consensus, miner: stale block mining support (#17506)

* consensus, miner: stale block supporting

* consensus, miner: refactor seal signature

* cmd, consensus, eth: add miner noverify flag

* cmd, consensus, miner: polish

7 files changed, 244 insertions, 107 deletions

diff --git a/consensus/clique/clique.go b/consensus/clique/clique.go
index 3730c91f6..547290984 100644
--- a/consensus/clique/clique.go
+++ b/consensus/clique/clique.go
@@ -590,17 +590,17 @@ func (c *Clique) Authorize(signer common.Address, signFn SignerFn) {
 
 // Seal implements consensus.Engine, attempting to create a sealed block using
 // the local signing credentials.
-func (c *Clique) Seal(chain consensus.ChainReader, block *types.Block, stop <-chan struct{}) (*types.Block, error) {
+func (c *Clique) Seal(chain consensus.ChainReader, block *types.Block, results chan<- *types.Block, stop <-chan struct{}) error {
 	header := block.Header()
 
 	// Sealing the genesis block is not supported
 	number := header.Number.Uint64()
 	if number == 0 {
-		return nil, errUnknownBlock
+		return errUnknownBlock
 	}
 	// For 0-period chains, refuse to seal empty blocks (no reward but would spin sealing)
 	if c.config.Period == 0 && len(block.Transactions()) == 0 {
-		return nil, errWaitTransactions
+		return errWaitTransactions
 	}
 	// Don't hold the signer fields for the entire sealing procedure
 	c.lock.RLock()
@@ -610,10 +610,10 @@ func (c *Clique) Seal(chain consensus.ChainReader, block *types.Block, stop <-ch
 	// Bail out if we're unauthorized to sign a block
 	snap, err := c.snapshot(chain, number-1, header.ParentHash, nil)
 	if err != nil {
-		return nil, err
+		return err
 	}
 	if _, authorized := snap.Signers[signer]; !authorized {
-		return nil, errUnauthorized
+		return errUnauthorized
 	}
 	// If we're amongst the recent signers, wait for the next block
 	for seen, recent := range snap.Recents {
@@ -621,8 +621,7 @@ func (c *Clique) Seal(chain consensus.ChainReader, block *types.Block, stop <-ch
 			// Signer is among recents, only wait if the current block doesn't shift it out
 			if limit := uint64(len(snap.Signers)/2 + 1); number < limit || seen > number-limit {
 				log.Info("Signed recently, must wait for others")
-				<-stop
-				return nil, nil
+				return nil
 			}
 		}
 	}
@@ -635,21 +634,29 @@ func (c *Clique) Seal(chain consensus.ChainReader, block *types.Block, stop <-ch
 
 		log.Trace("Out-of-turn signing requested", "wiggle", common.PrettyDuration(wiggle))
 	}
-	log.Trace("Waiting for slot to sign and propagate", "delay", common.PrettyDuration(delay))
-
-	select {
-	case <-stop:
-		return nil, nil
-	case <-time.After(delay):
-	}
 	// Sign all the things!
 	sighash, err := signFn(accounts.Account{Address: signer}, sigHash(header).Bytes())
 	if err != nil {
-		return nil, err
+		return err
 	}
 	copy(header.Extra[len(header.Extra)-extraSeal:], sighash)
+	// Wait until sealing is terminated or delay timeout.
+	log.Trace("Waiting for slot to sign and propagate", "delay", common.PrettyDuration(delay))
+	go func() {
+		select {
+		case <-stop:
+			return
+		case <-time.After(delay):
+		}
 
-	return block.WithSeal(header), nil
+		select {
+		case results <- block.WithSeal(header):
+		default:
+			log.Warn("Sealing result is not read by miner", "sealhash", c.SealHash(header))
+		}
+	}()
+
+	return nil
 }
 
 // CalcDifficulty is the difficulty adjustment algorithm. It returns the difficulty
diff --git a/consensus/consensus.go b/consensus/consensus.go
index 27799f13c..12ede7ff4 100644
--- a/consensus/consensus.go
+++ b/consensus/consensus.go
@@ -86,9 +86,12 @@ type Engine interface {
 	Finalize(chain ChainReader, header *types.Header, state *state.StateDB, txs []*types.Transaction,
 		uncles []*types.Header, receipts []*types.Receipt) (*types.Block, error)
 
