fix a race in the sync manager

1. SyncerState contains a mutex and should never be copied. Honestly, this case
was probably fine but it's just as easy to create a separate snapshot type and
easier to reason about.

2. We need to initialize the syncStates array once at start, before accessing
it. By each syncer state inside each worker, we were racing with calls to
`State()`. Again, this was probably benign, but I don't trust optimizing
compilers.

chain/sync.go

@@ -1726,7 +1726,7 @@ func VerifyElectionPoStVRF(ctx context.Context, worker address.Address, rand []b
 	return gen.VerifyVRF(ctx, worker, rand, evrf)
 }
 
-func (syncer *Syncer) State() []SyncerState {
+func (syncer *Syncer) State() []SyncerStateSnapshot {
 	return syncer.syncmgr.State()
 }
 

chain/sync_manager.go

@@ -38,7 +38,7 @@ type SyncManager interface {
 	SetPeerHead(ctx context.Context, p peer.ID, ts *types.TipSet)
 
 	// State retrieves the state of the sync workers.
-	State() []SyncerState
+	State() []SyncerStateSnapshot
 }
 
 type syncManager struct {
@@ -79,7 +79,7 @@ type syncResult struct {
 const syncWorkerCount = 3
 
 func NewSyncManager(sync SyncFunc) SyncManager {
-	return &syncManager{
+	sm := &syncManager{
 		bspThresh:       1,
 		peerHeads:       make(map[peer.ID]*types.TipSet),
 		syncTargets:     make(chan *types.TipSet),
@@ -90,6 +90,10 @@ func NewSyncManager(sync SyncFunc) SyncManager {
 		doSync:          sync,
 		stop:            make(chan struct{}),
 	}
+	for i := range sm.syncStates {
+		sm.syncStates[i] = new(SyncerState)
+	}
+	return sm
 }
 
 func (sm *syncManager) Start() {
@@ -128,8 +132,8 @@ func (sm *syncManager) SetPeerHead(ctx context.Context, p peer.ID, ts *types.Tip
 	sm.incomingTipSets <- ts
 }
 
-func (sm *syncManager) State() []SyncerState {
-	ret := make([]SyncerState, 0, len(sm.syncStates))
+func (sm *syncManager) State() []SyncerStateSnapshot {
+	ret := make([]SyncerStateSnapshot, 0, len(sm.syncStates))
 	for _, s := range sm.syncStates {
 		ret = append(ret, s.Snapshot())
 	}
@@ -405,8 +409,7 @@ func (sm *syncManager) scheduleWorkSent() {
 }
 
 func (sm *syncManager) syncWorker(id int) {
-	ss := &SyncerState{}
-	sm.syncStates[id] = ss
+	ss := sm.syncStates[id]
 	for {
 		select {
 		case ts, ok := <-sm.syncTargets:

chain/syncstate.go

@@ -11,8 +11,7 @@ import (
 	"github.com/filecoin-project/lotus/chain/types"
 )
 
-type SyncerState struct {
-	lk      sync.Mutex
+type SyncerStateSnapshot struct {
 	Target  *types.TipSet
 	Base    *types.TipSet
 	Stage   api.SyncStateStage
@@ -22,6 +21,11 @@ type SyncerState struct {
 	End     time.Time
 }
 
+type SyncerState struct {
+	lk   sync.Mutex
+	data SyncerStateSnapshot
+}
+
 func (ss *SyncerState) SetStage(v api.SyncStateStage) {
 	if ss == nil {
 		return
@@ -29,9 +33,9 @@ func (ss *SyncerState) SetStage(v api.SyncStateStage) {
 
 	ss.lk.Lock()
 	defer ss.lk.Unlock()
-	ss.Stage = v
+	ss.data.Stage = v
 	if v == api.StageSyncComplete {
-		ss.End = build.Clock.Now()
+		ss.data.End = build.Clock.Now()
 	}
 }
 
@@ -42,13 +46,13 @@ func (ss *SyncerState) Init(base, target *types.TipSet) {
 
 	ss.lk.Lock()
 	defer ss.lk.Unlock()
-	ss.Target = target
-	ss.Base = base
-	ss.Stage = api.StageHeaders
-	ss.Height = 0
-	ss.Message = ""
-	ss.Start = build.Clock.Now()
-	ss.End = time.Time{}
+	ss.data.Target = target
+	ss.data.Base = base
+	ss.data.Stage = api.StageHeaders
+	ss.data.Height = 0
+	ss.data.Message = ""
+	ss.data.Start = build.Clock.Now()
+	ss.data.End = time.Time{}
 }
 
 func (ss *SyncerState) SetHeight(h abi.ChainEpoch) {
@@ -58,7 +62,7 @@ func (ss *SyncerState) SetHeight(h abi.ChainEpoch) {
 
 	ss.lk.Lock()
 	defer ss.lk.Unlock()
-	ss.Height = h
+	ss.data.Height = h
 }
 
 func (ss *SyncerState) Error(err error) {
@@ -68,21 +72,13 @@ func (ss *SyncerState) Error(err error) {
 
 	ss.lk.Lock()
 	defer ss.lk.Unlock()
-	ss.Message = err.Error()
-	ss.Stage = api.StageSyncErrored
-	ss.End = build.Clock.Now()
+	ss.data.Message = err.Error()
+	ss.data.Stage = api.StageSyncErrored
+	ss.data.End = build.Clock.Now()
 }
 
-func (ss *SyncerState) Snapshot() SyncerState {
+func (ss *SyncerState) Snapshot() SyncerStateSnapshot {
 	ss.lk.Lock()
 	defer ss.lk.Unlock()
-	return SyncerState{
-		Base:    ss.Base,
-		Target:  ss.Target,
-		Stage:   ss.Stage,
-		Height:  ss.Height,
-		Message: ss.Message,
-		Start:   ss.Start,
-		End:     ss.End,
-	}
+	return ss.data
 }