Fixed _pickle.Unpickler to handle empty persistent IDs correctly.

Lib/test/pickletester.py

@@ -1499,30 +1499,34 @@ class AbstractPersistentPicklerTests(unittest.TestCase):
         if isinstance(object, int) and object % 2 == 0:
             self.id_count += 1
             return str(object)
+        elif object == "test_false_value":
+            self.false_count += 1
+            return ""
         else:
             return None
 
     def persistent_load(self, oid):
+        if not oid:
+            self.load_false_count += 1
+            return "test_false_value"
+        else:
             self.load_count += 1
             object = int(oid)
             assert object % 2 == 0
             return object
 
     def test_persistence(self):
+        L = list(range(10)) + ["test_false_value"]
+        for proto in protocols:
             self.id_count = 0
+            self.false_count = 0
+            self.load_false_count = 0
             self.load_count = 0
-        L = list(range(10))
-        self.assertEqual(self.loads(self.dumps(L)), L)
-        self.assertEqual(self.id_count, 5)
-        self.assertEqual(self.load_count, 5)
-
-    def test_bin_persistence(self):
-        self.id_count = 0
-        self.load_count = 0
-        L = list(range(10))
-        self.assertEqual(self.loads(self.dumps(L, 1)), L)
+            self.assertEqual(self.loads(self.dumps(L, proto)), L)
             self.assertEqual(self.id_count, 5)
+            self.assertEqual(self.false_count, 1)
             self.assertEqual(self.load_count, 5)
+            self.assertEqual(self.load_false_count, 1)
 
 
 class AbstractPicklerUnpicklerObjectTests(unittest.TestCase):

Misc/NEWS

@@ -21,6 +21,9 @@ Library
 - Issue #19088: Fixed incorrect caching of the copyreg module in
   object.__reduce__() and object.__reduce_ex__().
 
+- Fixed _pickle.Unpickler to not fail when loading empty strings as
+  persistent IDs.
+
 - Issue #11508: Fixed uuid.getnode() and uuid.uuid1() on environment with
   virtual interface.  Original patch by Kent Frazier.
 

Modules/_pickle.c

@@ -4665,7 +4665,7 @@ load_persid(UnpicklerObject *self)
     if (self->pers_func) {
         if ((len = _Unpickler_Readline(self, &s)) < 0)
             return -1;
-        if (len < 2)
+        if (len < 1)
             return bad_readline();
 
         pid = PyBytes_FromStringAndSize(s, len - 1);