-	// Seal generates a new block for the given input block with the local miner's
-	// seal place on top.
-	Seal(chain ChainReader, block *types.Block, stop <-chan struct{}) (*types.Block, error)
+	// Seal generates a new sealing request for the given input block and pushes
+	// the result into the given channel.
+	//
+	// Note, the method returns immediately and will send the result async. More
+	// than one result may also be returned depending on the consensus algorothm.
+	Seal(chain ChainReader, block *types.Block, results chan<- *types.Block, stop <-chan struct{}) error
 
 	// SealHash returns the hash of a block prior to it being sealed.
 	SealHash(header *types.Header) common.Hash
diff --git a/consensus/ethash/algorithm_test.go b/consensus/ethash/algorithm_test.go
index db22cccd0..c58479e28 100644
--- a/consensus/ethash/algorithm_test.go
+++ b/consensus/ethash/algorithm_test.go
@@ -729,7 +729,7 @@ func TestConcurrentDiskCacheGeneration(t *testing.T) {
 
 		go func(idx int) {
 			defer pend.Done()
-			ethash := New(Config{cachedir, 0, 1, "", 0, 0, ModeNormal}, nil)
+			ethash := New(Config{cachedir, 0, 1, "", 0, 0, ModeNormal}, nil, false)
 			defer ethash.Close()
 			if err := ethash.VerifySeal(nil, block.Header()); err != nil {
 				t.Errorf("proc %d: block verification failed: %v", idx, err)
diff --git a/consensus/ethash/ethash.go b/consensus/ethash/ethash.go
index d98c3371c..b4819ca38 100644
--- a/consensus/ethash/ethash.go
+++ b/consensus/ethash/ethash.go
@@ -50,7 +50,7 @@ var (
 	two256 = new(big.Int).Exp(big.NewInt(2), big.NewInt(256), big.NewInt(0))
 
 	// sharedEthash is a full instance that can be shared between multiple users.
-	sharedEthash = New(Config{"", 3, 0, "", 1, 0, ModeNormal}, nil)
+	sharedEthash = New(Config{"", 3, 0, "", 1, 0, ModeNormal}, nil, false)
 
 	// algorithmRevision is the data structure version used for file naming.
 	algorithmRevision = 23
@@ -405,6 +405,12 @@ type Config struct {
 	PowMode        Mode
 }
 
+// sealTask wraps a seal block with relative result channel for remote sealer thread.
+type sealTask struct {
+	block   *types.Block
+	results chan<- *types.Block
+}
+
 // mineResult wraps the pow solution parameters for the specified block.
 type mineResult struct {
 	nonce     types.BlockNonce
@@ -444,12 +450,11 @@ type Ethash struct {
 	hashrate metrics.Meter // Meter tracking the average hashrate
 
 	// Remote sealer related fields
-	workCh       chan *types.Block // Notification channel to push new work to remote sealer
-	resultCh     chan *types.Block // Channel used by mining threads to return result
-	fetchWorkCh  chan *sealWork    // Channel used for remote sealer to fetch mining work
-	submitWorkCh chan *mineResult  // Channel used for remote sealer to submit their mining result
-	fetchRateCh  chan chan uint64  // Channel used to gather submitted hash rate for local or remote sealer.
-	submitRateCh chan *hashrate    // Channel used for remote sealer to submit their mining hashrate
+	workCh       chan *sealTask   // Notification channel to push new work and relative result channel to remote sealer
+	fetchWorkCh  chan *sealWork   // Channel used for remote sealer to fetch mining work
+	submitWorkCh chan *mineResult // Channel used for remote sealer to submit their mining result
+	fetchRateCh  chan chan uint64 // Channel used to gather submitted hash rate for local or remote sealer.
+	submitRateCh chan *hashrate   // Channel used for remote sealer to submit their mining hashrate
 
