blockCh)
+
+	// Follow in-block transaction order.
+	for _, txExpected := range invBlock.Transactions {
+		tx = <-txCh
+		require.Equal(t, txExpected, tx)
+		if txExpected.Type == transaction.InvocationType {
+			exec := <-executionCh
+			require.Equal(t, tx.Hash(), exec.TxHash)
+			if exec.VMState == "HALT" {
+				notif := <-notificationCh
+				inv := tx.Data.(*transaction.InvocationTX)
+				require.Equal(t, hash.Hash160(inv.Script), notif.ScriptHash)
+			}
+		}
+	}
+	assert.Empty(t, txCh)
+	assert.Empty(t, notificationCh)
+	assert.Empty(t, executionCh)
+
+	bc.UnsubscribeFromBlocks(blockCh)
+	bc.UnsubscribeFromTransactions(txCh)
+	bc.UnsubscribeFromNotifications(notificationCh)
+	bc.UnsubscribeFromExecutions(executionCh)
+
+	// Ensure that new blocks are processed correctly after unsubscription.
+	_, err = bc.genBlocks(2 * chBufSize)
+	require.NoError(t, err)
+}

pkg/core/blockchainer.go

@@ -46,6 +46,14 @@ type Blockchainer interface {
 	References(t *transaction.Transaction) ([]transaction.InOut, error)
 	mempool.Feer // fee interface
 	PoolTx(*transaction.Transaction) error
+	SubscribeForBlocks(ch chan<- *block.Block)
+	SubscribeForExecutions(ch chan<- *state.AppExecResult)
+	SubscribeForNotifications(ch chan<- *state.NotificationEvent)
+	SubscribeForTransactions(ch chan<- *transaction.Transaction)
 	VerifyTx(*transaction.Transaction, *block.Block) error
 	GetMemPool() *mempool.Pool
+	UnsubscribeFromBlocks(ch chan<- *block.Block)
+	UnsubscribeFromExecutions(ch chan<- *state.AppExecResult)
+	UnsubscribeFromNotifications(ch chan<- *state.NotificationEvent)
+	UnsubscribeFromTransactions(ch chan<- *transaction.Transaction)
 }

pkg/core/doc.go

@@ -0,0 +1,29 @@
+/*
+Package core implements Neo ledger functionality.
+It's built around the Blockchain structure that maintains state of the ledger.
+
+Events
+
+You can subscribe to Blockchain events using a set of Subscribe and Unsubscribe
+methods. These methods accept channels that will be used to send appropriate
+events, so you can control buffering. Channels are never closed by Blockchain,
+you can close them after unsubscription.
+
+Unlike RPC-level subscriptions these don't allow event filtering because it
+doesn't improve overall efficiency much (when you're using Blockchain you're
+in the same process with it and filtering on your side is not that different
+from filtering on Blockchain side).
+
+The same level of ordering guarantees as with RPC subscriptions is provided,
+albeit for a set of event channels, so at first transaction execution is
+announced via appropriate channels, then followed by notifications generated
+during this execution, then followed by transaction announcement and then
+followed by block announcement. Transaction announcements are ordered the same
+way they're stored in the block.
+
+Be careful using these subscriptions, this mechanism is not intended to be used
+by lots of subscribers and failing to read from event channels can affect
+other Blockchain operations.
+
+*/
+package core

pkg/core/helper_test.go

@@ -59,6 +59,9 @@ func newBlock(cfg config.ProtocolConfiguration, index uint32, prev util.Uint256,
 	witness := transaction.Witness{
 		VerificationScript: valScript,
 	}
+	if len(txs) == 0 {
+		txs = []*transaction.Transaction{newMinerTX()}
+	}
 	b := &block.Block{
 		Base: block.Base{
 			Version:       0,
@@ -71,7 +74,10 @@ func newBlock(cfg config.ProtocolConfiguration, index uint32, prev util.Uint256,
 		},
 		Transactions: txs,
 	}
-	_ = b.RebuildMerkleRoot()
+	err := b.RebuildMerkleRoot()
+	if err != nil {
+		panic(err)
+	}
 
 	invScript := make([]byte, 0)
 	for _, wif := range privNetKeys {

pkg/network/helper_test.go

@@ -146,10 +146,36 @@ func (chain testChain) PoolTx(*transaction.Transaction) error {
 	panic("TODO")
 }
 
+func (chain testChain) SubscribeForBlocks(ch chan<- *block.Block) {
+	panic("TODO")
+}
+func (chain testChain) SubscribeForExecutions(ch chan<- *state.AppExecResult) {
+	panic("TODO")
+}
+func (chain testChain) SubscribeForNotifications(ch chan<- *state.NotificationEvent) {
+	panic("TODO")
+}
+func (chain testChain) SubscribeForTransactions(ch chan<- *transaction.Transaction) {
+	panic("TODO")
+}
+
 func (chain testChain) VerifyTx(*transaction.Transaction, *block.Block) error {
 	panic("TODO")
 }
 
+func (chain testChain) UnsubscribeFromBlocks(ch chan<- *block.Block) {
+	panic("TODO")
+}
+func (chain testChain) UnsubscribeFromExecutions(ch chan<- *state.AppExecResult) {
+	panic("TODO")
+}
+func (chain testChain) UnsubscribeFromNotifications(ch chan<- *state.NotificationEvent) {
+	panic("TODO")
+}
+func (chain testChain) UnsubscribeFromTransactions(ch chan<- *transaction.Transaction) {
+	panic("TODO")
+}
+
 type testDiscovery struct{}
 
 func (d testDiscovery) BackFill(addrs ...string)       {}

pkg/network/server.go

@@ -102,21 +102,17 @@ func NewServer(config ServerConfig, chain core.Blockchainer, log *zap.Logger) (*
 		transactions:     make(chan *transaction.Transaction, 64),
 	}
 	s.bQueue = newBlockQueue(maxBlockBatch, chain, log, func(b *block.Block) {
-		if s.consensusStarted.Load() {
+		if !s.consensusStarted.Load() {
-			s.consensus.OnNewBlock()
-		} else {
 			s.tryStartConsensus()
 		}
-		s.relayBlock(b)
 	})
 
 	srv, err := consensus.NewService(consensus.Config{
-		Logger:     log,
+		Logger:    log,
-		Broadcast:  s.handleNewPayload,
+		Broadcast: s.handleNewPayload,
-		RelayBlock: s.relayBlock,
+		Chain:     chain,
-		Chain:      chain,
+		RequestTx: s.requestTx,
-		RequestTx:  s.requestTx,
+		Wallet:    config.Wallet,
-		Wallet:     config.Wallet,
 
 		TimePerBlock: config.TimePerBlock,
 	})
@@ -178,6 +174,7 @@ func (s *Server) Start(errChan chan error) {
 	s.discovery.BackFill(s.Seeds...)
 
 	go s.broadcastTxLoop()
+	go s.relayBlocksLoop()
 	go s.bQueue.run()
 	go s.transport.Accept()
 	setServerAndNodeVersions(s.UserAgent, strconv.FormatUint(uint64(s.id), 10))
@@ -790,14 +787,25 @@ func (s *Server) broadcastHPMessage(msg *Message) {
 	s.iteratePeersWithSendMsg(msg, Peer.EnqueueHPPacket, nil)
 }
 
