Fix a race between splits and snapshots.

When a range is split, followers of that range may receive a snapshot
from the right-hand side of the split before they have caught up and
processed the left-hand side where the split originated. This results in
a "range already exists" panic.

The solution is to silently drop any snapshots which would cause a
conflict. They will be retried and will succeed once the left-hand range
has performed its split.

Fixes cockroachdb#1644.

multiraft/multiraft.go

@@ -743,9 +743,18 @@ func (s *state) handleMessage(req *RaftMessageRequest) {
 	case raftpb.MsgHeartbeat:
 		s.fanoutHeartbeat(req)
 		return
+
 	case raftpb.MsgHeartbeatResp:
 		s.fanoutHeartbeatResponse(req)
 		return
+
+	case raftpb.MsgSnap:
+		if !s.Storage.CanApplySnapshot(req.GroupID, req.Message.Snapshot) {
+			// If the storage cannot accept the snapshot, drop it before
+			// passing it to multiNode.Step, since our error handling
+			// options past that point are limited.
+			return
+		}
 	}
 
 	s.CacheReplicaDescriptor(req.GroupID, req.FromReplica)

multiraft/storage.go

@@ -53,6 +53,12 @@ type Storage interface {
 	ReplicaIDForStore(groupID roachpb.RangeID, storeID roachpb.StoreID) (roachpb.ReplicaID, error)
 	ReplicasFromSnapshot(snap raftpb.Snapshot) ([]roachpb.ReplicaDescriptor, error)
 
+	// CanApplySnapshot should return false if attempting to apply the
+	// given snapshot would result in an error. This allows snapshots to
+	// be dropped cleanly since errors deep inside raft often result in
+	// panics.
+	CanApplySnapshot(groupID roachpb.RangeID, snap raftpb.Snapshot) bool
+
 	// GroupLocker returns a lock which (if non-nil) will be acquired
 	// when a group is being created (which entails multiple calls to
 	// Storage and StateMachine methods and may race with the removal of
@@ -118,6 +124,11 @@ func (m *MemoryStorage) ReplicasFromSnapshot(_ raftpb.Snapshot) ([]roachpb.Repli
 	return nil, nil
 }
 
+// CanApplySnapshot implements the Storage interface.
+func (m *MemoryStorage) CanApplySnapshot(_ roachpb.RangeID, _ raftpb.Snapshot) bool {
+	return true
+}
+
 // GroupLocker implements the Storage interface by returning nil.
 func (m *MemoryStorage) GroupLocker() sync.Locker {
 	return nil

multiraft/storage_test.go

@@ -53,6 +53,10 @@ func (b *BlockableStorage) ReplicasFromSnapshot(snap raftpb.Snapshot) ([]roachpb
 	return b.storage.ReplicasFromSnapshot(snap)
 }
 
+func (b *BlockableStorage) CanApplySnapshot(groupID roachpb.RangeID, snap raftpb.Snapshot) bool {
+	return b.storage.CanApplySnapshot(groupID, snap)
+}
+
 func (b *BlockableStorage) GroupLocker() sync.Locker {
 	return b.storage.GroupLocker()
 }