 	// The fields below are hooks for testing
 	shared    *Ethash       // Shared PoW verifier to avoid cache regeneration
@@ -464,7 +469,7 @@ type Ethash struct {
 // New creates a full sized ethash PoW scheme and starts a background thread for
 // remote mining, also optionally notifying a batch of remote services of new work
 // packages.
-func New(config Config, notify []string) *Ethash {
+func New(config Config, notify []string, noverify bool) *Ethash {
 	if config.CachesInMem <= 0 {
 		log.Warn("One ethash cache must always be in memory", "requested", config.CachesInMem)
 		config.CachesInMem = 1
@@ -481,36 +486,34 @@ func New(config Config, notify []string) *Ethash {
 		datasets:     newlru("dataset", config.DatasetsInMem, newDataset),
 		update:       make(chan struct{}),
 		hashrate:     metrics.NewMeter(),
-		workCh:       make(chan *types.Block),
-		resultCh:     make(chan *types.Block),
+		workCh:       make(chan *sealTask),
 		fetchWorkCh:  make(chan *sealWork),
 		submitWorkCh: make(chan *mineResult),
 		fetchRateCh:  make(chan chan uint64),
 		submitRateCh: make(chan *hashrate),
 		exitCh:       make(chan chan error),
 	}
-	go ethash.remote(notify)
+	go ethash.remote(notify, noverify)
 	return ethash
 }
 
 // NewTester creates a small sized ethash PoW scheme useful only for testing
 // purposes.
-func NewTester(notify []string) *Ethash {
+func NewTester(notify []string, noverify bool) *Ethash {
 	ethash := &Ethash{
 		config:       Config{PowMode: ModeTest},
 		caches:       newlru("cache", 1, newCache),
 		datasets:     newlru("dataset", 1, newDataset),
 		update:       make(chan struct{}),
 		hashrate:     metrics.NewMeter(),
-		workCh:       make(chan *types.Block),
-		resultCh:     make(chan *types.Block),
+		workCh:       make(chan *sealTask),
 		fetchWorkCh:  make(chan *sealWork),
 		submitWorkCh: make(chan *mineResult),
 		fetchRateCh:  make(chan chan uint64),
 		submitRateCh: make(chan *hashrate),
 		exitCh:       make(chan chan error),
 	}
-	go ethash.remote(notify)
+	go ethash.remote(notify, noverify)
 	return ethash
 }
 
diff --git a/consensus/ethash/ethash_test.go b/consensus/ethash/ethash_test.go
index b190d63d6..8eded2ca8 100644
--- a/consensus/ethash/ethash_test.go
+++ b/consensus/ethash/ethash_test.go
@@ -34,17 +34,23 @@ import (
 func TestTestMode(t *testing.T) {
 	header := &types.Header{Number: big.NewInt(1), Difficulty: big.NewInt(100)}
 
-	ethash := NewTester(nil)
+	ethash := NewTester(nil, false)
 	defer ethash.Close()
 
-	block, err := ethash.Seal(nil, types.NewBlockWithHeader(header), nil)
+	results := make(chan *types.Block)
+	err := ethash.Seal(nil, types.NewBlockWithHeader(header), results, nil)
 	if err != nil {
 		t.Fatalf("failed to seal block: %v", err)
 	}
-	header.Nonce = types.EncodeNonce(block.Nonce())
-	header.MixDigest = block.MixDigest()
-	if err := ethash.VerifySeal(nil, header); err != nil {
-		t.Fatalf("unexpected verification error: %v", err)
+	select {
+	case block := <-results:
+		header.Nonce = types.EncodeNonce(block.Nonce())
+		header.MixDigest = block.MixDigest()
+		if err := ethash.VerifySeal(nil, header); err != nil {
+			t.Fatalf("unexpected verification error: %v", err)
+		}
+	case <-time.NewTimer(time.Second).C:
+		t.Error("sealing result timeout")
 	}
 }
 
@@ -56,7 +62,7 @@ func TestCacheFileEvict(t *testing.T) {
 		t.Fatal(err)
 	}
 	defer os.RemoveAll(tmpdir)
-	e := New(Config{CachesInMem: 3, CachesOnDisk: 10, CacheDir: tmpdir, PowMode: ModeTest}, nil)
+	e := New(Config{CachesInMem: 3, CachesOnDisk: 10, CacheDir: tmpdir, PowMode: ModeTest}, nil, false)
 	defer e.Close()
 
 	workers := 8
@@ -85,7 +91,7 @@ func verifyTest(wg *sync.WaitGroup, e *Ethash, workerIndex, epochs int) {
 }
 
 func TestRemoteSealer(t *testing.T) {
-	ethash := NewTester(nil)
+	ethash := NewTester(nil, false)
 	defer ethash.Close()
 
 	api := &API{ethash}
@@ -97,7 +103,8 @@ func TestRemoteSealer(t *testing.T) {
 	sealhash := ethash.SealHash(header)
 
