Handle nil closers (#726)

Change the methods Done and HasBeenNotified to be able to handle a nil closer.

Also fix a flaky test.

managed_db_test.go

@@ -428,7 +428,7 @@ func TestDropPrefixWithPendingTxn(t *testing.T) {
 	txn := db.NewTransaction(true)
 
 	var wg sync.WaitGroup
-	wg.Add(1)
+	wg.Add(2)
 	go func() {
 		defer wg.Done()
 		itr := txn.NewIterator(DefaultIteratorOptions)
@@ -464,6 +464,7 @@ func TestDropPrefixWithPendingTxn(t *testing.T) {
 	// Do not cancel txn.
 
 	go func() {
+		defer wg.Done()
 		time.Sleep(2 * time.Second)
 		require.NoError(t, db.DropPrefix([]byte("key0")))
 		require.NoError(t, db.DropPrefix([]byte("key00")))

y/y.go

@@ -48,6 +48,9 @@ var (
 
 	// CastagnoliCrcTable is a CRC32 polynomial table
 	CastagnoliCrcTable = crc32.MakeTable(crc32.Castagnoli)
+
+	// Dummy channel for nil closers.
+	dummyCloserChan = make(chan struct{})
 )
 
 // OpenExistingFile opens an existing file, errors if it doesn't exist.
@@ -203,11 +206,17 @@ func (lc *Closer) Signal() {
 
 // HasBeenClosed gets signaled when Signal() is called.
 func (lc *Closer) HasBeenClosed() <-chan struct{} {
+	if lc == nil {
+		return dummyCloserChan
+	}
 	return lc.closed
 }
 
 // Done calls Done() on the WaitGroup.
 func (lc *Closer) Done() {
+	if lc == nil {
+		return
+	}
 	lc.waiting.Done()
 }