storage/client_split_test.go

@@ -600,3 +600,170 @@ func TestStoreRangeSystemSplits(t *testing.T) {
 		t.Errorf("expected splits not found: %s", err)
 	}
 }
+
+// setupSplitSnapshotRace engineers a situation in which a range has
+// been split but node 3 hasn't processed it yet. There is a race
+// depending on whether node 3 learns of the split from its left or
+// right side. When this function returns most of the nodes will be
+// stopped, and depending on the order in which they are restarted, we
+// can arrange for both possible outcomes of the race.
+//
+// Range 1 is the system keyspace, located on node 0.
+// The range containing "a" is the left side of the split, located on nodes 1, 2, and 3.
+// The range containing "z" is the right side of the split, located on nodes 3, 4, and 5.
+// Nodes 1-5 are stopped; only node 0 is running.
+//
+// See https://github.com/cockroachdb/cockroach/issues/1644.
+func setupSplitSnapshotRace(t *testing.T) *multiTestContext {
+	mtc := startMultiTestContext(t, 6)
+
+	leftKey := roachpb.Key("a")
+	rightKey := roachpb.Key("z")
+
+	// First, do a couple of writes; we'll use these to determine when
+	// the dust has settled.
+	incArgs := incrementArgs(leftKey, 1)
+	if _, err := client.SendWrapped(rg1(mtc.stores[0]), nil, &incArgs); err != nil {
+		t.Fatal(err)
+	}
+	incArgs = incrementArgs(rightKey, 2)
+	if _, err := client.SendWrapped(rg1(mtc.stores[0]), nil, &incArgs); err != nil {
+		t.Fatal(err)
+	}
+
+	// Split the system range from the rest of the keyspace.
+	splitArgs := adminSplitArgs(roachpb.KeyMin, keys.SystemMax)
+	if _, err := client.SendWrapped(rg1(mtc.stores[0]), nil, &splitArgs); err != nil {
+		t.Fatal(err)
+	}
+
+	// Get the left range's ID. This is currently 2, but using
+	// LookupReplica is more future-proof (and see below for
+	// rightRangeID).
+	leftRangeID := mtc.stores[0].LookupReplica(roachpb.RKey("a"), nil).Desc().RangeID
+
+	// Replicate the left range onto nodes 1-3 and remove it from node 0.
+	mtc.replicateRange(leftRangeID, 0, 1, 2, 3)
+	mtc.unreplicateRange(leftRangeID, 0, 0)
+	mtc.expireLeaderLeases()
+
+	mtc.waitForValues(leftKey, 3*time.Second, []int64{0, 1, 1, 1, 0, 0})
+	mtc.waitForValues(rightKey, 3*time.Second, []int64{0, 2, 2, 2, 0, 0})
+
+	// Stop node 3 so it doesn't hear about the split.
+	mtc.stopStore(3)
+	mtc.expireLeaderLeases()
+
+	// Split the data range.
+	splitArgs = adminSplitArgs(keys.SystemMax, roachpb.Key("m"))
+	if _, err := client.SendWrapped(mtc.distSender, nil, &splitArgs); err != nil {
+		t.Fatal(err)
+	}
+
+	// Get the right range's ID. Since the split was performed on node
+	// 1, it is currently 11 and not 3 as might be expected.
+	rightRangeID := mtc.stores[1].LookupReplica(roachpb.RKey("z"), nil).Desc().RangeID
+
+	// Relocate the right range onto nodes 3-5.
+	mtc.replicateRange(rightRangeID, 1, 4, 5)
+	mtc.unreplicateRange(rightRangeID, 1, 2)
+	mtc.unreplicateRange(rightRangeID, 1, 1)
+
+	mtc.waitForValues(rightKey, 3*time.Second, []int64{0, 0, 0, 2, 2, 2})
+
+	// Stop the remaining data stores.
+	mtc.stopStore(1)
+	mtc.stopStore(2)
+	// 3 is already stopped.
