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, 2 insertions, 2 deletions

diff --git a/Python/bltinmodule.c b/Python/bltinmodule.c
index 06d8616..aa65ebc 100644
--- a/Python/bltinmodule.c
+++ b/Python/bltinmodule.c
@@ -1993,8 +1993,8 @@ filterstring(PyObject *func, PyObject *strobj)
 		Py_DECREF(item);
 	}
 
-	if (j < len && _PyString_Resize(&result, j) < 0)
-		return NULL;
+	if (j < len)
+		_PyString_Resize(&result, j);
 
 	return result;