-// relayBlock tells all the other connected nodes about the given block.
+// relayBlocksLoop subscribes to new blocks in the ledger and broadcasts them
-func (s *Server) relayBlock(b *block.Block) {
+// to the network. Intended to be run as a separate goroutine.
-	msg := s.MkMsg(CMDInv, payload.NewInventory(payload.BlockType, []util.Uint256{b.Hash()}))
+func (s *Server) relayBlocksLoop() {
-	// Filter out nodes that are more current (avoid spamming the network
+	ch := make(chan *block.Block, 2) // Some buffering to smooth out possible egressing delays.
-	// during initial sync).
+	s.chain.SubscribeForBlocks(ch)
-	s.iteratePeersWithSendMsg(msg, Peer.EnqueuePacket, func(p Peer) bool {
+	for {
-		return p.Handshaked() && p.LastBlockIndex() < b.Index
+		select {
-	})
+		case <-s.quit:
+			s.chain.UnsubscribeFromBlocks(ch)
+			return
+		case b := <-ch:
+			msg := s.MkMsg(CMDInv, payload.NewInventory(payload.BlockType, []util.Uint256{b.Hash()}))
+			// Filter out nodes that are more current (avoid spamming the network
+			// during initial sync).
+			s.iteratePeersWithSendMsg(msg, Peer.EnqueuePacket, func(p Peer) bool {
+				return p.Handshaked() && p.LastBlockIndex() < b.Index
+			})
+		}
+	}
 }
 
 // verifyAndPoolTX verifies the TX and adds it to the local mempool.

pkg/rpc/client/wsclient.go

@@ -7,8 +7,11 @@ import (
 	"time"
 
 	"github.com/gorilla/websocket"
+	"github.com/nspcc-dev/neo-go/pkg/core/block"
+	"github.com/nspcc-dev/neo-go/pkg/core/transaction"
 	"github.com/nspcc-dev/neo-go/pkg/rpc/request"
 	"github.com/nspcc-dev/neo-go/pkg/rpc/response"
+	"github.com/nspcc-dev/neo-go/pkg/rpc/response/result"
 )
 
 // WSClient is a websocket-enabled RPC client that can be used with appropriate
@@ -17,12 +20,28 @@ import (
 // that is only provided via websockets (like event subscription mechanism).
 type WSClient struct {
 	Client
+	// Notifications is a channel that is used to send events received from
+	// server. Client's code is supposed to be reading from this channel if
+	// it wants to use subscription mechanism, failing to do so will cause
+	// WSClient to block even regular requests. This channel is not buffered.
+	// In case of protocol error or upon connection closure this channel will
+	// be closed, so make sure to handle this.
+	Notifications chan Notification
+
 	ws            *websocket.Conn
 	done          chan struct{}
-	notifications chan *request.In
 	responses     chan *response.Raw
 	requests      chan *request.Raw
 	shutdown      chan struct{}
+	subscriptions map[string]bool
+}
+
+// Notification represents server-generated notification for client subscriptions.
+// Value can be one of block.Block, result.ApplicationLog, result.NotificationEvent
+// or transaction.Transaction based on Type.
+type Notification struct {
+	Type  response.EventID
+	Value interface{}
 }
 
 // requestResponse is a combined type for request and response since we can get
@@ -59,12 +78,15 @@ func NewWS(ctx context.Context, endpoint string, opts Options) (*WSClient, error
 		return nil, err
 	}
 	wsc := &WSClient{
-		Client:    *cl,
+		Client:        *cl,
-		ws:        ws,
+		Notifications: make(chan Notification),
-		shutdown:  make(chan struct{}),
+
-		done:      make(chan struct{}),
+		ws:            ws,
-		responses: make(chan *response.Raw),
+		shutdown:      make(chan struct{}),
-		requests:  make(chan *request.Raw),
+		done:          make(chan struct{}),
+		responses:     make(chan *response.Raw),
+		requests:      make(chan *request.Raw),
+		subscriptions: make(map[string]bool),
 	}
 	go wsc.wsReader()
 	go wsc.wsWriter()
@@ -86,6 +108,7 @@ func (c *WSClient) Close() {
 func (c *WSClient) wsReader() {
 	c.ws.SetReadLimit(wsReadLimit)
 	c.ws.SetPongHandler(func(string) error { c.ws.SetReadDeadline(time.Now().Add(wsPongLimit)); return nil })
+readloop:
 	for {
 		rr := new(requestResponse)
 		c.ws.SetReadDeadline(time.Now().Add(wsPongLimit))
@@ -95,9 +118,37 @@ func (c *WSClient) wsReader() {
 			break
 		}
 		if rr.RawID == nil && rr.Method != "" {
-			if c.notifications != nil {
+			event, err := response.GetEventIDFromString(rr.Method)
-				c.notifications <- &rr.In
+			if err != nil {
+				// Bad event received.
+				break
 			}
+			var slice []json.RawMessage
+			err = json.Unmarshal(rr.RawParams, &slice)
+			if err != nil || len(slice) != 1 {
+				// Bad event received.
+				break
+			}
+			var val interface{}
+			switch event {
+			case response.BlockEventID:
+				val = new(block.Block)
+			case response.TransactionEventID:
+				val = new(transaction.Transaction)
+			case response.NotificationEventID:
+				val = new(result.NotificationEvent)
+			case response.ExecutionEventID:
+				val = new(result.ApplicationLog)
+			default:
+				// Bad event received.
+				break readloop
+			}
+			err = json.Unmarshal(slice[0], val)
+			if err != nil || len(slice) != 1 {
+				// Bad event received.
+				break
+			}
+			c.Notifications <- Notification{event, val}
 		} else if rr.RawID != nil && (rr.Error != nil || rr.Result != nil) {
 			resp := new(response.Raw)
 			resp.ID = rr.RawID
@@ -112,9 +163,7 @@ func (c *WSClient) wsReader() {
 	}
 	close(c.done)
 	close(c.responses)
-	if c.notifications != nil {
+	close(c.Notifications)
-		close(c.notifications)
-	}
 }
 
 func (c *WSClient) wsWriter() {
@@ -158,3 +207,73 @@ func (c *WSClient) makeWsRequest(r *request.Raw) (*response.Raw, error) {
 		return resp, nil
 	}
 }
+
+func (c *WSClient) performSubscription(params request.RawParams) (string, error) {
+	var resp string
+
+	if err := c.performRequest("subscribe", params, &resp); err != nil {
+		return "", err
+	}
+	c.subscriptions[resp] = true
+	return resp, nil
+}
+
+func (c *WSClient) performUnsubscription(id string) error {
+	var resp bool
+
+	if !c.subscriptions[id] {
+		return errors.New("no subscription with this ID")
+	}
+	if err := c.performRequest("unsubscribe", request.NewRawParams(id), &resp); err != nil {
+		return err
+	}
+	if !resp {
+		return errors.New("unsubscribe method returned false result")
+	}
+	delete(c.subscriptions, id)
+	return nil
+}
+
+// SubscribeForNewBlocks adds subscription for new block events to this instance
+// of client.
+func (c *WSClient) SubscribeForNewBlocks() (string, error) {
+	params := request.NewRawParams("block_added")
+	return c.performSubscription(params)
+}
+
+// SubscribeForNewTransactions adds subscription for new transaction events to
+// this instance of client.
+func (c *WSClient) SubscribeForNewTransactions() (string, error) {
+	params := request.NewRawParams("transaction_added")
+	return c.performSubscription(params)
+}
+
+// SubscribeForExecutionNotifications adds subscription for notifications
+// generated during transaction execution to this instance of client.
+func (c *WSClient) SubscribeForExecutionNotifications() (string, error) {
+	params := request.NewRawParams("notification_from_execution")
+	return c.performSubscription(params)
+}
+
+// SubscribeForTransactionExecutions adds subscription for application execution
+// results generated during transaction execution to this instance of client.
+func (c *WSClient) SubscribeForTransactionExecutions() (string, error) {
+	params := request.NewRawParams("transaction_executed")
+	return c.performSubscription(params)
+}
+
+// Unsubscribe removes subscription for given event stream.
+func (c *WSClient) Unsubscribe(id string) error {
+	return c.performUnsubscription(id)
+}
+
+// UnsubscribeAll removes all active subscriptions of current client.
+func (c *WSClient) UnsubscribeAll() error {
+	for id := range c.subscriptions {
+		err := c.performUnsubscription(id)
+		if err != nil {
+			return err
+		}
+	}
+	return nil
+}

pkg/rpc/client/wsclient_test.go

@@ -2,8 +2,12 @@ package client
 
 import (
 	"context"
+	"net/http"
+	"net/http/httptest"
 	"testing"
+	"time"
 
