fix(engine): avoid panic with concurrent map read and map write (#503)

Author: agneum

engine/internal/cloning/snapshots.go

@@ -30,15 +30,13 @@ func (c *Base) fetchSnapshots() error {
 	var latestSnapshot *models.Snapshot
 
 	snapshots := make(map[string]*models.Snapshot, len(entries))
+	cloneCounter := c.cloneCounter()
 
 	for _, entry := range entries {
 		numClones := 0
 
-		for cloneName := range c.clones {
-			if c.clones[cloneName] != nil && c.clones[cloneName].Clone.Snapshot != nil &&
-				c.clones[cloneName].Clone.Snapshot.ID == entry.ID {
-				numClones++
-			}
+		if num, ok := cloneCounter[entry.ID]; ok {
+			numClones = num
 		}
 
 		currentSnapshot := &models.Snapshot{
@@ -61,6 +59,23 @@ func (c *Base) fetchSnapshots() error {
 
 	return nil
 }
+
+func (c *Base) cloneCounter() map[string]int {
+	cloneCounter := make(map[string]int)
+
+	c.cloneMutex.RLock()
+
+	for cloneName := range c.clones {
+		if c.clones[cloneName] != nil && c.clones[cloneName].Clone.Snapshot != nil {
+			cloneCounter[c.clones[cloneName].Clone.Snapshot.ID]++
+		}
+	}
+
+	c.cloneMutex.RUnlock()
+
+	return cloneCounter
+}
+
 func (c *Base) resetSnapshots(snapshotMap map[string]*models.Snapshot, latestSnapshot *models.Snapshot) {
 	c.snapshotBox.snapshotMutex.Lock()
 

engine/internal/cloning/snapshots_test.go

@@ -121,6 +121,50 @@ func TestCloneCounter(t *testing.T) {
 	require.Equal(t, 0, snapshot.NumClones)
 }
 
+func TestInitialCloneCounter(t *testing.T) {
+	c := &Base{}
+	c.clones = make(map[string]*CloneWrapper)
+
+	snapshot := &models.Snapshot{
+		ID: "testSnapshotID",
+	}
+
+	snapshot2 := &models.Snapshot{
+		ID: "testSnapshotID2",
+	}
+
+	cloneWrapper01 := &CloneWrapper{
+		Clone: &models.Clone{
+			ID:       "test_clone001",
+			Snapshot: snapshot,
+		},
+	}
+
+	cloneWrapper02 := &CloneWrapper{
+		Clone: &models.Clone{
+			ID:       "test_clone002",
+			Snapshot: snapshot,
+		},
+	}
+
+	cloneWrapper03 := &CloneWrapper{
+		Clone: &models.Clone{
+			ID:       "test_clone003",
+			Snapshot: snapshot2,
+		},
+	}
+
+	c.clones["test_clone001"] = cloneWrapper01
+	c.clones["test_clone002"] = cloneWrapper02
+	c.clones["test_clone003"] = cloneWrapper03
+
+	counters := c.cloneCounter()
+
+	require.Equal(t, 2, len(counters))
+	require.Equal(t, 2, counters["testSnapshotID"])
+	require.Equal(t, 1, counters["testSnapshotID2"])
+}
+
 func TestLatestSnapshots(t *testing.T) {
 	baseSnapshot := &models.Snapshot{
 		DataStateAt: &models.LocalTime{Time: time.Date(2020, 02, 19, 0, 0, 0, 0, time.UTC)},