eth/downloader: make fast sync resilient to critical section fails

eth/downloader/downloader.go

@@ -73,6 +73,7 @@ var (
 	fsHeaderForceVerify    = 24   // Number of headers to verify before and after the pivot to accept it
 	fsPivotInterval        = 512  // Number of headers out of which to randomize the pivot point
 	fsMinFullBlocks        = 1024 // Number of blocks to retrieve fully even in fast sync
+	fsCriticalTrials       = 10   // Number of times to retry in the cricical section before bailing
 )
 
 var (
@@ -103,13 +104,15 @@ var (
 )
 
 type Downloader struct {
-	mode   SyncMode       // Synchronisation mode defining the strategy used (per sync cycle)
+	mode SyncMode       // Synchronisation mode defining the strategy used (per sync cycle)
-	noFast bool           // Flag to disable fast syncing in case of a security error
+	mux  *event.TypeMux // Event multiplexer to announce sync operation events
-	mux    *event.TypeMux // Event multiplexer to announce sync operation events
 
 	queue *queue   // Scheduler for selecting the hashes to download
 	peers *peerSet // Set of active peers from which download can proceed
 
+	fsPivotLock  *types.Header // Pivot header on critical section entry (cannot change between retries)
+	fsPivotFails int           // Number of fast sync failures in the critical section
+
 	interrupt int32 // Atomic boolean to signal termination
 
 	// Statistics
@@ -314,6 +317,15 @@ func (d *Downloader) synchronise(id string, hash common.Hash, td *big.Int, mode
 		default:
 		}
 	}
+	for _, ch := range []chan dataPack{d.hashCh, d.blockCh, d.headerCh, d.bodyCh, d.receiptCh, d.stateCh} {
+		for empty := false; !empty; {
+			select {
+			case <-ch:
+			default:
+				empty = true
+			}
+		}
+	}
 	for empty := false; !empty; {
 		select {
 		case <-d.headerProcCh:
@@ -330,7 +342,7 @@ func (d *Downloader) synchronise(id string, hash common.Hash, td *big.Int, mode
 
 	// Set the requested sync mode, unless it's forbidden
 	d.mode = mode
-	if d.mode == FastSync && d.noFast {
+	if d.mode == FastSync && d.fsPivotFails >= fsCriticalTrials {
 		d.mode = FullSync
 	}
 	// Retrieve the origin peer and initiate the downloading process
@@ -413,12 +425,17 @@ func (d *Downloader) syncWithPeer(p *peer, hash common.Hash, td *big.Int) (err e
 			pivot = height
 		case FastSync:
 			// Calculate the new fast/slow sync pivot point
-			pivotOffset, err := rand.Int(rand.Reader, big.NewInt(int64(fsPivotInterval)))
+			if d.fsPivotLock == nil {
-			if err != nil {
+				pivotOffset, err := rand.Int(rand.Reader, big.NewInt(int64(fsPivotInterval)))
-				panic(fmt.Sprintf("Failed to access crypto random source: %v", err))
+				if err != nil {
-			}
+					panic(fmt.Sprintf("Failed to access crypto random source: %v", err))
-			if height > uint64(fsMinFullBlocks)+pivotOffset.Uint64() {
+				}
-				pivot = height - uint64(fsMinFullBlocks) - pivotOffset.Uint64()
+				if height > uint64(fsMinFullBlocks)+pivotOffset.Uint64() {
+					pivot = height - uint64(fsMinFullBlocks) - pivotOffset.Uint64()
+				}
+			} else {
+				// Pivot point locked in, use this and do not pick a new one!
+				pivot = d.fsPivotLock.Number.Uint64()
 			}
 			// If the point is below the origin, move origin back to ensure state download
 			if pivot < origin {
@@ -1218,8 +1235,12 @@ func (d *Downloader) fetchHeaders(p *peer, from uint64) error {
 			// If no more headers are inbound, notify the content fetchers and return
 			if packet.Items() == 0 {
 				glog.V(logger.Debug).Infof("%v: no available headers", p)
-				d.headerProcCh <- nil
+				select {
-				return nil
+				case d.headerProcCh <- nil:
+					return nil
+				case <-d.cancelCh:
+					return errCancelHeaderFetch
+				}
 			}
 			headers := packet.(*headerPack).headers
 
@@ -1611,9 +1632,18 @@ func (d *Downloader) processHeaders(origin uint64, td *big.Int) error {
 			glog.V(logger.Warn).Infof("Rolled back %d headers (LH: %d->%d, FB: %d->%d, LB: %d->%d)",
 				len(hashes), lastHeader, d.headHeader().Number, lastFastBlock, d.headFastBlock().Number(), lastBlock, d.headBlock().Number())
 
