X zodb: Clarify load semantic: for deleted data err="no data" and buf, serial = nil, 0

go/zodb/storage/cache_test.go

@@ -89,10 +89,11 @@ func (stor *tStorage) Load(_ context.Context, xid zodb.Xid) (buf *zodb.Buf, seri
 	}
 
 	s, e := datav[i].serial, datav[i].err
+	b := mkbuf(datav[i].data)
 	if e != nil {
-		s = 0 // obey protocol of returning 0 with error
+		b, s = nil, 0 // obey protocol of returning nil, 0 with error
 	}
-	return mkbuf(datav[i].data), s, e
+	return b, s, e
 }
 
 var ioerr = errors.New("input/output error")

go/zodb/storage/fs1/filestorage.go

@@ -146,7 +146,7 @@ func (dh *DataHeader) Free() {
 	dhPool.Put(dh)
 }
 
-func (fs *FileStorage) Load(_ context.Context, xid zodb.Xid) (buf *zodb.Buf, tid zodb.Tid, err error) {
+func (fs *FileStorage) Load(_ context.Context, xid zodb.Xid) (buf *zodb.Buf, serial zodb.Tid, err error) {
 	// lookup in index position of oid data record within latest transaction which changed this oid
 	dataPos, ok := fs.index.Get(xid.Oid)
 	if !ok {
@@ -162,9 +162,9 @@ func (fs *FileStorage) Load(_ context.Context, xid zodb.Xid) (buf *zodb.Buf, tid
 	dh.Tid = zodb.TidMax
 	dh.PrevRevPos = dataPos
 	//defer dh.Free()
-	buf, tid, err = fs._Load(dh, xid)
+	buf, serial, err = fs._Load(dh, xid)
 	dh.Free()
-	return buf, tid, err
+	return buf, serial, err
 }
 
 func (fs *FileStorage) _Load(dh *DataHeader, xid zodb.Xid) (*zodb.Buf, zodb.Tid, error) {
@@ -187,9 +187,9 @@ func (fs *FileStorage) _Load(dh *DataHeader, xid zodb.Xid) (*zodb.Buf, zodb.Tid,
 		}
 	}
 
-	// even if we will scan back via backpointers, the tid returned should
+	// even if we will scan back via backpointers, the serial returned should
 	// be of first-found transaction
-	tid := dh.Tid
+	serial := dh.Tid
 
 	buf, err := dh.LoadData(fs.file)
 	if err != nil {
@@ -200,7 +200,7 @@ func (fs *FileStorage) _Load(dh *DataHeader, xid zodb.Xid) (*zodb.Buf, zodb.Tid,
 		return nil, 0, &zodb.ErrXidMissing{Xid: xid}
 	}
 
-	return buf, tid, nil
+	return buf, serial, nil
 }
 
 // --- ZODB-level iteration ---

go/zodb/storage/fs1/format.go

@@ -657,7 +657,7 @@ func (dh *DataHeader) loadNext(r io.ReaderAt, txnh *TxnHeader) error {
 // LoadData loads data for the data record taking backpointers into account.
 //
 // NOTE on success dh state is changed to data header of original data transaction.
-// NOTE "deleted" records are indicated via returning buf with .Data=nil.
+// NOTE "deleted" records are indicated via returning buf with .Data=nil without error.
 func (dh *DataHeader) LoadData(r io.ReaderAt) (*zodb.Buf, error) {
 	// scan via backpointers
 	for dh.DataLen == 0 {

go/zodb/zodb.go

@@ -160,7 +160,6 @@ type IStorage interface {
 
 	// Load loads object data addressed by xid from database.
 	//
-	// XXX currently deleted data is returned as buf.Data=nil	-- is it ok?
 	// TODO specify error when data not found -> ErrOidMissing | ErrXidMissing
 	//
 	// NOTE ZODB/py provides 2 entrypoints in IStorage for loading: