Fix race in server LogEntry logger (#6449)

server/logging/log.go

@@ -67,6 +67,12 @@ func New() Log {
 
 func (l *logger) Open(_ context.Context, stepID int64) error {
 	l.Lock()
+	l.open(stepID)
+	l.Unlock()
+	return nil
+}
+
+func (l *logger) open(stepID int64) {
 	_, ok := l.streams[stepID]
 	if !ok {
 		l.streams[stepID] = &stream{
@@ -75,23 +81,18 @@ func (l *logger) Open(_ context.Context, stepID int64) error {
 			done:   make(chan struct{}),
 		}
 	}
-	l.Unlock()
-	return nil
 }
 
 func (l *logger) Write(ctx context.Context, stepID int64, entries []*model.LogEntry) error {
 	l.Lock()
 	s, ok := l.streams[stepID]
-	l.Unlock()
-
-	// auto open the stream if it does not exist
 	if !ok {
-		err := l.Open(ctx, stepID)
-		if err != nil {
-			return err
-		}
+		// Auto-open the stream while still holding the logger lock so that a
+		// concurrent Write for the same step cannot race on l.streams.
+		l.open(stepID)
 		s = l.streams[stepID]
 	}
+	l.Unlock()
 
 	s.Lock()
 	s.list = append(s.list, entries...)