-			// If we're already past the pivot point, this could be an attack, disable fast sync
+			// If we're already past the pivot point, this could be an attack, thread carefully
 			if rollback[len(rollback)-1].Number.Uint64() > pivot {
-				d.noFast = true
+				// If we didn't ever fail, lock in te pivot header (must! not! change!)
+				if d.fsPivotFails == 0 {
+					for _, header := range rollback {
+						if header.Number.Uint64() == pivot {
+							glog.V(logger.Warn).Infof("Fast-sync critical section failure, locked pivot to header #%d [%x…]", pivot, header.Hash().Bytes()[:4])
+							d.fsPivotLock = header
+						}
+					}
+				}
+				d.fsPivotFails++
 			}
 		}
 	}()
@@ -1712,6 +1742,13 @@ func (d *Downloader) processHeaders(origin uint64, td *big.Int) error {
 						rollback = append(rollback[:0], rollback[len(rollback)-fsHeaderSafetyNet:]...)
 					}
 				}
+				// If we're fast syncing and just pulled in the pivot, make sure it's the one locked in
+				if d.mode == FastSync && d.fsPivotLock != nil && chunk[0].Number.Uint64() <= pivot && chunk[len(chunk)-1].Number.Uint64() >= pivot {
+					if pivot := chunk[int(pivot-chunk[0].Number.Uint64())]; pivot.Hash() != d.fsPivotLock.Hash() {
+						glog.V(logger.Warn).Infof("Pivot doesn't match locked in version: have #%v [%x…], want #%v [%x…]", pivot.Number, pivot.Hash().Bytes()[:4], d.fsPivotLock.Number, d.fsPivotLock.Hash().Bytes()[:4])
+						return errInvalidChain
+					}
+				}
 				// Unless we're doing light chains, schedule the headers for associated content retrieval
 				if d.mode == FullSync || d.mode == FastSync {
 					// If we've reached the allowed number of pending headers, stall a bit

eth/downloader/downloader_test.go

@@ -149,22 +149,25 @@ type downloadTester struct {
 	peerReceipts map[string]map[common.Hash]types.Receipts // Receipts belonging to different test peers
 	peerChainTds map[string]map[common.Hash]*big.Int       // Total difficulties of the blocks in the peer chains
 
+	peerMissingStates map[string]map[common.Hash]bool // State entries that fast sync should not return
+
 	lock sync.RWMutex
 }
 
 // newTester creates a new downloader test mocker.
 func newTester() *downloadTester {
 	tester := &downloadTester{
-		ownHashes:    []common.Hash{genesis.Hash()},
+		ownHashes:         []common.Hash{genesis.Hash()},
-		ownHeaders:   map[common.Hash]*types.Header{genesis.Hash(): genesis.Header()},
+		ownHeaders:        map[common.Hash]*types.Header{genesis.Hash(): genesis.Header()},
-		ownBlocks:    map[common.Hash]*types.Block{genesis.Hash(): genesis},
+		ownBlocks:         map[common.Hash]*types.Block{genesis.Hash(): genesis},
-		ownReceipts:  map[common.Hash]types.Receipts{genesis.Hash(): nil},
+		ownReceipts:       map[common.Hash]types.Receipts{genesis.Hash(): nil},
-		ownChainTd:   map[common.Hash]*big.Int{genesis.Hash(): genesis.Difficulty()},
+		ownChainTd:        map[common.Hash]*big.Int{genesis.Hash(): genesis.Difficulty()},
-		peerHashes:   make(map[string][]common.Hash),
+		peerHashes:        make(map[string][]common.Hash),
-		peerHeaders:  make(map[string]map[common.Hash]*types.Header),
+		peerHeaders:       make(map[string]map[common.Hash]*types.Header),
-		peerBlocks:   make(map[string]map[common.Hash]*types.Block),
+		peerBlocks:        make(map[string]map[common.Hash]*types.Block),
-		peerReceipts: make(map[string]map[common.Hash]types.Receipts),
+		peerReceipts:      make(map[string]map[common.Hash]types.Receipts),
-		peerChainTds: make(map[string]map[common.Hash]*big.Int),
+		peerChainTds:      make(map[string]map[common.Hash]*big.Int),
+		peerMissingStates: make(map[string]map[common.Hash]bool),
 	}
 	tester.stateDb, _ = ethdb.NewMemDatabase()
 	tester.stateDb.Put(genesis.Root().Bytes(), []byte{0x00})
@@ -408,6 +411,7 @@ func (dl *downloadTester) newSlowPeer(id string, version int, hashes []common.Ha
 		dl.peerBlocks[id] = make(map[common.Hash]*types.Block)
 		dl.peerReceipts[id] = make(map[common.Hash]types.Receipts)
 		dl.peerChainTds[id] = make(map[common.Hash]*big.Int)
+		dl.peerMissingStates[id] = make(map[common.Hash]bool)
 