 	// Push new work.
-	ethash.Seal(nil, block, nil)
+	results := make(chan *types.Block)
+	ethash.Seal(nil, block, results, nil)
 
 	var (
 		work [3]string
@@ -114,20 +121,11 @@ func TestRemoteSealer(t *testing.T) {
 	header = &types.Header{Number: big.NewInt(1), Difficulty: big.NewInt(1000)}
 	block = types.NewBlockWithHeader(header)
 	sealhash = ethash.SealHash(header)
-	ethash.Seal(nil, block, nil)
+	ethash.Seal(nil, block, results, nil)
 
 	if work, err = api.GetWork(); err != nil || work[0] != sealhash.Hex() {
 		t.Error("expect to return the latest pushed work")
 	}
-	// Push block with higher block number.
-	newHead := &types.Header{Number: big.NewInt(2), Difficulty: big.NewInt(100)}
-	newBlock := types.NewBlockWithHeader(newHead)
-	newSealhash := ethash.SealHash(newHead)
-	ethash.Seal(nil, newBlock, nil)
-
-	if res := api.SubmitWork(types.BlockNonce{}, newSealhash, common.Hash{}); res {
-		t.Error("expect to return false when submit a stale solution")
-	}
 }
 
 func TestHashRate(t *testing.T) {
@@ -136,7 +134,7 @@ func TestHashRate(t *testing.T) {
 		expect   uint64
 		ids      = []common.Hash{common.HexToHash("a"), common.HexToHash("b"), common.HexToHash("c")}
 	)
-	ethash := NewTester(nil)
+	ethash := NewTester(nil, false)
 	defer ethash.Close()
 
 	if tot := ethash.Hashrate(); tot != 0 {
@@ -156,7 +154,7 @@ func TestHashRate(t *testing.T) {
 }
 
 func TestClosedRemoteSealer(t *testing.T) {
-	ethash := NewTester(nil)
+	ethash := NewTester(nil, false)
 	time.Sleep(1 * time.Second) // ensure exit channel is listening
 	ethash.Close()
 
diff --git a/consensus/ethash/sealer.go b/consensus/ethash/sealer.go
index a458c60f6..06c98a781 100644
--- a/consensus/ethash/sealer.go
+++ b/consensus/ethash/sealer.go
@@ -35,6 +35,11 @@ import (
 	"github.com/ethereum/go-ethereum/log"
 )
 
+const (
+	// staleThreshold is the maximum depth of the acceptable stale but valid ethash solution.
+	staleThreshold = 7
+)
+
 var (
 	errNoMiningWork      = errors.New("no mining work available yet")
 	errInvalidSealResult = errors.New("invalid or stale proof-of-work solution")
@@ -42,16 +47,21 @@ var (
 
