Issue #7084: Fix a (very unlikely) crash when printing a list from one

thread, and mutating it from another one.  Patch by Scott Dial.

Misc/ACKS

@@ -175,6 +175,7 @@ Arnaud Delobelle
 Erik Demaine
 Roger Dev
 Raghuram Devarakonda
+Scott Dial
 Toby Dickenson
 Mark Dickinson
 Jack Diederich

Misc/NEWS

@@ -12,6 +12,9 @@ What's New in Python 2.7 alpha 1
 Core and Builtins
 -----------------
 
+- Issue #7084: Fix a (very unlikely) crash when printing a list from one
+  thread, and mutating it from another one.  Patch by Scott Dial.
+
 - Issue #1571184: The Unicode database contains properties for more characters.
   The tables for code points representing numeric values, white spaces or line
   breaks are now generated from the official Unicode Character Database files,

Objects/listobject.c

@@ -319,6 +319,7 @@ list_print(PyListObject *op, FILE *fp, int flags)
 {
 	int rc;
 	Py_ssize_t i;
+	PyObject *item;
 
 	rc = Py_ReprEnter((PyObject*)op);
 	if (rc != 0) {
@@ -333,15 +334,19 @@ list_print(PyListObject *op, FILE *fp, int flags)
 	fprintf(fp, "[");
 	Py_END_ALLOW_THREADS
 	for (i = 0; i < Py_SIZE(op); i++) {
+		item = op->ob_item[i];
+		Py_INCREF(item);
 		if (i > 0) {
 			Py_BEGIN_ALLOW_THREADS
 			fprintf(fp, ", ");
 			Py_END_ALLOW_THREADS
 		}
-		if (PyObject_Print(op->ob_item[i], fp, 0) != 0) {
+		if (PyObject_Print(item, fp, 0) != 0) {
+			Py_DECREF(item);
 			Py_ReprLeave((PyObject *)op);
 			return -1;
 		}
+		Py_DECREF(item);
 	}
 	Py_BEGIN_ALLOW_THREADS
 	fprintf(fp, "]");