+	mtc.stopStore(4)
+	mtc.stopStore(5)
+	mtc.expireLeaderLeases()
+
+	return mtc
+}
+
+// TestSplitSnapshotRace_SplitWins exercises one outcome of the
+// split/snapshot race: The left side of the split propagates first,
+// so the split completes before it sees a competing snapshot. This is
+// the more common outcome in practice.
+func TestSplitSnapshotRace_SplitWins(t *testing.T) {
+	defer leaktest.AfterTest(t)
+	mtc := setupSplitSnapshotRace(t)
+	defer mtc.Stop()
+
+	// Bring the left range up first so that the split happens before it sees a snapshot.
+	for i := 1; i <= 3; i++ {
+		mtc.restartStore(i)
+	}
+
+	leftKey := roachpb.Key("a")
+	rightKey := roachpb.Key("z")
+
+	// Perform a write on the left range and wait for it to propagate.
+	incArgs := incrementArgs(leftKey, 10)
+	if _, err := client.SendWrapped(mtc.distSender, nil, &incArgs); err != nil {
+		t.Fatal(err)
+	}
+	mtc.waitForValues(leftKey, 3*time.Second, []int64{0, 11, 11, 11, 0, 0})
+
+	// Now wake the other stores up.
+	mtc.restartStore(4)
+	mtc.restartStore(5)
+
+	// Write to the right range.
+	incArgs = incrementArgs(rightKey, 20)
+	if _, err := client.SendWrapped(mtc.distSender, nil, &incArgs); err != nil {
+		t.Fatal(err)
+	}
+	mtc.waitForValues(rightKey, 3*time.Second, []int64{0, 0, 0, 22, 22, 22})
+}
+
+// TestSplitSnapshotRace_SplitWins exercises one outcome of the
+// split/snapshot race: The right side of the split replicates first,
+// so target node sees a raft snapshot before it has processed the split,
+// so it still has a conflicting range.
+func TestSplitSnapshotRace_SnapshotWins(t *testing.T) {
+	defer leaktest.AfterTest(t)
+	mtc := setupSplitSnapshotRace(t)
+	defer mtc.Stop()
+
+	// Bring the right range up first.
+	for i := 3; i <= 5; i++ {
+		mtc.restartStore(i)
+	}
+
+	leftKey := roachpb.Key("a")
+	rightKey := roachpb.Key("z")
+
+	// Perform a write on the right range.
+	incArgs := incrementArgs(rightKey, 20)
+	if _, err := client.SendWrapped(mtc.distSender, nil, &incArgs); err != nil {
+		t.Fatal(err)
+	}
+
+	// It immediately propagates between nodes 4 and 5, but node 3 remains
+	// at its old value.  It can't accept the right-hand range because
+	// it conflicts with its not-yet-split copy of the left-hand range.
+	mtc.waitForValues(rightKey, 3*time.Second, []int64{0, 0, 0, 2, 22, 22})
+
+	// Wake up the left-hand range. This will allow the left-hand
+	// range's split to complete and unblock the right-hand range.
+	mtc.restartStore(1)
+	mtc.restartStore(2)
+
+	// Perform writes on both sides. This is not strictly necessary but
+	// it helps wake up dormant ranges that would otherwise have to wait
+	// for retry timeouts.
+	incArgs = incrementArgs(leftKey, 10)
+	if _, err := client.SendWrapped(mtc.distSender, nil, &incArgs); err != nil {
+		t.Fatal(err)
+	}
+	mtc.waitForValues(leftKey, 3*time.Second, []int64{0, 11, 11, 11, 0, 0})
+
+	incArgs = incrementArgs(rightKey, 200)
+	if _, err := client.SendWrapped(mtc.distSender, nil, &incArgs); err != nil {
+		t.Fatal(err)
+	}
+	mtc.waitForValues(rightKey, 3*time.Second, []int64{0, 0, 0, 222, 222, 222})
+
+}