 // Seal implements consensus.Engine, attempting to find a nonce that satisfies
 // the block's difficulty requirements.
-func (ethash *Ethash) Seal(chain consensus.ChainReader, block *types.Block, stop <-chan struct{}) (*types.Block, error) {
+func (ethash *Ethash) Seal(chain consensus.ChainReader, block *types.Block, results chan<- *types.Block, stop <-chan struct{}) error {
 	// If we're running a fake PoW, simply return a 0 nonce immediately
 	if ethash.config.PowMode == ModeFake || ethash.config.PowMode == ModeFullFake {
 		header := block.Header()
 		header.Nonce, header.MixDigest = types.BlockNonce{}, common.Hash{}
-		return block.WithSeal(header), nil
+		select {
+		case results <- block.WithSeal(header):
+		default:
+			log.Warn("Sealing result is not read by miner", "mode", "fake", "sealhash", ethash.SealHash(block.Header()))
+		}
+		return nil
 	}
 	// If we're running a shared PoW, delegate sealing to it
 	if ethash.shared != nil {
-		return ethash.shared.Seal(chain, block, stop)
+		return ethash.shared.Seal(chain, block, results, stop)
 	}
 	// Create a runner and the multiple search threads it directs
 	abort := make(chan struct{})
@@ -62,7 +72,7 @@ func (ethash *Ethash) Seal(chain consensus.ChainReader, block *types.Block, stop
 		seed, err := crand.Int(crand.Reader, big.NewInt(math.MaxInt64))
 		if err != nil {
 			ethash.lock.Unlock()
-			return nil, err
+			return err
 		}
 		ethash.rand = rand.New(rand.NewSource(seed.Int64()))
 	}
@@ -75,34 +85,45 @@ func (ethash *Ethash) Seal(chain consensus.ChainReader, block *types.Block, stop
 	}
 	// Push new work to remote sealer
 	if ethash.workCh != nil {
-		ethash.workCh <- block
+		ethash.workCh <- &sealTask{block: block, results: results}
 	}
-	var pend sync.WaitGroup
+	var (
+		pend   sync.WaitGroup
+		locals = make(chan *types.Block)
+	)
 	for i := 0; i < threads; i++ {
 		pend.Add(1)
 		go func(id int, nonce uint64) {
 			defer pend.Done()
-			ethash.mine(block, id, nonce, abort, ethash.resultCh)
+			ethash.mine(block, id, nonce, abort, locals)
 		}(i, uint64(ethash.rand.Int63()))
 	}
 	// Wait until sealing is terminated or a nonce is found
-	var result *types.Block
-	select {
-	case <-stop:
-		// Outside abort, stop all miner threads
-		close(abort)
-	case result = <-ethash.resultCh:
-		// One of the threads found a block, abort all others
-		close(abort)
-	case <-ethash.update:
-		// Thread count was changed on user request, restart
-		close(abort)
+	go func() {
+		var result *types.Block
+		select {
+		case <-stop:
+			// Outside abort, stop all miner threads
+			close(abort)
+		case result = <-locals:
+			// One of the threads found a block, abort all others
+			select {
+			case results <- result:
+			default:
+				log.Warn("Sealing result is not read by miner", "mode", "local", "sealhash", ethash.SealHash(block.Header()))
+			}
+			close(abort)
+		case <-ethash.update:
+			// Thread count was changed on user request, restart
+			close(abort)
+			if err := ethash.Seal(chain, block, results, stop); err != nil {
+				log.Error("Failed to restart sealing after update", "err", err)
+			}
+		}
+		// Wait for all miners to terminate and return the block
 		pend.Wait()
-		return ethash.Seal(chain, block, stop)
-	}
-	// Wait for all miners to terminate and return the block
-	pend.Wait()
-	return result, nil
+	}()
+	return nil
 }
 
 // mine is the actual proof-of-work miner that searches for a nonce starting from
@@ -165,11 +186,12 @@ search:
 }
 
 // remote is a standalone goroutine to handle remote mining related stuff.
-func (ethash *Ethash) remote(notify []string) {
+func (ethash *Ethash) remote(notify []string, noverify bool) {
 	var (
 		works = make(map[common.Hash]*types.Block)
 		rates = make(map[common.Hash]hashrate)
 
+		results      chan<- *types.Block
 		currentBlock *types.Block
 		currentWork  [3]string
 
@@ -226,11 +248,15 @@ func (ethash *Ethash) remote(notify []string) {
 	// submitWork verifies the submitted pow solution, returning
 	// whether the solution was accepted or not (not can be both a bad pow as well as
 	// any other error, like no pending work or stale mining result).
-	submitWork := func(nonce types.BlockNonce, mixDigest common.Hash, hash common.Hash) bool {
+	submitWork := func(nonce types.BlockNonce, mixDigest common.Hash, sealhash common.Hash) bool {
+		if currentBlock == nil {
+			log.Error("Pending work without block", "sealhash", sealhash)
+			return false
+		}
 		// Make sure the work submitted is present
-		block := works[hash]
+		block := works[sealhash]
 		if block == nil {
-			log.Info("Work submitted but none pending", "hash", hash)
+			log.Warn("Work submitted but none pending", "sealhash", sealhash, "curnumber", currentBlock.NumberU64())
 			return false
 		}
 		// Verify the correctness of submitted result.
@@ -239,26 +265,36 @@ func (ethash *Ethash) remote(notify []string) {
 		header.MixDigest = mixDigest
 
 		start := time.Now()
-		if err := ethash.verifySeal(nil, header, true); err != nil {
-			log.Warn("Invalid proof-of-work submitted", "hash", hash, "elapsed", time.Since(start), "err", err)
-			return false
+		if !noverify {
+			if err := ethash.verifySeal(nil, header, true); err != nil {
+				log.Warn("Invalid proof-of-work submitted", "sealhash", sealhash, "elapsed", time.Since(start), "err", err)
+				return false
+			}
 		}
-		// Make sure the result channel is created.
-		if ethash.resultCh == nil {
+		// Make sure the result channel is assigned.
+		if results == nil {
 			log.Warn("Ethash result channel is empty, submitted mining result is rejected")
 			return false
 		}
-		log.Trace("Verified correct proof-of-work", "hash", hash, "elapsed", time.Since(start))
+		log.Trace("Verified correct proof-of-work", "sealhash", sealhash, "elapsed", time.Since(start))
 