+	"github.com/gorilla/websocket"
 	"github.com/stretchr/testify/require"
 )
 
@@ -14,3 +18,129 @@ func TestWSClientClose(t *testing.T) {
 	require.NoError(t, err)
 	wsc.Close()
 }
+
+func TestWSClientSubscription(t *testing.T) {
+	var cases = map[string]func(*WSClient) (string, error){
+		"blocks":        (*WSClient).SubscribeForNewBlocks,
+		"transactions":  (*WSClient).SubscribeForNewTransactions,
+		"notifications": (*WSClient).SubscribeForExecutionNotifications,
+		"executions":    (*WSClient).SubscribeForTransactionExecutions,
+	}
+	t.Run("good", func(t *testing.T) {
+		for name, f := range cases {
+			t.Run(name, func(t *testing.T) {
+				srv := initTestServer(t, `{"jsonrpc": "2.0", "id": 1, "result": "55aaff00"}`)
+				defer srv.Close()
+				wsc, err := NewWS(context.TODO(), httpURLtoWS(srv.URL), Options{})
+				require.NoError(t, err)
+				id, err := f(wsc)
+				require.NoError(t, err)
+				require.Equal(t, "55aaff00", id)
+			})
+		}
+	})
+	t.Run("bad", func(t *testing.T) {
+		for name, f := range cases {
+			t.Run(name, func(t *testing.T) {
+				srv := initTestServer(t, `{"jsonrpc": "2.0", "id": 1, "error":{"code":-32602,"message":"Invalid Params"}}`)
+				defer srv.Close()
+				wsc, err := NewWS(context.TODO(), httpURLtoWS(srv.URL), Options{})
+				require.NoError(t, err)
+				_, err = f(wsc)
+				require.Error(t, err)
+			})
+		}
+	})
+}
+
+func TestWSClientUnsubscription(t *testing.T) {
+	type responseCheck struct {
+		response string
+		code     func(*testing.T, *WSClient)
+	}
+	var cases = map[string]responseCheck{
+		"good": {`{"jsonrpc": "2.0", "id": 1, "result": true}`, func(t *testing.T, wsc *WSClient) {
+			// We can't really subscribe using this stub server, so set up wsc internals.
+			wsc.subscriptions["0"] = true
+			err := wsc.Unsubscribe("0")
+			require.NoError(t, err)
+		}},
+		"all": {`{"jsonrpc": "2.0", "id": 1, "result": true}`, func(t *testing.T, wsc *WSClient) {
+			// We can't really subscribe using this stub server, so set up wsc internals.
+			wsc.subscriptions["0"] = true
+			err := wsc.UnsubscribeAll()
+			require.NoError(t, err)
+			require.Equal(t, 0, len(wsc.subscriptions))
+		}},
+		"not subscribed": {`{"jsonrpc": "2.0", "id": 1, "result": true}`, func(t *testing.T, wsc *WSClient) {
+			err := wsc.Unsubscribe("0")
+			require.Error(t, err)
+		}},
+		"error returned": {`{"jsonrpc": "2.0", "id": 1, "error":{"code":-32602,"message":"Invalid Params"}}`, func(t *testing.T, wsc *WSClient) {
+			// We can't really subscribe using this stub server, so set up wsc internals.
+			wsc.subscriptions["0"] = true
+			err := wsc.Unsubscribe("0")
+			require.Error(t, err)
+		}},
+		"false returned": {`{"jsonrpc": "2.0", "id": 1, "result": false}`, func(t *testing.T, wsc *WSClient) {
+			// We can't really subscribe using this stub server, so set up wsc internals.
+			wsc.subscriptions["0"] = true
+			err := wsc.Unsubscribe("0")
+			require.Error(t, err)
+		}},
+	}
+	for name, rc := range cases {
+		t.Run(name, func(t *testing.T) {
+			srv := initTestServer(t, rc.response)
+			defer srv.Close()
+			wsc, err := NewWS(context.TODO(), httpURLtoWS(srv.URL), Options{})
+			require.NoError(t, err)
+			rc.code(t, wsc)
+		})
+	}
+}
+
+func TestWSClientEvents(t *testing.T) {
+	var ok bool
+	// Events from RPC server test chain.
+	var events = []string{
+		`{"jsonrpc":"2.0","method":"transaction_executed","params":[{"txid":"0x93670859cc8a42f6ea994869c944879678d33d7501d388f5a446a8c7de147df7","executions":[{"trigger":"Application","contract":"0x0000000000000000000000000000000000000000","vmstate":"HALT","gas_consumed":"1.048","stack":[{"type":"Integer","value":"1"}],"notifications":[{"contract":"0xc2789e5ab9bab828743833965b1df0d5fbcc206f","state":{"type":"Array","value":[{"type":"ByteArray","value":"636f6e74726163742063616c6c"},{"type":"ByteArray","value":"507574"},{"type":"Array","value":[{"type":"ByteArray","value":"746573746b6579"},{"type":"ByteArray","value":"7465737476616c7565"}]}]}}]}]}]}`,
+		`{"jsonrpc":"2.0","method":"notification_from_execution","params":[{"contract":"0xc2789e5ab9bab828743833965b1df0d5fbcc206f","state":{"type":"Array","value":[{"type":"ByteArray","value":"636f6e74726163742063616c6c"},{"type":"ByteArray","value":"507574"},{"type":"Array","value":[{"type":"ByteArray","value":"746573746b6579"},{"type":"ByteArray","value":"7465737476616c7565"}]}]}}]}`,
+		`{"jsonrpc":"2.0","method":"transaction_added","params":[{"txid":"0x93670859cc8a42f6ea994869c944879678d33d7501d388f5a446a8c7de147df7","size":60,"type":"InvocationTransaction","version":1,"attributes":[],"vin":[],"vout":[],"scripts":[],"script":"097465737476616c756507746573746b657952c103507574676f20ccfbd5f01d5b9633387428b8bab95a9e78c2"}]}`,
+		`{"jsonrpc":"2.0","method":"block_added","params":[{"version":0,"previousblockhash":"0x33f3e0e24542b2ec3b6420e6881c31f6460a39a4e733d88f7557cbcc3b5ed560","merkleroot":"0x9d922c5cfd4c8cd1da7a6b2265061998dc438bd0dea7145192e2858155e6c57a","time":1586154525,"height":205,"nonce":1111,"next_consensus":"0xa21e4f7178607089e4fe9fab1300d1f5a3d348be","script":{"invocation":"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","verification":"532102103a7f7dd016558597f7960d27c516a4394fd968b9e65155eb4b013e4040406e2102a7bc55fe8684e0119768d104ba30795bdcc86619e864add26156723ed185cd622102b3622bf4017bdfe317c58aed5f4c753f206b7db896046fa7d774bbc4bf7f8dc22103d90c07df63e690ce77912e10ab51acc944b66860237b608c4f8f8309e71ee69954ae"},"tx":[{"txid":"0xf9adfde059810f37b3d0686d67f6b29034e0c669537df7e59b40c14a0508b9ed","size":10,"type":"MinerTransaction","version":0,"attributes":[],"vin":[],"vout":[],"scripts":[]},{"txid":"0x93670859cc8a42f6ea994869c944879678d33d7501d388f5a446a8c7de147df7","size":60,"type":"InvocationTransaction","version":1,"attributes":[],"vin":[],"vout":[],"scripts":[],"script":"097465737476616c756507746573746b657952c103507574676f20ccfbd5f01d5b9633387428b8bab95a9e78c2"}]}]}`,
+	}
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
+		if req.URL.Path == "/ws" && req.Method == "GET" {
+			var upgrader = websocket.Upgrader{}
+			ws, err := upgrader.Upgrade(w, req, nil)
+			require.NoError(t, err)
+			for _, event := range events {
+				ws.SetWriteDeadline(time.Now().Add(2 * time.Second))
+				err = ws.WriteMessage(1, []byte(event))
+				if err != nil {
+					break
+				}
+			}
+			ws.Close()
+			return
+		}
+	}))
+
+	wsc, err := NewWS(context.TODO(), httpURLtoWS(srv.URL), Options{})
+	require.NoError(t, err)
+	for range events {
+		select {
+		case _, ok = <-wsc.Notifications:
+		case <-time.After(time.Second):
+			t.Fatal("timeout waiting for event")
+		}
+		require.Equal(t, true, ok)
+	}
+	select {
+	case _, ok = <-wsc.Notifications:
+	case <-time.After(time.Second):
+		t.Fatal("timeout waiting for event")
+	}
+	// Connection closed by server.
+	require.Equal(t, false, ok)
+}

