.

go/zodb/storage/fs1/filestorage_test.go

-// Copyright (C) 2017-2020  Nexedi SA and Contributors.
+// Copyright (C) 2017-2021  Nexedi SA and Contributors.
 //                          Kirill Smelkov <kirr@nexedi.com>
 //
 // This program is free software: you can Use, Study, Modify and Redistribute
@@ -370,3 +370,37 @@ func TestOpenRecovery(t *testing.T) {
 		})
 	}
 }
+
+
+// TestLoadWhiteout verifies access to whiteout data record.
+//
+// Whiteout is data record with deletion when object was not previously there.
+// It has both len(data)=0 and backpointer=0.
+//
+// TODO merge into regilat tests on testdata/1.fs when
+// FileStorage/py.deleteObject allows to create whiteouts instead of raising
+// POSKeyError.
+func TestLoadWhiteout(t *testing.T) {
+	fs, _ := xfsopen(t, "testdata/whiteout.fs")
+	defer exc.XRun(fs.Close)
+
+	xid := zodb.Xid{At: zodb.Tid(0x17), Oid: zodb.Oid(1)}
+	buf, serial, err := fs.Load(context.Background(), xid)
+
+	errOk := &zodb.OpError{
+		URL:  fs.URL(),
+		Op:   "load",
+		Args: xid,
+		Err:  &zodb.NoDataError{Oid: xid.Oid, DeletedAt: xid.At},
+	}
+
+	if !reflect.DeepEqual(err, errOk) {
+		t.Errorf("load %s: bad err:\nhave: %v\nwant: %v", xid, err, errOk)
+	}
+	if buf != nil {
+		t.Errorf("load %s: buf != nil", xid)
+	}
+	if serial != 0 {
+		t.Errorf("load %s: bad serial %s  ; want 0", xid, serial)
+	}
+}

go/zodb/storage/fs1/format.go

-// Copyright (C) 2017-2020  Nexedi SA and Contributors.
+// Copyright (C) 2017-2021  Nexedi SA and Contributors.
 //                          Kirill Smelkov <kirr@nexedi.com>
 //
 // This program is free software: you can Use, Study, Modify and Redistribute
@@ -575,7 +575,10 @@ func (dh *DataHeader) LoadBackRef(r io.ReaderAt) (backPos int64, err error) {
 	}
 
 	backPos = int64(binary.BigEndian.Uint64(dh.workMem[0:]))
-	if !(backPos == 0 || backPos >= dataValidFrom) {
+	if backPos == 0 {
+		return 0, nil // deletion
+	}
+	if backPos < dataValidFrom {
 		return 0, checkErr(r, dh, "invalid backpointer: %v", backPos)
 	}
 	if backPos + DataHeaderSize > dh.TxnPos - 8 {

go/zodb/storage/fs1/py/gen-testdata

 #!/usr/bin/env python2
 # -*- coding: utf-8 -*-
-# Copyright (C) 2017-2020  Nexedi SA and Contributors.
+# Copyright (C) 2017-2021  Nexedi SA and Contributors.
 #                          Kirill Smelkov <kirr@nexedi.com>
 #
 # This program is free software: you can Use, Study, Modify and Redistribute
@@ -21,9 +21,10 @@
 """generate reference fs1 database and index for tests"""
 
 from ZODB.FileStorage import FileStorage
+from ZODB.FileStorage.FileStorage import FILESTORAGE_MAGIC, TxnHeader, DataHeader, TRANS_HDR_LEN
 from ZODB import DB
 from ZODB.Connection import TransactionMetaData
-from zodbtools.test.gen_testdata import gen_testdb, precommit, run_with_zodb3py2_compat
+from zodbtools.test.gen_testdata import gen_testdb, precommit, run_with_zodb4py2_compat
 from os import stat, remove
 from shutil import copyfile
 from golang.gcompat import qq
@@ -156,7 +157,7 @@ def main():
         vstor.store(vroot._p_oid, vroot._p_serial, '000 data 000', '', txn_stormeta)
         vstor.tpc_vote(txn_stormeta)
         # NO tpc_finish here so that status remain 'c' (voted) instead of ' ' (committed)
-    run_with_zodb3py2_compat(_)
+    run_with_zodb4py2_compat(_)
 
     st = stat(outfs)
     l  = st.st_size
@@ -174,5 +175,37 @@ def main():
     remove(voted+".lock")
 
 
+    # prepare file with whiteout (deletion of previously non-existing object)
+    whiteout = "testdata/whiteout.fs"
+    # as of 20210317 FileStorage.deleteObject verifies that object exists
+    # -> prepare magic/transaction/data records manually
+    with open(whiteout, "wb") as f:
+        oid = p64(1)
+        tid = p64(0x17)
+
+        # file header
+        f.write(FILESTORAGE_MAGIC)
+        tpos = f.tell()
+
+        # data record (see FileStorage.deleteObject)
+        dh = DataHeader(oid, tid, 0, tpos, 0, 0)
+        drec = dh.asString() + p64(0)
+
+        # emit txn header (see FileStorage.tpc_vote)
+        tlen = TRANS_HDR_LEN + 0 + 0 + 0 + len(drec) # empty u,d,e
+        th = TxnHeader(tid, tlen, ' ', 0, 0, 0)
+        th.user  = b''
+        th.descr = b''
+        th.ext   = b''
+        f.write(th.asString())
+
+        # emit data record
+        f.write(drec)
+
+        # emit txn tail
+        f.write(p64(tlen))
+
+
+
 if __name__ == '__main__':
     main()