gh-159739: Fix reference and error handling in warnings (#159758)

Do not release the borrowed skip_file_prefixes argument, and stop if
allocation of the fallback filename fails.

Author: jelle-openai

Lib/test/test_warnings/__init__.py

@@ -698,6 +698,20 @@ def test_skip_file_prefixes_type_errors(self):
             with self.assertRaises(TypeError):
                 warn("msg", skip_file_prefixes="a sequence of strs")
 
+    def test_skip_file_prefixes_empty_tuple_subclass(self):
+        with warnings_state(self.module):
+            self.module.simplefilter("always")
+            warn = warning_tests.warnings.warn
+            class EmptyTuple(tuple):
+                pass
+            # Passing an empty tuple subclass used to lead to a crash.
+            for _ in range(3):
+                with self.module.catch_warnings(record=True) as w:
+                    warn("msg", skip_file_prefixes=EmptyTuple())
+                self.assertEqual(len(w), 1)
+                self.assertEqual(w[-1].filename, __file__)
+                self.assertEqual(w[-1].module, __name__)
+
     def test_exec_filename_module(self):
         filename = "<warnings-test>"
         codeobj = compile(("import warnings\n"

Misc/NEWS.d/next/Core_and_Builtins/2026-10-19-19-33-35.gh-issue-159739.f81734.rst

@@ -0,0 +1,2 @@
+Fix a crash in :func:`warnings.warn` when ``skip_file_prefixes`` is an empty tuple subclass.
+Propagate allocation errors when creating the fallback filename.

Python/_warnings.c

@@ -1053,6 +1053,9 @@ setup_context(Py_ssize_t stack_level,
     if (f == NULL) {
         globals = interp->sysdict;
         *filename = PyUnicode_FromString("<sys>");
+        if (*filename == NULL) {
+            return 0;
+        }
         *lineno = 0;
     }
     else {
@@ -1196,7 +1199,6 @@ warnings_warn_impl(PyObject *module, PyObject *message, PyObject *category,
                 stacklevel = 2;
             }
         } else {
-            Py_DECREF((PyObject *)skip_file_prefixes);
             skip_file_prefixes = NULL;
         }
     }