pkg/rpc/response/events.go

@@ -0,0 +1,79 @@
+package response
+
+import (
+	"encoding/json"
+
+	"github.com/pkg/errors"
+)
+
+type (
+	// EventID represents an event type happening on the chain.
+	EventID byte
+)
+
+const (
+	// InvalidEventID is an invalid event id that is the default value of
+	// EventID. It's only used as an initial value similar to nil.
+	InvalidEventID EventID = iota
+	// BlockEventID is a `block_added` event.
+	BlockEventID
+	// TransactionEventID corresponds to `transaction_added` event.
+	TransactionEventID
+	// NotificationEventID represents `notification_from_execution` events.
+	NotificationEventID
+	// ExecutionEventID is used for `transaction_executed` events.
+	ExecutionEventID
+)
+
+// String is a good old Stringer implementation.
+func (e EventID) String() string {
+	switch e {
+	case BlockEventID:
+		return "block_added"
+	case TransactionEventID:
+		return "transaction_added"
+	case NotificationEventID:
+		return "notification_from_execution"
+	case ExecutionEventID:
+		return "transaction_executed"
+	default:
+		return "unknown"
+	}
+}
+
+// GetEventIDFromString converts input string into an EventID if it's possible.
+func GetEventIDFromString(s string) (EventID, error) {
+	switch s {
+	case "block_added":
+		return BlockEventID, nil
+	case "transaction_added":
+		return TransactionEventID, nil
+	case "notification_from_execution":
+		return NotificationEventID, nil
+	case "transaction_executed":
+		return ExecutionEventID, nil
+	default:
+		return 255, errors.New("invalid stream name")
+	}
+}
+
+// MarshalJSON implements json.Marshaler interface.
+func (e EventID) MarshalJSON() ([]byte, error) {
+	return json.Marshal(e.String())
+}
+
+// UnmarshalJSON implements json.Unmarshaler interface.
+func (e *EventID) UnmarshalJSON(b []byte) error {
+	var s string
+
+	err := json.Unmarshal(b, &s)
+	if err != nil {
+		return err
+	}
+	id, err := GetEventIDFromString(s)
+	if err != nil {
+		return err
+	}
+	*e = id
+	return nil
+}

pkg/rpc/response/result/application_log.go

@@ -30,16 +30,22 @@ type NotificationEvent struct {
 	Item     smartcontract.Parameter `json:"state"`
 }
 
+// StateEventToResultNotification converts state.NotificationEvent to
+// result.NotificationEvent.
+func StateEventToResultNotification(event state.NotificationEvent) NotificationEvent {
+	seen := make(map[vm.StackItem]bool)
+	item := event.Item.ToContractParameter(seen)
+	return NotificationEvent{
+		Contract: event.ScriptHash,
+		Item:     item,
+	}
+}
+
 // NewApplicationLog creates a new ApplicationLog wrapper.
 func NewApplicationLog(appExecRes *state.AppExecResult, scriptHash util.Uint160) ApplicationLog {
 	events := make([]NotificationEvent, 0, len(appExecRes.Events))
 	for _, e := range appExecRes.Events {
-		seen := make(map[vm.StackItem]bool)
+		events = append(events, StateEventToResultNotification(e))
-		item := e.Item.ToContractParameter(seen)
-		events = append(events, NotificationEvent{
-			Contract: e.ScriptHash,
-			Item:     item,
-		})
 	}
 
 	triggerString := appExecRes.Trigger.String()

pkg/rpc/response/types.go

@@ -37,3 +37,12 @@ type GetRawTx struct {
 	HeaderAndError
 	Result *result.TransactionOutputRaw `json:"result"`
 }
+
+// Notification is a type used to represent wire format of events, they're
+// special in that they look like requests but they don't have IDs and their
+// "method" is actually an event name.
+type Notification struct {
+	JSONRPC string        `json:"jsonrpc"`
+	Event   EventID       `json:"method"`
+	Payload []interface{} `json:"params"`
+}

pkg/rpc/server/server.go

@@ -9,6 +9,7 @@ import (
 	"net"
 	"net/http"
 	"strconv"
+	"sync"
 	"time"
 
 	"github.com/gorilla/websocket"
@@ -41,6 +42,19 @@ type (
 		coreServer *network.Server
 		log        *zap.Logger
 		https      *http.Server
+		shutdown   chan struct{}
+
+		subsLock         sync.RWMutex
+		subscribers      map[*subscriber]bool
+		subsGroup        sync.WaitGroup
+		blockSubs        int
+		executionSubs    int
+		notificationSubs int
+		transactionSubs  int
+		blockCh          chan *block.Block
+		executionCh      chan *state.AppExecResult
+		notificationCh   chan *state.NotificationEvent
+		transactionCh    chan *transaction.Transaction
 	}
 )
 
@@ -56,6 +70,11 @@ const (
 
 	// Write deadline.
 	wsWriteLimit = wsPingPeriod / 2
+
+	// Maximum number of subscribers per Server. Each websocket client is
+	// treated like subscriber, so technically it's a limit on websocket
+	// connections.
+	maxSubscribers = 64
 )
 
 var rpcHandlers = map[string]func(*Server, request.Params) (interface{}, *response.Error){
@@ -91,6 +110,11 @@ var rpcHandlers = map[string]func(*Server, request.Params) (interface{}, *respon
 	"validateaddress":      (*Server).validateAddress,
 }
 
+var rpcWsHandlers = map[string]func(*Server, request.Params, *subscriber) (interface{}, *response.Error){
+	"subscribe":   (*Server).subscribe,
+	"unsubscribe": (*Server).unsubscribe,
+}
+
 var invalidBlockHeightError = func(index int, height int) *response.Error {
 	return response.NewRPCError(fmt.Sprintf("Param at index %d should be greater than or equal to 0 and less then or equal to current block height, got: %d", index, height), "", nil)
 }
@@ -119,11 +143,20 @@ func New(chain core.Blockchainer, conf rpc.Config, coreServer *network.Server, l
 		coreServer: coreServer,
 		log:        log,
 		https:      tlsServer,
+		shutdown:   make(chan struct{}),
+
+		subscribers: make(map[*subscriber]bool),
+		// These are NOT buffered to preserve original order of events.
+		blockCh:        make(chan *block.Block),
+		executionCh:    make(chan *state.AppExecResult),
+		notificationCh: make(chan *state.NotificationEvent),
+		transactionCh:  make(chan *transaction.Transaction),
 	}
 }
 
-// Start creates a new JSON-RPC server
+// Start creates a new JSON-RPC server listening on the configured port. It's
-// listening on the configured port.
+// supposed to be run as a separate goroutine (like http.Server's Serve) and it
+// returns its errors via given errChan.
 func (s *Server) Start(errChan chan error) {
 	if !s.config.Enabled {
 		s.log.Info("RPC server is not enabled")
@@ -132,6 +165,7 @@ func (s *Server) Start(errChan chan error) {
 	s.Handler = http.HandlerFunc(s.handleHTTPRequest)
 	s.log.Info("starting rpc-server", zap.String("endpoint", s.Addr))
 
