lib/db: Wrap errors from leveldb iterators (fixes #6263) (#6264)

lib/db/backend/backend_test.go

@@ -14,6 +14,7 @@ import "testing"
 func testBackendBehavior(t *testing.T, open func() Backend) {
 	t.Run("WriteIsolation", func(t *testing.T) { testWriteIsolation(t, open) })
 	t.Run("DeleteNonexisten", func(t *testing.T) { testDeleteNonexistent(t, open) })
+	t.Run("IteratorClosedDB", func(t *testing.T) { testIteratorClosedDB(t, open) })
 }
 
 func testWriteIsolation(t *testing.T, open func() Backend) {
@@ -51,3 +52,25 @@ func testDeleteNonexistent(t *testing.T, open func() Backend) {
 		t.Error(err)
 	}
 }
+
+// Either creating the iterator or the .Error() method of the returned iterator
+// should return an error and IsClosed(err) == true.
+func testIteratorClosedDB(t *testing.T, open func() Backend) {
+	db := open()
+
+	_ = db.Put([]byte("a"), []byte("a"))
+
+	db.Close()
+
+	it, err := db.NewPrefixIterator(nil)
+	if err != nil {
+		if !IsClosed(err) {
+			t.Error("NewPrefixIterator: IsClosed(err) == false:", err)
+		}
+		return
+	}
+	it.Next()
+	if err := it.Error(); !IsClosed(err) {
+		t.Error("Next: IsClosed(err) == false:", err)
+	}
+}

lib/db/backend/leveldb_backend.go

@@ -10,6 +10,7 @@ import (
 	"sync"
 
 	"github.com/syndtr/goleveldb/leveldb"
+	"github.com/syndtr/goleveldb/leveldb/iterator"
 	"github.com/syndtr/goleveldb/leveldb/util"
 )
 
@@ -65,11 +66,11 @@ func (b *leveldbBackend) Get(key []byte) ([]byte, error) {
 }
 
 func (b *leveldbBackend) NewPrefixIterator(prefix []byte) (Iterator, error) {
-	return b.ldb.NewIterator(util.BytesPrefix(prefix), nil), nil
+	return &leveldbIterator{b.ldb.NewIterator(util.BytesPrefix(prefix), nil)}, nil
 }
 
 func (b *leveldbBackend) NewRangeIterator(first, last []byte) (Iterator, error) {
-	return b.ldb.NewIterator(&util.Range{Start: first, Limit: last}, nil), nil
+	return &leveldbIterator{b.ldb.NewIterator(&util.Range{Start: first, Limit: last}, nil)}, nil
 }
 
 func (b *leveldbBackend) Put(key, val []byte) error {
@@ -158,6 +159,14 @@ func (t *leveldbTransaction) flush() error {
 	return nil
 }
 
+type leveldbIterator struct {
+	iterator.Iterator
+}
+
+func (it *leveldbIterator) Error() error {
+	return wrapLeveldbErr(it.Iterator.Error())
+}
+
 // wrapLeveldbErr wraps errors so that the backend package can recognize them
 func wrapLeveldbErr(err error) error {
 	if err == nil {