fix a race in tipset cache usage

This tipset cache is shared between multiple services and is called from
multiple places.

chain/events/tscache.go

@@ -2,6 +2,7 @@ package events
 
 import (
 	"context"
+	"sync"
 
 	"github.com/filecoin-project/go-state-types/abi"
 	"golang.org/x/xerrors"
@@ -17,6 +18,8 @@ type tsCacheAPI interface {
 // tipSetCache implements a simple ring-buffer cache to keep track of recent
 // tipsets
 type tipSetCache struct {
+	mu sync.RWMutex
+
 	cache []*types.TipSet
 	start int
 	len   int
@@ -35,6 +38,9 @@ func newTSCache(cap abi.ChainEpoch, storage tsCacheAPI) *tipSetCache {
 }
 
 func (tsc *tipSetCache) add(ts *types.TipSet) error {
+	tsc.mu.Lock()
+	defer tsc.mu.Unlock()
+
 	if tsc.len > 0 {
 		if tsc.cache[tsc.start].Height() >= ts.Height() {
 			return xerrors.Errorf("tipSetCache.add: expected new tipset height to be at least %d, was %d", tsc.cache[tsc.start].Height()+1, ts.Height())
@@ -65,6 +71,13 @@ func (tsc *tipSetCache) add(ts *types.TipSet) error {
 }
 
 func (tsc *tipSetCache) revert(ts *types.TipSet) error {
+	tsc.mu.Lock()
+	defer tsc.mu.Unlock()
+
+	return tsc.revertUnlocked(ts)
+}
+
+func (tsc *tipSetCache) revertUnlocked(ts *types.TipSet) error {
 	if tsc.len == 0 {
 		return nil // this can happen, and it's fine
 	}
@@ -77,7 +90,7 @@ func (tsc *tipSetCache) revert(ts *types.TipSet) error {
 	tsc.start = normalModulo(tsc.start-1, len(tsc.cache))
 	tsc.len--
 
-	_ = tsc.revert(nil) // revert null block gap
+	_ = tsc.revertUnlocked(nil) // revert null block gap
 	return nil
 }
 
@@ -95,14 +108,18 @@ func (tsc *tipSetCache) getNonNull(height abi.ChainEpoch) (*types.TipSet, error)
 }
 
 func (tsc *tipSetCache) get(height abi.ChainEpoch) (*types.TipSet, error) {
+	tsc.mu.RLock()
+
 	if tsc.len == 0 {
+		tsc.mu.RUnlock()
 		log.Warnf("tipSetCache.get: cache is empty, requesting from storage (h=%d)", height)
 		return tsc.storage.ChainGetTipSetByHeight(context.TODO(), height, types.EmptyTSK)
 	}
 
 	headH := tsc.cache[tsc.start].Height()
 
 	if height > headH {
+		tsc.mu.RUnlock()
 		return nil, xerrors.Errorf("tipSetCache.get: requested tipset not in cache (req: %d, cache head: %d)", height, headH)
 	}
 
@@ -116,15 +133,20 @@ func (tsc *tipSetCache) get(height abi.ChainEpoch) (*types.TipSet, error) {
 	}
 
 	if height < tail.Height() {
+		tsc.mu.RUnlock()
 		log.Warnf("tipSetCache.get: requested tipset not in cache, requesting from storage (h=%d; tail=%d)", height, tail.Height())
 		return tsc.storage.ChainGetTipSetByHeight(context.TODO(), height, tail.Key())
 	}
 
-	return tsc.cache[normalModulo(tsc.start-int(headH-height), clen)], nil
+	ts := tsc.cache[normalModulo(tsc.start-int(headH-height), clen)]
+	tsc.mu.RUnlock()
+	return ts, nil
 }
 
 func (tsc *tipSetCache) best() (*types.TipSet, error) {
+	tsc.mu.RLock()
 	best := tsc.cache[tsc.start]
+	tsc.mu.RUnlock()
 	if best == nil {
 		return tsc.storage.ChainHead(context.TODO())
 	}