storage/client_test.go

@@ -141,6 +141,7 @@ type multiTestContext struct {
 	gossip       *gossip.Gossip
 	storePool    *storage.StorePool
 	transport    multiraft.Transport
+	distSender   *kv.DistSender
 	db           *client.DB
 	feed         *util.Feed
 	// The per-store clocks slice normally contains aliases of
@@ -199,12 +200,12 @@ func (m *multiTestContext) Start(t *testing.T, numStores int) {
 	m.senders = append(m.senders, kv.NewLocalSender())
 
 	if m.db == nil {
-		distSender := kv.NewDistSender(&kv.DistSenderContext{
+		m.distSender = kv.NewDistSender(&kv.DistSenderContext{
 			Clock:             m.clock,
 			RangeDescriptorDB: m.senders[0],
 			RPCSend:           m.rpcSend,
 		}, m.gossip)
-		sender := kv.NewTxnCoordSender(distSender, m.clock, false, nil, m.clientStopper)
+		sender := kv.NewTxnCoordSender(m.distSender, m.clock, false, nil, m.clientStopper)
 		m.db = client.NewDB(sender)
 	}
 
@@ -276,7 +277,13 @@ func (m *multiTestContext) rpcSend(_ rpc.Options, _ string, addrs []net.Addr,
 		if stErr != nil {
 			m.t.Fatal(stErr)
 		}
-		br, pErr = m.senders[nodeID-1].Send(context.Background(), ba)
+		nodeIndex := nodeID - 1
+		if m.stores[nodeIndex] == nil {
+			pErr = &roachpb.Error{}
+			pErr.SetGoError(rpc.NewSendError("store is stopped", true))
+			continue
+		}
+		br, pErr = m.senders[nodeIndex].Send(context.Background(), ba)
 		if pErr == nil {
 			return []proto.Message{br}, nil
 		}
@@ -289,6 +296,9 @@ func (m *multiTestContext) rpcSend(_ rpc.Options, _ string, addrs []net.Addr,
 				// from the group. Move the manual clock forward in an attempt to
 				// expire the lease.
 				m.expireLeaderLeases()
+			} else if m.stores[tErr.Leader.NodeID-1] == nil {
+				// The leader is known but down, so expire its lease.
+				m.expireLeaderLeases()
 			}
 		default:
 			if testutils.IsError(tErr, `store \d+ not found`) {

storage/store.go

@@ -910,6 +910,13 @@ func (s *Store) SplitRange(origRng, newRng *Replica) error {
 	if s.replicasByKey.ReplaceOrInsert(origRng) != nil {
 		return util.Errorf("couldn't insert range %v in rangesByKey btree", origRng)
 	}
+
+	// If we have an uninitialized replica of the new range, delete it to make
+	// way for the complete one created by the split.
+	if _, ok := s.uninitReplicas[newDesc.RangeID]; ok {
+		delete(s.uninitReplicas, newDesc.RangeID)
+		delete(s.replicas, newDesc.RangeID)
+	}
 	if err := s.addReplicaInternal(newRng); err != nil {
 		return util.Errorf("couldn't insert range %v in rangesByKey btree: %s", newRng, err)
 	}
@@ -1584,6 +1591,35 @@ func (s *Store) GroupLocker() sync.Locker {
 	return &s.raftGroupLocker
 }
 
+// CanApplySnapshot implements the multiraft.Storage interface.
+func (s *Store) CanApplySnapshot(rangeID roachpb.RangeID, snap raftpb.Snapshot) bool {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	if r, ok := s.replicas[rangeID]; ok && r.isInitialized() {
+		// We have the range and it's initialized, so let the snapshot
+		// through. We'll decide later if it's newer or older than our
+		// current state.
+		return true
+	}
+
+	// We don't have the range (or we have an uninitialized
+	// placeholder). Will we be able to create/initialize it?
+	// TODO(bdarnell): can we avoid parsing this twice?
+	var parsedSnap roachpb.RaftSnapshotData
+	if err := parsedSnap.Unmarshal(snap.Data); err != nil {
+		return false
+	}
+
+	if s.replicasByKey.Has(rangeBTreeKey(parsedSnap.RangeDescriptor.EndKey)) {
+		// We have a conflicting range, so we must block the snapshot.
+		// When such a conflict exists, it will be resolved by one range
+		// either being split or garbage collected.
+		return false
+	}
+
+	return true
+}
+
 // AppliedIndex implements the multiraft.StateMachine interface.
 func (s *Store) AppliedIndex(groupID roachpb.RangeID) (uint64, error) {
 	s.mu.RLock()