Issue #16230: Fix a crash in select.select() when one the lists changes size while iterated on.

Patch by Serhiy Storchaka.

Lib/test/test_select.py

@@ -65,6 +65,16 @@ class SelectTestCase(unittest.TestCase):
             self.fail('Unexpected return values from select():', rfd, wfd, xfd)
         p.close()
 
+    # Issue 16230: Crash on select resized list
+    def test_select_mutated(self):
+        a = []
+        class F:
+            def fileno(self):
+                del a[-1]
+                return sys.__stdout__.fileno()
+        a[:] = [F()] * 10
+        self.assertEqual(select.select([], a, []), ([], a[:5], []))
+
 def test_main():
     support.run_unittest(SelectTestCase)
     support.reap_children()

Misc/NEWS

@@ -85,6 +85,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #16230: Fix a crash in select.select() when one the lists changes
+  size while iterated on.  Patch by Serhiy Storchaka.
+
 - Issue #16228: Fix a crash in the json module where a list changes size
   while it is being encoded.  Patch by Serhiy Storchaka.
 

Modules/selectmodule.c

@@ -84,7 +84,7 @@ seq2set(PyObject *seq, fd_set *set, pylist fd2obj[FD_SETSIZE + 1])
 {
     int max = -1;
     int index = 0;
-    Py_ssize_t i, len = -1;
+    Py_ssize_t i;
     PyObject* fast_seq = NULL;
     PyObject* o = NULL;
 
@@ -95,9 +95,7 @@ seq2set(PyObject *seq, fd_set *set, pylist fd2obj[FD_SETSIZE + 1])
     if (!fast_seq)
         return -1;
 
-    len = PySequence_Fast_GET_SIZE(fast_seq);
-
-    for (i = 0; i < len; i++)  {
+    for (i = 0; i < PySequence_Fast_GET_SIZE(fast_seq); i++)  {
         SOCKET v;
 
         /* any intervening fileno() calls could decr this refcnt */