fix: close chain head subscription when the reader is slow

The reader can just re-subscribe when they're ready to catch up. This
prevents a slow reader from bogging down the entire system.

chain/store/store.go

@@ -293,27 +293,36 @@ func (cs *ChainStore) SubHeadChanges(ctx context.Context) chan []*api.HeadChange
 	}}
 
 	go func() {
-		defer close(out)
-		var unsubOnce sync.Once
+		defer func() {
+			// Tell the caller we're done first, the following may block for a bit.
+			close(out)
+
+			// Unsubscribe.
+			cs.bestTips.Unsub(subch)
+
+			// Drain the channel.
+			for range subch {
+			}
+		}()
 
 		for {
 			select {
 			case val, ok := <-subch:
 				if !ok {
-					log.Warn("chain head sub exit loop")
+					// Shutting down.
+					return
+				}
+				select {
+				case out <- val.([]*api.HeadChange):
+				default:
+					log.Errorf("closing head change subscription due to slow reader")
 					return
 				}
 				if len(out) > 5 {
 					log.Warnf("head change sub is slow, has %d buffered entries", len(out))
 				}
-				select {
-				case out <- val.([]*api.HeadChange):
-				case <-ctx.Done():
-				}
 			case <-ctx.Done():
-				unsubOnce.Do(func() {
-					go cs.bestTips.Unsub(subch)
-				})
+				return
 			}
 		}
 	}()

itests/api_test.go

@@ -15,6 +15,7 @@ import (
 	"github.com/filecoin-project/lotus/build"
 	"github.com/filecoin-project/lotus/chain/types"
 	"github.com/filecoin-project/lotus/itests/kit"
+	logging "github.com/ipfs/go-log/v2"
 )
 
 func TestAPI(t *testing.T) {
@@ -39,6 +40,7 @@ func runAPITest(t *testing.T, opts ...interface{}) {
 	t.Run("testConnectTwo", ts.testConnectTwo)
 	t.Run("testMining", ts.testMining)
 	t.Run("testMiningReal", ts.testMiningReal)
+	t.Run("testSlowNotify", ts.testSlowNotify)
 	t.Run("testSearchMsg", ts.testSearchMsg)
 	t.Run("testNonGenesisMiner", ts.testNonGenesisMiner)
 }
@@ -169,6 +171,51 @@ func (ts *apiSuite) testMiningReal(t *testing.T) {
 	ts.testMining(t)
 }
 
+func (ts *apiSuite) testSlowNotify(t *testing.T) {
+	_ = logging.SetLogLevel("rpc", "ERROR")
+
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	full, miner, _ := kit.EnsembleMinimal(t, ts.opts...)
+
+	// Subscribe a bunch of times to make sure we fill up any RPC buffers.
+	var newHeadsChans []<-chan []*lapi.HeadChange
+	for i := 0; i < 100; i++ {
+		newHeads, err := full.ChainNotify(ctx)
+		require.NoError(t, err)
+		newHeadsChans = append(newHeadsChans, newHeads)
+	}
+
+	initHead := (<-newHeadsChans[0])[0]
+	baseHeight := initHead.Val.Height()
+
+	bm := kit.NewBlockMiner(t, miner)
+	bm.MineBlocks(ctx, time.Microsecond)
+
+	full.WaitTillChain(ctx, kit.HeightAtLeast(baseHeight+100))
+
+	// Make sure they were all closed.
+	for _, ch := range newHeadsChans {
+		var ok bool
+		for ok {
+			select {
+			case _, ok = <-ch:
+			default:
+				t.Fatal("expected new heads channel to be closed")
+			}
+		}
+	}
+
+	// Make sure we can resubscribe and everything still works.
+	newHeads, err := full.ChainNotify(ctx)
+	require.NoError(t, err)
+	for i := 0; i < 10; i++ {
+		_, ok := <-newHeads
+		require.True(t, ok, "notify channel closed")
+	}
+}
+
 func (ts *apiSuite) testNonGenesisMiner(t *testing.T) {
 	ctx := context.Background()
 

tools/stats/rpc.go

@@ -139,7 +139,10 @@ func GetTips(ctx context.Context, api v0api.FullNode, lastHeight abi.ChainEpoch,
 
 		for {
 			select {
-			case changes := <-notif:
+			case changes, ok := <-notif:
+				if !ok {
+					return
+				}
 				for _, change := range changes {
 					log.Infow("Head event", "height", change.Val.Height(), "type", change.Type)