Fix and add test syzygy DTZ bug

See official-stockfish/Stockfish@6e2ca97d93812b2.

Author: niklasf

chess/syzygy.py

@@ -1465,10 +1465,10 @@ def probe_dtz_no_ep(self, board):
 
                 board.push(move)
 
-                v_plus, success = self.probe_ab(board, -2, -wdl + 1)
+                v_plus = self.probe_wdl(board)
                 board.pop()
 
-                if v_plus is None or not success:
+                if v_plus is None:
                     return None
 
                 v = -v_plus

test.py

@@ -2437,7 +2437,7 @@ def test_probe_dtz_table_pawn(self):
 
     def test_probe_wdl_tablebase(self):
         tablebases = chess.syzygy.Tablebases(max_fds=2)
-        self.assertEqual(tablebases.open_directory("data/syzygy"), 70)
+        self.assertTrue(tablebases.open_directory("data/syzygy") >= 70)
 
         # Winning KRvKB.
         board = chess.Board("7k/6b1/6K1/8/8/8/8/3R4 b - - 12 7")
@@ -2502,6 +2502,15 @@ def test_testsuite(self):
 
         tablebases.close()
 
+    def test_stockfish_dtz_bug(self):
+        board = chess.Board("3K4/8/3k4/8/4p3/4B3/5P2/8 w - - 0 5")
+        tablebases = chess.syzygy.Tablebases("data/syzygy")
+        dtz = tablebases.probe_dtz(board)
+        if dtz is None:
+            self.skipTest("need KBPvKP.rtbz and its children")
+        self.assertEqual(tablebases.probe_dtz(board), 15)
+        tablebases.close()
+
 
 class NativeGaviotaTestCase(unittest.TestCase):