Fix obscure segfault condition in PL/Python

In PLy_output(), when the elog() call in the TRY branch throws an exception
(this can happen when a statement timeout kicks in, for example), the
PyErr_SetString() call in the CATCH branch can cause a segfault, because the
Py_XDECREF(so) call before it releases memory that is still used by the sv
variable that PyErr_SetString() uses as argument, because sv points into
memory owned by so.

Backpatched back to 8.0, where this code was introduced.

I also threw in a couple of volatile declarations for variables that are used
before and after the TRY.  I don't think they caused the crash that I
observed, but they could become issues.

diff --git a/src/pl/plpython/plpython.c b/src/pl/plpython/plpython.c
index c379938..43b1db7 100644 (file)
--- a/src/pl/plpython/plpython.c
+++ b/src/pl/plpython/plpython.c
@@ -1,7 +1,7 @@
 /**********************************************************************
  * plpython.c - python as a procedural language for PostgreSQL
  *
- *     $PostgreSQL: pgsql/src/pl/plpython/plpython.c,v 1.130 2009/09/13 22:07:06 petere Exp $
+ *     $PostgreSQL: pgsql/src/pl/plpython/plpython.c,v 1.131 2009/11/03 09:35:18 petere Exp $
  *
  *********************************************************************
  */
@@ -3076,9 +3076,9 @@ PLy_fatal(PyObject *self, PyObject *args)
 static PyObject *
 PLy_output(volatile int level, PyObject *self, PyObject *args)
 {
-       PyObject   *so;
+       PyObject   *volatile so;
        char       *volatile sv;
-       MemoryContext oldcontext;
+       volatile MemoryContext oldcontext;
 
        so = PyObject_Str(args);
        if (so == NULL || ((sv = PyString_AsString(so)) == NULL))
@@ -3097,6 +3097,10 @@ PLy_output(volatile int level, PyObject *self, PyObject *args)
                MemoryContextSwitchTo(oldcontext);
                PLy_error_in_progress = CopyErrorData();
                FlushErrorState();
+
+               PyErr_SetString(PLy_exc_error, sv);
+               /* Note: If sv came from PyString_AsString(), it points into
+                * storage owned by so.  So free so after using sv. */
                Py_XDECREF(so);
 
                /*
@@ -3104,7 +3108,6 @@ PLy_output(volatile int level, PyObject *self, PyObject *args)
                 * control passes back to PLy_procedure_call, we check for PG
                 * exceptions and re-throw the error.
                 */
-               PyErr_SetString(PLy_exc_error, sv);
                return NULL;
        }
        PG_END_TRY();