 		// Solutions seems to be valid, return to the miner and notify acceptance.
-		select {
-		case ethash.resultCh <- block.WithSeal(header):
-			delete(works, hash)
-			return true
-		default:
-			log.Info("Work submitted is stale", "hash", hash)
-			return false
+		solution := block.WithSeal(header)
+
+		// The submitted solution is within the scope of acceptance.
+		if solution.NumberU64()+staleThreshold > currentBlock.NumberU64() {
+			select {
+			case results <- solution:
+				log.Debug("Work submitted is acceptable", "number", solution.NumberU64(), "sealhash", sealhash, "hash", solution.Hash())
+				return true
+			default:
+				log.Warn("Sealing result is not read by miner", "mode", "remote", "sealhash", sealhash)
+				return false
+			}
 		}
+		// The submitted block is too old to accept, drop it.
+		log.Warn("Work submitted is too old", "number", solution.NumberU64(), "sealhash", sealhash, "hash", solution.Hash())
+		return false
 	}
 
 	ticker := time.NewTicker(5 * time.Second)
@@ -266,14 +302,12 @@ func (ethash *Ethash) remote(notify []string) {
 
 	for {
 		select {
-		case block := <-ethash.workCh:
-			if currentBlock != nil && block.ParentHash() != currentBlock.ParentHash() {
-				// Start new round mining, throw out all previous work.
-				works = make(map[common.Hash]*types.Block)
-			}
+		case work := <-ethash.workCh:
 			// Update current work with new received block.
 			// Note same work can be past twice, happens when changing CPU threads.
-			makeWork(block)
+			results = work.results
+
+			makeWork(work.block)
 
 			// Notify and requested URLs of the new work availability
 			notifyWork()
@@ -315,6 +349,14 @@ func (ethash *Ethash) remote(notify []string) {
 					delete(rates, id)
 				}
 			}
+			// Clear stale pending blocks
+			if currentBlock != nil {
+				for hash, block := range works {
+					if block.NumberU64()+staleThreshold <= currentBlock.NumberU64() {
+						delete(works, hash)
+					}
+				}
+			}
 
 		case errc := <-ethash.exitCh:
 			// Exit remote loop if ethash is closed and return relevant error.
diff --git a/consensus/ethash/sealer_test.go b/consensus/ethash/sealer_test.go
index d1b66f9cf..31d18b67c 100644
--- a/consensus/ethash/sealer_test.go
+++ b/consensus/ethash/sealer_test.go
@@ -41,14 +41,14 @@ func TestRemoteNotify(t *testing.T) {
 	go server.Serve(listener)
 
 	// Create the custom ethash engine
-	ethash := NewTester([]string{"http://" + listener.Addr().String()})
+	ethash := NewTester([]string{"http://" + listener.Addr().String()}, false)
 	defer ethash.Close()
 
 	// Stream a work task and ensure the notification bubbles out
 	header := &types.Header{Number: big.NewInt(1), Difficulty: big.NewInt(100)}
 	block := types.NewBlockWithHeader(header)
 
-	ethash.Seal(nil, block, nil)
+	ethash.Seal(nil, block, nil, nil)
 	select {
 	case work := <-sink:
 		if want := ethash.SealHash(header).Hex(); work[0] != want {
@@ -66,7 +66,7 @@ func TestRemoteNotify(t *testing.T) {
 	}
 }
 
-// Tests that pushing work packages fast to the miner doesn't cause any daa race
+// Tests that pushing work packages fast to the miner doesn't cause any data race
 // issues in the notifications.
 func TestRemoteMultiNotify(t *testing.T) {
 	// Start a simple webserver to capture notifications
@@ -95,7 +95,7 @@ func TestRemoteMultiNotify(t *testing.T) {
 	go server.Serve(listener)
 
 	// Create the custom ethash engine
-	ethash := NewTester([]string{"http://" + listener.Addr().String()})
+	ethash := NewTester([]string{"http://" + listener.Addr().String()}, false)
 	defer ethash.Close()
 
 	// Stream a lot of work task and ensure all the notifications bubble out
@@ -103,7 +103,7 @@ func TestRemoteMultiNotify(t *testing.T) {
 		header := &types.Header{Number: big.NewInt(int64(i)), Difficulty: big.NewInt(100)}
 		block := types.NewBlockWithHeader(header)
 