+	go s.handleSubEvents()
 	if cfg := s.config.TLSConfig; cfg.Enabled {
 		s.https.Handler = http.HandlerFunc(s.handleHTTPRequest)
 		s.log.Info("starting rpc-server (https)", zap.String("endpoint", s.https.Addr))
@@ -154,6 +188,10 @@ func (s *Server) Start(errChan chan error) {
 // method.
 func (s *Server) Shutdown() error {
 	var httpsErr error
+
+	// Signal to websocket writer routines and handleSubEvents.
+	close(s.shutdown)
+
 	if s.config.TLSConfig.Enabled {
 		s.log.Info("shutting down rpc-server (https)", zap.String("endpoint", s.https.Addr))
 		httpsErr = s.https.Shutdown(context.Background())
@@ -161,6 +199,10 @@ func (s *Server) Shutdown() error {
 
 	s.log.Info("shutting down rpc-server", zap.String("endpoint", s.Addr))
 	err := s.Server.Shutdown(context.Background())
+
+	// Wait for handleSubEvents to finish.
+	<-s.executionCh
+
 	if err == nil {
 		return httpsErr
 	}
@@ -168,20 +210,40 @@ func (s *Server) Shutdown() error {
 }
 
 func (s *Server) handleHTTPRequest(w http.ResponseWriter, httpRequest *http.Request) {
+	req := request.NewIn()
+
 	if httpRequest.URL.Path == "/ws" && httpRequest.Method == "GET" {
+		// Technically there is a race between this check and
+		// s.subscribers modification 20 lines below, but it's tiny
+		// and not really critical to bother with it. Some additional
+		// clients may sneak in, no big deal.
+		s.subsLock.RLock()
+		numOfSubs := len(s.subscribers)
+		s.subsLock.RUnlock()
+		if numOfSubs >= maxSubscribers {
+			s.writeHTTPErrorResponse(
+				req,
+				w,
+				response.NewInternalServerError("websocket users limit reached", nil),
+			)
+			return
+		}
 		ws, err := upgrader.Upgrade(w, httpRequest, nil)
 		if err != nil {
 			s.log.Info("websocket connection upgrade failed", zap.Error(err))
 			return
 		}
 		resChan := make(chan response.Raw)
-		go s.handleWsWrites(ws, resChan)
+		subChan := make(chan *websocket.PreparedMessage, notificationBufSize)
-		s.handleWsReads(ws, resChan)
+		subscr := &subscriber{writer: subChan, ws: ws}
+		s.subsLock.Lock()
+		s.subscribers[subscr] = true
+		s.subsLock.Unlock()
+		go s.handleWsWrites(ws, resChan, subChan)
+		s.handleWsReads(ws, resChan, subscr)
 		return
 	}
 
-	req := request.NewIn()
-
 	if httpRequest.Method != "POST" {
 		s.writeHTTPErrorResponse(
 			req,
@@ -199,11 +261,14 @@ func (s *Server) handleHTTPRequest(w http.ResponseWriter, httpRequest *http.Requ
 		return
 	}
 
-	resp := s.handleRequest(req)
+	resp := s.handleRequest(req, nil)
 	s.writeHTTPServerResponse(req, w, resp)
 }
 
-func (s *Server) handleRequest(req *request.In) response.Raw {
+func (s *Server) handleRequest(req *request.In, sub *subscriber) response.Raw {
+	var res interface{}
+	var resErr *response.Error
+
 	reqParams, err := req.Params()
 	if err != nil {
 		return s.packResponseToRaw(req, nil, response.NewInvalidParamsError("Problem parsing request parameters", err))
@@ -215,20 +280,37 @@ func (s *Server) handleRequest(req *request.In) response.Raw {
 
 	incCounter(req.Method)
 
+	resErr = response.NewMethodNotFoundError(fmt.Sprintf("Method '%s' not supported", req.Method), nil)
 	handler, ok := rpcHandlers[req.Method]
-	if !ok {
+	if ok {
-		return s.packResponseToRaw(req, nil, response.NewMethodNotFoundError(fmt.Sprintf("Method '%s' not supported", req.Method), nil))
+		res, resErr = handler(s, *reqParams)
+	} else if sub != nil {
+		handler, ok := rpcWsHandlers[req.Method]
+		if ok {
+			res, resErr = handler(s, *reqParams, sub)
+		}
 	}
-	res, resErr := handler(s, *reqParams)
 	return s.packResponseToRaw(req, res, resErr)
 }
 
-func (s *Server) handleWsWrites(ws *websocket.Conn, resChan <-chan response.Raw) {
+func (s *Server) handleWsWrites(ws *websocket.Conn, resChan <-chan response.Raw, subChan <-chan *websocket.PreparedMessage) {
 	pingTicker := time.NewTicker(wsPingPeriod)
 	defer ws.Close()
 	defer pingTicker.Stop()
 	for {
 		select {
+		case <-s.shutdown:
+			// Signal to the reader routine.
+			ws.Close()
+			return
+		case event, ok := <-subChan:
+			if !ok {
+				return
+			}
+			ws.SetWriteDeadline(time.Now().Add(wsWriteLimit))
+			if err := ws.WritePreparedMessage(event); err != nil {
+				return
+			}
 		case res, ok := <-resChan:
 			if !ok {
 				return
@@ -246,22 +328,36 @@ func (s *Server) handleWsWrites(ws *websocket.Conn, resChan <-chan response.Raw)
 	}
 }
 
-func (s *Server) handleWsReads(ws *websocket.Conn, resChan chan<- response.Raw) {
+func (s *Server) handleWsReads(ws *websocket.Conn, resChan chan<- response.Raw, subscr *subscriber) {
 	ws.SetReadLimit(wsReadLimit)
 	ws.SetReadDeadline(time.Now().Add(wsPongLimit))
 	ws.SetPongHandler(func(string) error { ws.SetReadDeadline(time.Now().Add(wsPongLimit)); return nil })
+requestloop:
 	for {
 		req := new(request.In)
 		err := ws.ReadJSON(req)
 		if err != nil {
 			break
 		}
-		res := s.handleRequest(req)
+		res := s.handleRequest(req, subscr)
 		if res.Error != nil {
 			s.logRequestError(req, res.Error)
 		}
-		resChan <- res
+		select {
+		case <-s.shutdown:
+			break requestloop
+		case resChan <- res:
+		}
+
 	}
+	s.subsLock.Lock()
+	delete(s.subscribers, subscr)
+	for _, e := range subscr.feeds {
+		if e != response.InvalidEventID {
+			s.unsubscribeFromChannel(e)
+		}
+	}
+	s.subsLock.Unlock()
 	close(resChan)
 	ws.Close()
 }
@@ -1024,6 +1120,201 @@ func (s *Server) sendrawtransaction(reqParams request.Params) (interface{}, *res
 	return results, resultsErr
 }
 
