Repair widespread misuse of _PyString_Resize. Since it's clear people

don't understand how this function works, also beefed up the docs.  The
most common usage error is of this form (often spread out across gotos):

	if (_PyString_Resize(&s, n) < 0) {
		Py_DECREF(s);
		s = NULL;
		goto outtahere;
	}

The error is that if _PyString_Resize runs out of memory, it automatically
decrefs the input string object s (which also deallocates it, since its
refcount must be 1 upon entry), and sets s to NULL.  So if the "if"
branch ever triggers, it's an error to call Py_DECREF(s):  s is already
NULL!  A correct way to write the above is the simpler (and intended)

	if (_PyString_Resize(&s, n) < 0)
		goto outtahere;

Bugfix candidate.

1 files changed, 8 insertions, 26 deletions

diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index c4c8d8d..9a3e456 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -927,10 +927,7 @@ PyObject *PyUnicode_EncodeUTF7(const Py_UNICODE *s,
         *out++ = '-';
     }
 
-    if (_PyString_Resize(&v, out - start)) {
-        Py_DECREF(v);
-        return NULL;
-    }
+    _PyString_Resize(&v, out - start);
     return v;
 }
 
@@ -1764,7 +1761,7 @@ PyObject *unicodeescape_string(const Py_UNICODE *s,
 	    /* Resize the string if necessary */
 	    if (offset + 12 > PyString_GET_SIZE(repr)) {
 		if (_PyString_Resize(&repr, PyString_GET_SIZE(repr) + 100))
-		    goto onError;
+		    return NULL;
 		p = PyString_AS_STRING(repr) + offset;
 	    }
 
@@ -1847,14 +1844,8 @@ PyObject *unicodeescape_string(const Py_UNICODE *s,
         *p++ = PyString_AS_STRING(repr)[1];
 
     *p = '\0';
-    if (_PyString_Resize(&repr, p - PyString_AS_STRING(repr)))
-	goto onError;
-
+    _PyString_Resize(&repr, p - PyString_AS_STRING(repr));
     return repr;
-
- onError:
-    Py_DECREF(repr);
-    return NULL;
 }
 
 PyObject *PyUnicode_EncodeUnicodeEscape(const Py_UNICODE *s,
@@ -1985,14 +1976,8 @@ PyObject *PyUnicode_EncodeRawUnicodeEscape(const Py_UNICODE *s,
             *p++ = (char) ch;
     }
     *p = '\0';
-    if (_PyString_Resize(&repr, p - q))
-	goto onError;
-
+    _PyString_Resize(&repr, p - q);
     return repr;
-
- onError:
-    Py_DECREF(repr);
-    return NULL;
 }
 
 PyObject *PyUnicode_AsRawUnicodeEscapeString(PyObject *unicode)
@@ -2092,8 +2077,7 @@ PyObject *PyUnicode_EncodeLatin1(const Py_UNICODE *p,
     }
     /* Resize if error handling skipped some characters */
     if (s - start < PyString_GET_SIZE(repr))
-	if (_PyString_Resize(&repr, s - start))
-	    goto onError;
+	_PyString_Resize(&repr, s - start);
     return repr;
 
  onError:
@@ -2240,8 +2224,7 @@ PyObject *PyUnicode_EncodeASCII(const Py_UNICODE *p,
     }
     /* Resize if error handling skipped some characters */
     if (s - start < PyString_GET_SIZE(repr))
-	if (_PyString_Resize(&repr, s - start))
-	    goto onError;
+	_PyString_Resize(&repr, s - start);
     return repr;
 
  onError:
@@ -2588,12 +2571,11 @@ PyObject *PyUnicode_EncodeCharmap(const Py_UNICODE *p,
 	Py_DECREF(x);
     }
     if (s - PyString_AS_STRING(v) < PyString_GET_SIZE(v))
-	if (_PyString_Resize(&v, (int)(s - PyString_AS_STRING(v))))
-	    goto onError;
+	_PyString_Resize(&v, (int)(s - PyString_AS_STRING(v)));
     return v;
 
  onError:
-    Py_DECREF(v);
+    Py_XDECREF(v);
     return NULL;
 }