bail in unicode error's __str__ methods if the objects are not properly initialized (closes #21134)

Lib/test/test_exceptions.py

@@ -431,6 +431,12 @@ class ExceptionTests(unittest.TestCase):
         u.start = 1000
         self.assertEqual(str(u), "can't translate characters in position 1000-4: 965230951443685724997")
 
+    def test_unicode_errors_no_object(self):
+        # See issue #21134.
+        klasses = UnicodeDecodeError, UnicodeDecodeError, UnicodeTranslateError
+        for klass in klasses:
+            self.assertEqual(str(klass.__new__(klass)), "")
+
     def test_badisinstance(self):
         # Bug #2542: if issubclass(e, MyException) raises an exception,
         # it should be ignored

Misc/NEWS

@@ -12,6 +12,9 @@ Core and Builtins
 
 - Issue #20437: Fixed 43 potential bugs when deleting objects references.
 
+- Issue #21134: Fix segfault when str is called on an uninitialized
+  UnicodeEncodeError, UnicodeDecodeError, or UnicodeTranslateError object.
+
 - Issue #20494: Ensure that free()d memory arenas are really released on POSIX
   systems supporting anonymous memory mappings.  Patch by Charles-François
   Natali.

Objects/exceptions.c

@@ -1648,6 +1648,10 @@ UnicodeEncodeError_str(PyObject *self)
     PyObject *reason_str = NULL;
     PyObject *encoding_str = NULL;
 
+    if (!uself->object)
+        /* Not properly initialized. */
+        return PyUnicode_FromString("");
+
     /* Get reason and encoding as strings, which they might not be if
        they've been modified after we were contructed. */
     reason_str = PyObject_Str(uself->reason);
@@ -1733,6 +1737,10 @@ UnicodeDecodeError_str(PyObject *self)
     PyObject *reason_str = NULL;
     PyObject *encoding_str = NULL;
 
+    if (!uself->object)
+        /* Not properly initialized. */
+        return PyUnicode_FromString("");
+
     /* Get reason and encoding as strings, which they might not be if
        they've been modified after we were contructed. */
     reason_str = PyObject_Str(uself->reason);
@@ -1830,6 +1838,10 @@ UnicodeTranslateError_str(PyObject *self)
     PyObject *result = NULL;
     PyObject *reason_str = NULL;
 
+    if (!uself->object)
+        /* Not properly initialized. */
+        return PyUnicode_FromString("");
+
     /* Get reason as a string, which it might not be if it's been
        modified after we were contructed. */
     reason_str = PyObject_Str(uself->reason);