+// subscribe handles subscription requests from websocket clients.
+func (s *Server) subscribe(reqParams request.Params, sub *subscriber) (interface{}, *response.Error) {
+	p, ok := reqParams.Value(0)
+	if !ok {
+		return nil, response.ErrInvalidParams
+	}
+	streamName, err := p.GetString()
+	if err != nil {
+		return nil, response.ErrInvalidParams
+	}
+	event, err := response.GetEventIDFromString(streamName)
+	if err != nil {
+		return nil, response.ErrInvalidParams
+	}
+	s.subsLock.Lock()
+	defer s.subsLock.Unlock()
+	select {
+	case <-s.shutdown:
+		return nil, response.NewInternalServerError("server is shutting down", nil)
+	default:
+	}
+	var id int
+	for ; id < len(sub.feeds); id++ {
+		if sub.feeds[id] == response.InvalidEventID {
+			break
+		}
+	}
+	if id == len(sub.feeds) {
+		return nil, response.NewInternalServerError("maximum number of subscriptions is reached", nil)
+	}
+	sub.feeds[id] = event
+	s.subscribeToChannel(event)
+	return strconv.FormatInt(int64(id), 10), nil
+}
+
+// subscribeToChannel subscribes RPC server to appropriate chain events if
+// it's not yet subscribed for them. It's supposed to be called with s.subsLock
+// taken by the caller.
+func (s *Server) subscribeToChannel(event response.EventID) {
+	switch event {
+	case response.BlockEventID:
+		if s.blockSubs == 0 {
+			s.chain.SubscribeForBlocks(s.blockCh)
+		}
+		s.blockSubs++
+	case response.TransactionEventID:
+		if s.transactionSubs == 0 {
+			s.chain.SubscribeForTransactions(s.transactionCh)
+		}
+		s.transactionSubs++
+	case response.NotificationEventID:
+		if s.notificationSubs == 0 {
+			s.chain.SubscribeForNotifications(s.notificationCh)
+		}
+		s.notificationSubs++
+	case response.ExecutionEventID:
+		if s.executionSubs == 0 {
+			s.chain.SubscribeForExecutions(s.executionCh)
+		}
+		s.executionSubs++
+	}
+}
+
+// unsubscribe handles unsubscription requests from websocket clients.
+func (s *Server) unsubscribe(reqParams request.Params, sub *subscriber) (interface{}, *response.Error) {
+	p, ok := reqParams.Value(0)
+	if !ok {
+		return nil, response.ErrInvalidParams
+	}
+	id, err := p.GetInt()
+	if err != nil || id < 0 {
+		return nil, response.ErrInvalidParams
+	}
+	s.subsLock.Lock()
+	defer s.subsLock.Unlock()
+	if len(sub.feeds) <= id || sub.feeds[id] == response.InvalidEventID {
+		return nil, response.ErrInvalidParams
+	}
+	event := sub.feeds[id]
+	sub.feeds[id] = response.InvalidEventID
+	s.unsubscribeFromChannel(event)
+	return true, nil
+}
+
+// unsubscribeFromChannel unsubscribes RPC server from appropriate chain events
+// if there are no other subscribers for it. It's supposed to be called with
+// s.subsLock taken by the caller.
+func (s *Server) unsubscribeFromChannel(event response.EventID) {
+	switch event {
+	case response.BlockEventID:
+		s.blockSubs--
+		if s.blockSubs == 0 {
+			s.chain.UnsubscribeFromBlocks(s.blockCh)
+		}
+	case response.TransactionEventID:
+		s.transactionSubs--
+		if s.transactionSubs == 0 {
+			s.chain.UnsubscribeFromTransactions(s.transactionCh)
+		}
+	case response.NotificationEventID:
+		s.notificationSubs--
+		if s.notificationSubs == 0 {
+			s.chain.UnsubscribeFromNotifications(s.notificationCh)
+		}
+	case response.ExecutionEventID:
+		s.executionSubs--
+		if s.executionSubs == 0 {
+			s.chain.UnsubscribeFromExecutions(s.executionCh)
+		}
+	}
+}
+
+func (s *Server) handleSubEvents() {
+chloop:
+	for {
+		var resp = response.Notification{
+			JSONRPC: request.JSONRPCVersion,
+			Payload: make([]interface{}, 1),
+		}
+		var msg *websocket.PreparedMessage
+		select {
+		case <-s.shutdown:
+			break chloop
+		case b := <-s.blockCh:
+			resp.Event = response.BlockEventID
+			resp.Payload[0] = b
+		case execution := <-s.executionCh:
+			resp.Event = response.ExecutionEventID
+			resp.Payload[0] = result.NewApplicationLog(execution, util.Uint160{})
+		case notification := <-s.notificationCh:
+			resp.Event = response.NotificationEventID
+			resp.Payload[0] = result.StateEventToResultNotification(*notification)
+		case tx := <-s.transactionCh:
+			resp.Event = response.TransactionEventID
+			resp.Payload[0] = tx
+		}
+		s.subsLock.RLock()
+	subloop:
+		for sub := range s.subscribers {
+			for _, subID := range sub.feeds {
+				if subID == resp.Event {
+					if msg == nil {
+						b, err := json.Marshal(resp)
+						if err != nil {
+							s.log.Error("failed to marshal notification",
+								zap.Error(err),
+								zap.String("type", resp.Event.String()))
+							break subloop
+						}
+						msg, err = websocket.NewPreparedMessage(websocket.TextMessage, b)
+						if err != nil {
+							s.log.Error("failed to prepare notification message",
+								zap.Error(err),
+								zap.String("type", resp.Event.String()))
+							break subloop
+						}
+					}
+					sub.writer <- msg
+					// The message is sent only once per subscriber.
+					break
+				}
+			}
+		}
+		s.subsLock.RUnlock()
+	}
+	// It's important to do it with lock held because no subscription routine
+	// should be running concurrently to this one. And even if one is to run
+	// after unlock, it'll see closed s.shutdown and won't subscribe.
+	s.subsLock.Lock()
+	// There might be no subscription in reality, but it's not a problem as
+	// core.Blockchain allows unsubscribing non-subscribed channels.
+	s.chain.UnsubscribeFromBlocks(s.blockCh)
+	s.chain.UnsubscribeFromTransactions(s.transactionCh)
+	s.chain.UnsubscribeFromNotifications(s.notificationCh)
+	s.chain.UnsubscribeFromExecutions(s.executionCh)
+	s.subsLock.Unlock()
+drainloop:
+	for {
+		select {
+		case <-s.blockCh:
+		case <-s.executionCh:
+		case <-s.notificationCh:
+		case <-s.transactionCh:
+		default:
+			break drainloop
+		}
+	}
+	// It's not required closing these, but since they're drained already
+	// this is safe and it also allows to give a signal to Shutdown routine.
+	close(s.blockCh)
+	close(s.transactionCh)
+	close(s.notificationCh)
+	close(s.executionCh)
+}
+
 func (s *Server) blockHeightFromParam(param *request.Param) (int, *response.Error) {
 	num, err := param.GetInt()
 	if err != nil {

pkg/rpc/server/server_helper_test.go

@@ -15,12 +15,11 @@ import (
 	"github.com/nspcc-dev/neo-go/pkg/network"
 	"github.com/nspcc-dev/neo-go/pkg/util"
 	"github.com/stretchr/testify/require"
+	"go.uber.org/zap"
 	"go.uber.org/zap/zaptest"
 )
 
-func initServerWithInMemoryChain(t *testing.T) (*core.Blockchain, *httptest.Server) {
+func getUnitTestChain(t *testing.T) (*core.Blockchain, config.Config, *zap.Logger) {
-	var nBlocks uint32
-
 	net := config.ModeUnitTestNet
 	configPath := "../../../config"
 	cfg, err := config.Load(configPath, net)
@@ -33,6 +32,11 @@ func initServerWithInMemoryChain(t *testing.T) (*core.Blockchain, *httptest.Serv
 
 	go chain.Run()
 
+	return chain, cfg, logger
+}
+
+func getTestBlocks(t *testing.T) []*block.Block {
+	blocks := make([]*block.Block, 0)
 	// File "./testdata/testblocks.acc" was generated by function core._
 	// ("neo-go/pkg/core/helper_test.go").
 	// To generate new "./testdata/testblocks.acc", follow the steps:
@@ -42,25 +46,41 @@ func initServerWithInMemoryChain(t *testing.T) (*core.Blockchain, *httptest.Serv
 	f, err := os.Open("testdata/testblocks.acc")
 	require.Nil(t, err)
 	br := io.NewBinReaderFromIO(f)
-	nBlocks = br.ReadU32LE()
+	nBlocks := br.ReadU32LE()
 	require.Nil(t, br.Err)
 	for i := 0; i < int(nBlocks); i++ {
 		_ = br.ReadU32LE()
 		b := &block.Block{}
 		b.DecodeBinary(br)
 		require.Nil(t, br.Err)
-		require.NoError(t, chain.AddBlock(b))
+		blocks = append(blocks, b)
 	}
+	return blocks
+}
+
+func initClearServerWithInMemoryChain(t *testing.T) (*core.Blockchain, *Server, *httptest.Server) {
+	chain, cfg, logger := getUnitTestChain(t)
 
 	serverConfig := network.NewServerConfig(cfg)
 	server, err := network.NewServer(serverConfig, chain, logger)
 	require.NoError(t, err)
 	rpcServer := New(chain, cfg.ApplicationConfiguration.RPC, server, logger)
+	errCh := make(chan error, 2)
+	go rpcServer.Start(errCh)
 
 	handler := http.HandlerFunc(rpcServer.handleHTTPRequest)
 	srv := httptest.NewServer(handler)
 
