[3.11] gh-107915: Handle errors in C API functions PyErr_Set*() and PyErr_Format() (GH-107918) (GH-108135)

Such C API functions as PyErr_SetString(), PyErr_Format(),
PyErr_SetFromErrnoWithFilename() and many others no longer crash or
ignore errors if it failed to format the error message or decode the
filename. Instead, they keep a corresponding error.
(cherry picked from commit 633ea21)

Author: serhiy-storchaka

Misc/NEWS.d/next/C API/2023-08-13-12-33-00.gh-issue-107915.jQ0wOi.rst

@@ -0,0 +1,4 @@
+Such C API functions as ``PyErr_SetString()``, ``PyErr_Format()``,
+``PyErr_SetFromErrnoWithFilename()`` and many others no longer crash or
+ignore errors if it failed to format the error message or decode the
+filename. Instead, they keep a corresponding error.

Python/errors.c

@@ -225,8 +225,10 @@ _PyErr_SetString(PyThreadState *tstate, PyObject *exception,
                  const char *string)
 {
     PyObject *value = PyUnicode_FromString(string);
-    _PyErr_SetObject(tstate, exception, value);
-    Py_XDECREF(value);
+    if (value != NULL) {
+        _PyErr_SetObject(tstate, exception, value);
+        Py_DECREF(value);
+    }
 }
 
 void
@@ -852,7 +854,13 @@ PyErr_SetFromErrnoWithFilenameObjects(PyObject *exc, PyObject *filenameObject, P
 PyObject *
 PyErr_SetFromErrnoWithFilename(PyObject *exc, const char *filename)
 {
-    PyObject *name = filename ? PyUnicode_DecodeFSDefault(filename) : NULL;
+    PyObject *name = NULL;
+    if (filename) {
+        name = PyUnicode_DecodeFSDefault(filename);
+        if (name == NULL) {
+            return NULL;
+        }
+    }
     PyObject *result = PyErr_SetFromErrnoWithFilenameObjects(exc, name, NULL);
     Py_XDECREF(name);
     return result;
@@ -949,7 +957,13 @@ PyObject *PyErr_SetExcFromWindowsErrWithFilename(
     int ierr,
     const char *filename)
 {
-    PyObject *name = filename ? PyUnicode_DecodeFSDefault(filename) : NULL;
+    PyObject *name = NULL;
+    if (filename) {
+        name = PyUnicode_DecodeFSDefault(filename);
+        if (name == NULL) {
+            return NULL;
+        }
+    }
     PyObject *ret = PyErr_SetExcFromWindowsErrWithFilenameObjects(exc,
                                                                  ierr,
                                                                  name,
@@ -973,7 +987,13 @@ PyObject *PyErr_SetFromWindowsErrWithFilename(
     int ierr,
     const char *filename)
 {
-    PyObject *name = filename ? PyUnicode_DecodeFSDefault(filename) : NULL;
+    PyObject *name = NULL;
+    if (filename) {
+        name = PyUnicode_DecodeFSDefault(filename);
+        if (name == NULL) {
+            return NULL;
+        }
+    }
     PyObject *result = PyErr_SetExcFromWindowsErrWithFilenameObjects(
                                                   PyExc_OSError,
                                                   ierr, name, NULL);
@@ -1076,9 +1096,10 @@ _PyErr_FormatV(PyThreadState *tstate, PyObject *exception,
     _PyErr_Clear(tstate);
 
     string = PyUnicode_FromFormatV(format, vargs);
-
-    _PyErr_SetObject(tstate, exception, string);
-    Py_XDECREF(string);
+    if (string != NULL) {
+        _PyErr_SetObject(tstate, exception, string);
+        Py_DECREF(string);
+    }
     return NULL;
 }