 		genesis := hashes[len(hashes)-1]
 		if header := headers[genesis]; header != nil {
@@ -648,7 +652,9 @@ func (dl *downloadTester) peerGetNodeDataFn(id string, delay time.Duration) func
 		results := make([][]byte, 0, len(hashes))
 		for _, hash := range hashes {
 			if data, err := testdb.Get(hash.Bytes()); err == nil {
-				results = append(results, data)
+				if !dl.peerMissingStates[id][hash] {
+					results = append(results, data)
+				}
 			}
 		}
 		go dl.downloader.DeliverNodeData(id, results)
@@ -1288,7 +1294,7 @@ func testInvalidHeaderRollback(t *testing.T, protocol int, mode SyncMode) {
 	tester.newPeer("withhold-attack", protocol, hashes, headers, blocks, receipts)
 	missing = 3*fsHeaderSafetyNet + MaxHeaderFetch + 1
 
-	tester.downloader.noFast = false
+	tester.downloader.fsPivotFails = 0
 	tester.downloader.syncInitHook = func(uint64, uint64) {
 		for i := missing; i <= len(hashes); i++ {
 			delete(tester.peerHeaders["withhold-attack"], hashes[len(hashes)-i])
@@ -1307,6 +1313,8 @@ func testInvalidHeaderRollback(t *testing.T, protocol int, mode SyncMode) {
 			t.Errorf("fast sync pivot block #%d not rolled back", head)
 		}
 	}
+	tester.downloader.fsPivotFails = fsCriticalTrials
+
 	// Synchronise with the valid peer and make sure sync succeeds. Since the last
 	// rollback should also disable fast syncing for this process, verify that we
 	// did a fresh full sync. Note, we can't assert anything about the receipts
@@ -1749,3 +1757,41 @@ func testDeliverHeadersHang(t *testing.T, protocol int, mode SyncMode) {
 		}
 	}
 }
+
+// Tests that if fast sync aborts in the critical section, it can restart a few
+// times before giving up.
+func TestFastCriticalRestarts63(t *testing.T) { testFastCriticalRestarts(t, 63) }
+func TestFastCriticalRestarts64(t *testing.T) { testFastCriticalRestarts(t, 64) }
+
+func testFastCriticalRestarts(t *testing.T, protocol int) {
+	t.Parallel()
+
+	// Create a large enough blockchin to actually fast sync on
+	targetBlocks := fsMinFullBlocks + 2*fsPivotInterval - 15
+	hashes, headers, blocks, receipts := makeChain(targetBlocks, 0, genesis, nil, false)
+
+	// Create a tester peer with the critical section state roots missing (force failures)
+	tester := newTester()
+	tester.newPeer("peer", protocol, hashes, headers, blocks, receipts)
+
+	for i := 0; i < fsPivotInterval; i++ {
+		tester.peerMissingStates["peer"][headers[hashes[fsMinFullBlocks+i]].Root] = true
+	}
+	// Synchronise with the peer a few times and make sure they fail until the retry limit
+	for i := 0; i < fsCriticalTrials; i++ {
+		// Attempt a sync and ensure it fails properly
+		if err := tester.sync("peer", nil, FastSync); err == nil {
+			t.Fatalf("failing fast sync succeeded: %v", err)
+		}
+		// If it's the first failure, pivot should be locked => reenable all others to detect pivot changes
+		if i == 0 {
+			tester.peerMissingStates["peer"] = map[common.Hash]bool{tester.downloader.fsPivotLock.Root: true}
+		}
+		time.Sleep(100 * time.Millisecond) // Make sure no in-flight requests remain
+	}
+	// Retry limit exhausted, downloader will switch to full sync, should succeed
+	if err := tester.sync("peer", nil, FastSync); err != nil {
+		t.Fatalf("failed to synchronise blocks in slow sync: %v", err)
+	}
+	assertOwnChain(t, tester, targetBlocks+1)
+}