-	return chain, srv
+	return chain, &rpcServer, srv
+}
+
+func initServerWithInMemoryChain(t *testing.T) (*core.Blockchain, *Server, *httptest.Server) {
+	chain, rpcServer, srv := initClearServerWithInMemoryChain(t)
+
+	for _, b := range getTestBlocks(t) {
+		require.NoError(t, chain.AddBlock(b))
+	}
+	return chain, rpcServer, srv
 }
 
 type FeerStub struct{}

pkg/rpc/server/server_test.go

@@ -896,9 +896,10 @@ func TestRPC(t *testing.T) {
 // calls. Some tests change the chain state, thus we reinitialize the chain from
 // scratch here.
 func testRPCProtocol(t *testing.T, doRPCCall func(string, string, *testing.T) []byte) {
-	chain, httpSrv := initServerWithInMemoryChain(t)
+	chain, rpcSrv, httpSrv := initServerWithInMemoryChain(t)
 
 	defer chain.Close()
+	defer rpcSrv.Shutdown()
 
 	e := &executor{chain: chain, httpSrv: httpSrv}
 	for method, cases := range rpcTestCases {

pkg/rpc/server/subscription.go

@@ -0,0 +1,35 @@
+package server
+
+import (
+	"github.com/gorilla/websocket"
+	"github.com/nspcc-dev/neo-go/pkg/rpc/response"
+)
+
+type (
+	// subscriber is an event subscriber.
+	subscriber struct {
+		writer chan<- *websocket.PreparedMessage
+		ws     *websocket.Conn
+
+		// These work like slots as there is not a lot of them (it's
+		// cheaper doing it this way rather than creating a map),
+		// pointing to EventID is an obvious overkill at the moment, but
+		// that's not for long.
+		feeds [maxFeeds]response.EventID
+	}
+)
+
+const (
+	// Maximum number of subscriptions per one client.
+	maxFeeds = 16
+
+	// This sets notification messages buffer depth, it may seem to be quite
+	// big, but there is a big gap in speed between internal event processing
+	// and networking communication that is combined with spiky nature of our
+	// event generation process, which leads to lots of events generated in
+	// short time and they will put some pressure to this buffer (consider
+	// ~500 invocation txs in one block with some notifications). At the same
+	// time this channel is about sending pointers, so it's doesn't cost
+	// a lot in terms of memory used.
+	notificationBufSize = 1024
+)

pkg/rpc/server/subscription_test.go

@@ -0,0 +1,227 @@
+package server
+
+import (
+	"encoding/json"
+	"fmt"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/gorilla/websocket"
+	"github.com/nspcc-dev/neo-go/pkg/core"
+	"github.com/nspcc-dev/neo-go/pkg/core/transaction"
+	"github.com/nspcc-dev/neo-go/pkg/rpc/response"
+	"github.com/stretchr/testify/require"
+	"go.uber.org/atomic"
+)
+
+func wsReader(t *testing.T, ws *websocket.Conn, msgCh chan<- []byte, isFinished *atomic.Bool) {
+	for {
+		ws.SetReadDeadline(time.Now().Add(time.Second))
+		_, body, err := ws.ReadMessage()
+		if isFinished.Load() {
+			require.Error(t, err)
+			break
+		}
+		require.NoError(t, err)
+		msgCh <- body
+	}
+}
+
+func callWSGetRaw(t *testing.T, ws *websocket.Conn, msg string, respCh <-chan []byte) *response.Raw {
+	var resp = new(response.Raw)
+
+	ws.SetWriteDeadline(time.Now().Add(time.Second))
+	require.NoError(t, ws.WriteMessage(websocket.TextMessage, []byte(msg)))
+
+	body := <-respCh
+	require.NoError(t, json.Unmarshal(body, resp))
+	return resp
+}
+
+func getNotification(t *testing.T, respCh <-chan []byte) *response.Notification {
+	var resp = new(response.Notification)
+	body := <-respCh
+	require.NoError(t, json.Unmarshal(body, resp))
+	return resp
+}
+
+func initCleanServerAndWSClient(t *testing.T) (*core.Blockchain, *Server, *websocket.Conn, chan []byte, *atomic.Bool) {
+	chain, rpcSrv, httpSrv := initClearServerWithInMemoryChain(t)
+
+	dialer := websocket.Dialer{HandshakeTimeout: time.Second}
+	url := "ws" + strings.TrimPrefix(httpSrv.URL, "http") + "/ws"
+	ws, _, err := dialer.Dial(url, nil)
+	require.NoError(t, err)
+
+	// Use buffered channel to read server's messages and then read expected
+	// responses from it.
+	respMsgs := make(chan []byte, 16)
+	finishedFlag := atomic.NewBool(false)
+	go wsReader(t, ws, respMsgs, finishedFlag)
+	return chain, rpcSrv, ws, respMsgs, finishedFlag
+}
+
+func TestSubscriptions(t *testing.T) {
+	var subIDs = make([]string, 0)
+	var subFeeds = []string{"block_added", "transaction_added", "notification_from_execution", "transaction_executed"}
+
+	chain, rpcSrv, c, respMsgs, finishedFlag := initCleanServerAndWSClient(t)
+
+	defer chain.Close()
+	defer rpcSrv.Shutdown()
+
+	for _, feed := range subFeeds {
+		var s string
+		resp := callWSGetRaw(t, c, fmt.Sprintf(`{
+  "jsonrpc": "2.0",
+  "method": "subscribe",
+  "params": ["%s"],
+  "id": 1
+}`, feed), respMsgs)
+		require.Nil(t, resp.Error)
+		require.NotNil(t, resp.Result)
+		require.NoError(t, json.Unmarshal(resp.Result, &s))
+		subIDs = append(subIDs, s)
+	}
+
+	for _, b := range getTestBlocks(t) {
+		require.NoError(t, chain.AddBlock(b))
+		for _, tx := range b.Transactions {
+			var mayNotify bool
+
+			if tx.Type == transaction.InvocationType {
+				resp := getNotification(t, respMsgs)
+				require.Equal(t, response.ExecutionEventID, resp.Event)
+				mayNotify = true
+			}
+			for {
+				resp := getNotification(t, respMsgs)
+				if mayNotify && resp.Event == response.NotificationEventID {
+					continue
+				}
+				require.Equal(t, response.TransactionEventID, resp.Event)
+				break
+			}
+		}
+		resp := getNotification(t, respMsgs)
+		require.Equal(t, response.BlockEventID, resp.Event)
+	}
+
+	for _, id := range subIDs {
+		var b bool
+
+		resp := callWSGetRaw(t, c, fmt.Sprintf(`{
+  "jsonrpc": "2.0",
+  "method": "unsubscribe",
+  "params": ["%s"],
+  "id": 1
+}`, id), respMsgs)
+		require.Nil(t, resp.Error)
+		require.NotNil(t, resp.Result)
+		require.NoError(t, json.Unmarshal(resp.Result, &b))
+		require.Equal(t, true, b)
+	}
+	finishedFlag.CAS(false, true)
+	c.Close()
+}
+
+func TestMaxSubscriptions(t *testing.T) {
+	var subIDs = make([]string, 0)
+	chain, rpcSrv, c, respMsgs, finishedFlag := initCleanServerAndWSClient(t)
+
+	defer chain.Close()
+	defer rpcSrv.Shutdown()
+
+	for i := 0; i < maxFeeds+1; i++ {
+		var s string
+		resp := callWSGetRaw(t, c, `{"jsonrpc": "2.0", "method": "subscribe", "params": ["block_added"], "id": 1}`, respMsgs)
+		if i < maxFeeds {
+			require.Nil(t, resp.Error)
+			require.NotNil(t, resp.Result)
+			require.NoError(t, json.Unmarshal(resp.Result, &s))
+			// Each ID must be unique.
+			for _, id := range subIDs {
+				require.NotEqual(t, id, s)
+			}
+			subIDs = append(subIDs, s)
+		} else {
+			require.NotNil(t, resp.Error)
+			require.Nil(t, resp.Result)
+		}
+	}
+
+	finishedFlag.CAS(false, true)
+	c.Close()
+}
+
+func TestBadSubUnsub(t *testing.T) {
+	var subCases = map[string]string{
+		"no params":              `{"jsonrpc": "2.0", "method": "subscribe", "params": [], "id": 1}`,
+		"bad (non-string) event": `{"jsonrpc": "2.0", "method": "subscribe", "params": [1], "id": 1}`,
+		"bad (wrong) event":      `{"jsonrpc": "2.0", "method": "subscribe", "params": ["block_removed"], "id": 1}`,
+	}
+	var unsubCases = map[string]string{
+		"no params":         `{"jsonrpc": "2.0", "method": "unsubscribe", "params": [], "id": 1}`,
+		"bad id":            `{"jsonrpc": "2.0", "method": "unsubscribe", "params": ["vasiliy"], "id": 1}`,
+		"not subscribed id": `{"jsonrpc": "2.0", "method": "unsubscribe", "params": ["7"], "id": 1}`,
+	}
+	chain, rpcSrv, c, respMsgs, finishedFlag := initCleanServerAndWSClient(t)
+
+	defer chain.Close()
+	defer rpcSrv.Shutdown()
+
+	testF := func(t *testing.T, cases map[string]string) func(t *testing.T) {
+		return func(t *testing.T) {
+			for n, s := range cases {
+				t.Run(n, func(t *testing.T) {
+					resp := callWSGetRaw(t, c, s, respMsgs)
+					require.NotNil(t, resp.Error)
+					require.Nil(t, resp.Result)
+				})
+			}
+		}
+	}
+	t.Run("subscribe", testF(t, subCases))
+	t.Run("unsubscribe", testF(t, unsubCases))
+
+	finishedFlag.CAS(false, true)
+	c.Close()
+}
+
+func doSomeWSRequest(t *testing.T, ws *websocket.Conn) {
+	ws.SetWriteDeadline(time.Now().Add(time.Second))
+	// It could be just about anything including invalid request,
+	// we only care about server handling being active.
+	require.NoError(t, ws.WriteMessage(websocket.TextMessage, []byte(`{"jsonrpc": "2.0", "method": "getversion", "params": [], "id": 1}`)))
+	ws.SetReadDeadline(time.Now().Add(time.Second))
+	_, _, err := ws.ReadMessage()
+	require.NoError(t, err)
+}
+
+func TestWSClientsLimit(t *testing.T) {
+	chain, rpcSrv, httpSrv := initClearServerWithInMemoryChain(t)
+	defer chain.Close()
+	defer rpcSrv.Shutdown()
+
+	dialer := websocket.Dialer{HandshakeTimeout: time.Second}
+	url := "ws" + strings.TrimPrefix(httpSrv.URL, "http") + "/ws"
+	wss := make([]*websocket.Conn, maxSubscribers)
+
+	for i := 0; i < len(wss)+1; i++ {
+		ws, _, err := dialer.Dial(url, nil)
+		if i < maxSubscribers {
+			require.NoError(t, err)
+			wss[i] = ws
+			// Check that it's completely ready.
+			doSomeWSRequest(t, ws)
+		} else {
+			require.Error(t, err)
+		}
+	}
+	// Check connections are still alive (it actually is necessary to add
+	// some use of wss to keep connections alive).
+	for i := 0; i < len(wss); i++ {
+		doSomeWSRequest(t, wss[i])
+	}
+}

