fix: break graph api notifiers across a sync map

lib/graph_gateway.go

@@ -83,15 +83,22 @@ type Notifier interface {
 	NotifyNewBlocks(ctx context.Context, blocks ...blocks.Block) error
 }
 
+// notifiersForRootCid is used for reducing lock contention by only notifying
+// exchanges related to the same content root CID
+type notifiersForRootCid struct {
+	lk        sync.RWMutex
+	deleted   int8
+	notifiers []Notifier
+}
+
 type GraphGateway struct {
 	fetcher      CarFetcher
 	blockFetcher exchange.Fetcher
 	routing      routing.ValueStore
 	namesys      namesys.NameSystem
 	bstore       blockstore.Blockstore
 
-	lk        sync.RWMutex
-	notifiers map[Notifier]struct{}
+	notifiers sync.Map // cid -> notifiersForRootCid
 	metrics   *GraphGatewayMetrics
 }
 
@@ -152,7 +159,7 @@ func NewGraphGatewayBackend(f CarFetcher, blockFetcher exchange.Fetcher, opts ..
 		routing:      vs,
 		namesys:      ns,
 		bstore:       bs,
-		notifiers:    make(map[Notifier]struct{}),
+		notifiers:    sync.Map{},
 		metrics:      registerGraphGatewayMetrics(),
 	}, nil
 }
@@ -242,6 +249,18 @@ func registerGraphGatewayMetrics() *GraphGatewayMetrics {
 	}
 }
 
+func (api *GraphGateway) getRootOfPath(path string) string {
+	pth, err := ipfspath.ParsePath(path)
+	if err != nil {
+		return path
+	}
+	if pth.IsJustAKey() {
+		return pth.Segments()[0]
+	} else {
+		return pth.Segments()[1]
+	}
+}
+
 /*
 Implementation iteration plan:
 
@@ -263,9 +282,25 @@ func (api *GraphGateway) loadRequestIntoSharedBlockstoreAndBlocksGateway(ctx con
 		metrics:          api.metrics,
 	}
 
-	api.lk.Lock()
-	api.notifiers[exch] = struct{}{}
-	api.lk.Unlock()
+	notifierKey := api.getRootOfPath(path)
+	var notifier *notifiersForRootCid
+	for {
+		notifiers, _ := api.notifiers.LoadOrStore(notifierKey, &notifiersForRootCid{notifiers: []Notifier{}})
+		if n, ok := notifiers.(*notifiersForRootCid); ok {
+			n.lk.Lock()
+			// could have been deleted after our load. try again.
+			if n.deleted != 0 {
+				n.lk.Unlock()
+				continue
+			}
+			notifier = n
+			n.notifiers = append(n.notifiers, exch)
+			n.lk.Unlock()
+			break
+		} else {
+			return nil, nil, errors.New("failed to get notifier")
+		}
+	}
 
 	go func(metrics *GraphGatewayMetrics) {
 		defer func() {
@@ -297,7 +332,7 @@ func (api *GraphGateway) loadRequestIntoSharedBlockstoreAndBlocksGateway(ctx con
 					return err
 				}
 				metrics.carBlocksFetchedMetric.Inc()
-				api.notifyAllOngoingRequests(ctx, blk)
+				api.notifyOngoingRequests(ctx, notifierKey, blk)
 			}
 		})
 		if err != nil {
@@ -317,21 +352,37 @@ func (api *GraphGateway) loadRequestIntoSharedBlockstoreAndBlocksGateway(ctx con
 	}
 
 	return blkgw, func() {
-		api.lk.Lock()
-		delete(api.notifiers, exch)
-		api.lk.Unlock()
+		notifier.lk.Lock()
+		for i, e := range notifier.notifiers {
+			if e == exch {
+				notifier.notifiers = append(notifier.notifiers[0:i], notifier.notifiers[i+1:]...)
+				break
+			}
+		}
+		if len(notifier.notifiers) == 0 {
+			notifier.deleted = 1
+			api.notifiers.Delete(notifierKey)
+		}
+		notifier.lk.Unlock()
 	}, nil
 }
 
-func (api *GraphGateway) notifyAllOngoingRequests(ctx context.Context, blks ...blocks.Block) {
-	api.lk.RLock()
-	for n := range api.notifiers {
-		err := n.NotifyNewBlocks(ctx, blks...)
-		if err != nil {
-			graphLog.Errorw("notifyAllOngoingRequests failed", "error", err)
+func (api *GraphGateway) notifyOngoingRequests(ctx context.Context, key string, blks ...blocks.Block) {
+	if notifiers, ok := api.notifiers.Load(key); ok {
+		notifier, ok := notifiers.(*notifiersForRootCid)
+		if !ok {
+			graphLog.Errorw("notifyOngoingRequests failed", "key", key, "error", "could not get notifiersForRootCid")
+			return
+		}
+		notifier.lk.RLock()
+		for _, n := range notifier.notifiers {
+			err := n.NotifyNewBlocks(ctx, blks...)
+			if err != nil {
+				graphLog.Errorw("notifyOngoingRequests failed", "key", key, "error", err)
+			}
 		}
+		notifier.lk.RUnlock()
 	}
-	api.lk.RUnlock()
 }
 
 type fileCloseWrapper struct {