-		ethash.Seal(nil, block, nil)
+		ethash.Seal(nil, block, nil, nil)
 	}
 	for i := 0; i < cap(sink); i++ {
 		select {
@@ -113,3 +113,87 @@ func TestRemoteMultiNotify(t *testing.T) {
 		}
 	}
 }
+
+// Tests whether stale solutions are correctly processed.
+func TestStaleSubmission(t *testing.T) {
+	ethash := NewTester(nil, true)
+	defer ethash.Close()
+	api := &API{ethash}
+
+	fakeNonce, fakeDigest := types.BlockNonce{0x01, 0x02, 0x03}, common.HexToHash("deadbeef")
+
+	testcases := []struct {
+		headers     []*types.Header
+		submitIndex int
+		submitRes   bool
+	}{
+		// Case1: submit solution for the latest mining package
+		{
+			[]*types.Header{
+				{ParentHash: common.BytesToHash([]byte{0xa}), Number: big.NewInt(1), Difficulty: big.NewInt(100000000)},
+			},
+			0,
+			true,
+		},
+		// Case2: submit solution for the previous package but have same parent.
+		{
+			[]*types.Header{
+				{ParentHash: common.BytesToHash([]byte{0xb}), Number: big.NewInt(2), Difficulty: big.NewInt(100000000)},
+				{ParentHash: common.BytesToHash([]byte{0xb}), Number: big.NewInt(2), Difficulty: big.NewInt(100000001)},
+			},
+			0,
+			true,
+		},
+		// Case3: submit stale but acceptable solution
+		{
+			[]*types.Header{
+				{ParentHash: common.BytesToHash([]byte{0xc}), Number: big.NewInt(3), Difficulty: big.NewInt(100000000)},
+				{ParentHash: common.BytesToHash([]byte{0xd}), Number: big.NewInt(9), Difficulty: big.NewInt(100000000)},
+			},
+			0,
+			true,
+		},
+		// Case4: submit very old solution
+		{
+			[]*types.Header{
+				{ParentHash: common.BytesToHash([]byte{0xe}), Number: big.NewInt(10), Difficulty: big.NewInt(100000000)},
+				{ParentHash: common.BytesToHash([]byte{0xf}), Number: big.NewInt(17), Difficulty: big.NewInt(100000000)},
+			},
+			0,
+			false,
+		},
+	}
+	results := make(chan *types.Block, 16)
+
+	for id, c := range testcases {
+		for _, h := range c.headers {
+			ethash.Seal(nil, types.NewBlockWithHeader(h), results, nil)
+		}
+		if res := api.SubmitWork(fakeNonce, ethash.SealHash(c.headers[c.submitIndex]), fakeDigest); res != c.submitRes {
+			t.Errorf("case %d submit result mismatch, want %t, get %t", id+1, c.submitRes, res)
+		}
+		if !c.submitRes {
+			continue
+		}
+		select {
+		case res := <-results:
+			if res.Header().Nonce != fakeNonce {
+				t.Errorf("case %d block nonce mismatch, want %s, get %s", id+1, fakeNonce, res.Header().Nonce)
+			}
+			if res.Header().MixDigest != fakeDigest {
+				t.Errorf("case %d block digest mismatch, want %s, get %s", id+1, fakeDigest, res.Header().MixDigest)
+			}
+			if res.Header().Difficulty.Uint64() != c.headers[c.submitIndex].Difficulty.Uint64() {
+				t.Errorf("case %d block difficulty mismatch, want %d, get %d", id+1, c.headers[c.submitIndex].Difficulty, res.Header().Difficulty)
+			}
+			if res.Header().Number.Uint64() != c.headers[c.submitIndex].Number.Uint64() {
+				t.Errorf("case %d block number mismatch, want %d, get %d", id+1, c.headers[c.submitIndex].Number.Uint64(), res.Header().Number.Uint64())
+			}
+			if res.Header().ParentHash != c.headers[c.submitIndex].ParentHash {
+				t.Errorf("case %d block parent hash mismatch, want %s, get %s", id+1, c.headers[c.submitIndex].ParentHash.Hex(), res.Header().ParentHash.Hex())
+			}
+		case <-time.NewTimer(time.Second).C:
+			t.Errorf("case %d fetch ethash result timeout", id+1)
+		}
+	}
+}