pkg/smartcontract/parameter.go

@@ -85,6 +85,8 @@ func (p *Parameter) MarshalJSON() ([]byte, error) {
 	case MapType:
 		ppair := p.Value.([]ParameterPair)
 		resultRawValue, resultErr = json.Marshal(ppair)
+	case InteropInterfaceType:
+		resultRawValue = []byte("null")
 	default:
 		resultErr = errors.Errorf("Marshaller for type %s not implemented", p.Type)
 	}
@@ -166,6 +168,9 @@ func (p *Parameter) UnmarshalJSON(data []byte) (err error) {
 			return
 		}
 		p.Value = h
+	case InteropInterfaceType:
+		// stub, ignore value, it can only be null
+		p.Value = nil
 	default:
 		return errors.Errorf("Unmarshaller for type %s not implemented", p.Type)
 	}

pkg/smartcontract/parameter_test.go

@@ -122,6 +122,13 @@ var marshalJSONTestCases = []struct {
 		},
 		result: `{"type":"Hash256","value":"0xf037308fa0ab18155bccfc08485468c112409ea5064595699e98c545f245f32d"}`,
 	},
+	{
+		input: Parameter{
+			Type:  InteropInterfaceType,
+			Value: nil,
+		},
+		result: `{"type":"InteropInterface","value":null}`,
+	},
 }
 
 var marshalJSONErrorCases = []Parameter{
@@ -129,10 +136,6 @@ var marshalJSONErrorCases = []Parameter{
 		Type:  UnknownType,
 		Value: nil,
 	},
-	{
-		Type:  InteropInterfaceType,
-		Value: nil,
-	},
 	{
 		Type:  IntegerType,
 		Value: math.Inf(1),
@@ -252,6 +255,27 @@ var unmarshalJSONTestCases = []struct {
 		},
 		input: `{"type":"PublicKey","value":"03b3bf1502fbdc05449b506aaf04579724024b06542e49262bfaa3f70e200040a9"}`,
 	},
+	{
+		input: `{"type":"InteropInterface","value":null}`,
+		result: Parameter{
+			Type:  InteropInterfaceType,
+			Value: nil,
+		},
+	},
+	{
+		input: `{"type":"InteropInterface","value":""}`,
+		result: Parameter{
+			Type:  InteropInterfaceType,
+			Value: nil,
+		},
+	},
+	{
+		input: `{"type":"InteropInterface","value":"Hundertwasser"}`,
+		result: Parameter{
+			Type:  InteropInterfaceType,
+			Value: nil,
+		},
+	},
 }
 
 var unmarshalJSONErrorCases = []string{
@@ -272,8 +296,6 @@ var unmarshalJSONErrorCases = []string{
 	`{"type": "Map","value": ["key": {}]}`, // incorrect Map value
 	`{"type": "Map","value": ["key": {"type":"String", "value":"qwer"}, "value": {"type":"Boolean"}]}`, // incorrect Map Value value
 	`{"type": "Map","value": ["key": {"type":"String"}, "value": {"type":"Boolean", "value":true}]}`,   // incorrect Map Key value
-
-	`{"type": "InteropInterface","value": ""}`, // ununmarshable type
 }
 
 func TestParam_UnmarshalJSON(t *testing.T) {

pkg/wallet/account.go

@@ -132,6 +132,9 @@ func (a *Account) SignTx(t *transaction.Transaction) error {
 		return errors.New("account is not unlocked")
 	}
 	data := t.GetSignedPart()
+	if data == nil {
+		return errors.New("failed to get transaction's signed part")
+	}
 	sign := a.privateKey.Sign(data)
 
 	t.Scripts = append(t.Scripts, transaction.Witness{