bpo-35441: Remove dead and buggy code related to PyList_SetItem(). (GH-11033)

In _localemodule.c and selectmodule.c, remove dead code that would
cause double decrefs if run.

In addition, replace PyList_SetItem() with PyList_SET_ITEM() in cases
where a new list is populated and there is no possibility of an error.

In addition, check if the list changed size in the loop in array_array_fromlist().
(cherry picked from commit 99d56b53560b3867844472ae381fb3f858760621)

Co-authored-by: Zackery Spytz <zspytz@gmail.com>

7 files changed, 25 insertions, 30 deletions

diff --git a/Modules/_localemodule.c b/Modules/_localemodule.c
index f3421af..eba8289 100644
--- a/Modules/_localemodule.c
+++ b/Modules/_localemodule.c
@@ -70,20 +70,13 @@ copy_grouping(const char* s)
     do {
         i++;
         val = PyLong_FromLong(s[i]);
-        if (!val)
-            break;
-        if (PyList_SetItem(result, i, val)) {
-            Py_DECREF(val);
-            val = NULL;
-            break;
+        if (val == NULL) {
+            Py_DECREF(result);
+            return NULL;
         }
+        PyList_SET_ITEM(result, i, val);
     } while (s[i] != '\0' && s[i] != CHAR_MAX);
 
-    if (!val) {
-        Py_DECREF(result);
-        return NULL;
-    }
-
     return result;
 }
 
diff --git a/Modules/arraymodule.c b/Modules/arraymodule.c
index 9254af5..bc3f62d 100644
--- a/Modules/arraymodule.c
+++ b/Modules/arraymodule.c
@@ -1499,12 +1499,18 @@ array_array_fromlist(arrayobject *self, PyObject *list)
         if (array_resize(self, old_size + n) == -1)
             return NULL;
         for (i = 0; i < n; i++) {
-            PyObject *v = PyList_GetItem(list, i);
+            PyObject *v = PyList_GET_ITEM(list, i);
             if ((*self->ob_descr->setitem)(self,
                             Py_SIZE(self) - n + i, v) != 0) {
                 array_resize(self, old_size);
                 return NULL;
             }
+            if (n != PyList_GET_SIZE(list)) {
+                PyErr_SetString(PyExc_RuntimeError,
+                                "list changed size during iteration");
+                array_resize(self, old_size);
+                return NULL;
+            }
         }
     }
     Py_INCREF(Py_None);
@@ -1530,8 +1536,7 @@ array_array_tolist_impl(arrayobject *self)
         PyObject *v = getarrayitem((PyObject *)self, i);
         if (v == NULL)
             goto error;
-        if (PyList_SetItem(list, i, v) < 0)
-            goto error;
+        PyList_SET_ITEM(list, i, v);
     }
     return list;
 
diff --git a/Modules/readline.c b/Modules/readline.c
index 575479c..0965715 100644
--- a/Modules/readline.c
+++ b/Modules/readline.c
@@ -947,8 +947,7 @@ on_completion_display_matches_hook(char **matches,
         s = decode(matches[i+1]);
         if (s == NULL)
             goto error;
-        if (PyList_SetItem(m, i, s) == -1)
-            goto error;
+        PyList_SET_ITEM(m, i, s);
     }
     sub = decode(matches[0]);
     r = PyObject_CallFunction(readlinestate_global->completion_display_matches_hook,
diff --git a/Modules/selectmodule.c b/Modules/selectmodule.c
index 4b99657..2bc7b9f 100644
--- a/Modules/selectmodule.c
+++ b/Modules/selectmodule.c
@@ -655,10 +655,7 @@ poll_poll(pollObject *self, PyObject *args)
             goto error;
         }
         PyTuple_SET_ITEM(value, 1, num);
-        if ((PyList_SetItem(result_list, j, value)) == -1) {
-            Py_DECREF(value);
-            goto error;
-        }
+        PyList_SET_ITEM(result_list, j, value);
         i++;
     }
     return result_list;
@@ -984,10 +981,7 @@ devpoll_poll(devpollObject *self, PyObject *args)
         Py_DECREF(num2);
         if (value == NULL)
             goto error;
-        if ((PyList_SetItem(result_list, i, value)) == -1) {
-            Py_DECREF(value);
-            goto error;
-        }
+        PyList_SET_ITEM(result_list, i, value);
     }
 
     return result_list;
diff --git a/PC/winreg.c b/PC/winreg.c
index 5efdc5e..3fde04d 100644
--- a/PC/winreg.c
+++ b/PC/winreg.c
@@ -757,9 +757,13 @@ Reg2Py(BYTE *retDataBuf, DWORD retDataSize, DWORD typ)
                         PyMem_Free(str);
                         return NULL;
                     }
-                    PyList_SetItem(obData,
-                                   index,
-                                   PyUnicode_FromWideChar(str[index], len));
+                    PyObject *uni = PyUnicode_FromWideChar(str[index], len);
+                    if (uni == NULL) {
+                        Py_DECREF(obData);
+                        PyMem_Free(str);
+                        return NULL;
+                    }
+                    PyList_SET_ITEM(obData, index, uni);
                 }
                 PyMem_Free(str);
 
diff --git a/Python/ceval.c b/Python/ceval.c
index 50834f8..38d1d73 100644
--- a/Python/ceval.c
+++ b/Python/ceval.c
@@ -5500,7 +5500,7 @@ getarray(long a[256])
             Py_DECREF(l);
             return NULL;
         }
-        PyList_SetItem(l, i, x);
+        PyList_SET_ITEM(l, i, x);
     }
     for (i = 0; i < 256; i++)
         a[i] = 0;
@@ -5522,7 +5522,7 @@ _Py_GetDXProfile(PyObject *self, PyObject *args)
             Py_DECREF(l);
             return NULL;
         }
-        PyList_SetItem(l, i, x);
+        PyList_SET_ITEM(l, i, x);
     }
     return l;
 #endif
diff --git a/Python/sysmodule.c b/Python/sysmodule.c
index b09268b..7d1493c 100644
--- a/Python/sysmodule.c
+++ b/Python/sysmodule.c
@@ -2109,7 +2109,7 @@ makepathobject(const wchar_t *path, wchar_t delim)
             Py_DECREF(v);
             return NULL;
         }
-        PyList_SetItem(v, i, w);
+        PyList_SET_ITEM(v, i, w);
         if (*p == '\0')
             break;
         path = p+1;