gh-151673: Fix crash in warnings.warn() under memory pressure

Ijtihed · Ijtihed · commit e01d44b6255f · 2026-06-20T14:19:24.000+03:00
diff --git a/Lib/test/test_warnings/__init__.py b/Lib/test/test_warnings/__init__.py
@@ -24,7 +24,6 @@
 import warnings as original_warnings
 from warnings import deprecated
 
-
 py_warnings = import_helper.import_fresh_module('_py_warnings')
 py_warnings._set_module(py_warnings)
 
@@ -1237,6 +1236,25 @@ def test_issue31566(self):
                  support.swap_item(globals(), '__file__', None):
                 self.assertRaises(UserWarning, self.module.warn, 'bar')
 
+    @support.cpython_only
+    # Python built with Py_TRACE_REFS fail with a fatal error in
+    # _PyRefchain_Trace() on memory allocation error.
+    @unittest.skipIf(support.Py_TRACE_REFS, 'cannot test Py_TRACE_REFS build')
+    def test_issue151673(self):
+        # Skip this test if the _testcapi module isn't available.
+        _testcapi = import_helper.import_module('_testcapi')
+        # warn() shouldn't crash when the "<sys>" fallback filename
+        # can't be allocated under memory pressure.
+        code = """if 1:
+            import _testcapi, warnings
+            warnings.simplefilter("always")
+            _testcapi.set_nomemory(0)
+            warnings.warn("boom")
+        """
+        rc, out, err = assert_python_failure("-c", code)
+        self.assertIn(rc, (1, 120))
+        self.assertIn(b'MemoryError', err)
+
 
 class WarningsDisplayTests(BaseTest):
 
diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2026-06-19-21-58-01.gh-issue-151673.Kx9mPq.rst b/Misc/NEWS.d/next/Core_and_Builtins/2026-06-19-21-58-01.gh-issue-151673.Kx9mPq.rst
@@ -0,0 +1,3 @@
+Fix a crash in :func:`warnings.warn` when the ``"<sys>"`` fallback filename
+could not be allocated under memory pressure: :c:func:`!setup_context` left
+the filename ``NULL`` and it was later passed to :c:func:`Py_DECREF`.
diff --git a/Python/_warnings.c b/Python/_warnings.c
@@ -1031,9 +1031,18 @@ setup_context(Py_ssize_t stack_level,
         }
     }
 
+    /* Initialize the output references so handle_error can Py_XDECREF them
+       even when we bail out before they are assigned. */
+    *filename = NULL;
+    *registry = NULL;
+    *module = NULL;
+
     if (f == NULL) {
         globals = interp->sysdict;
         *filename = PyUnicode_FromString("<sys>");
+        if (*filename == NULL) {
+            goto handle_error;
+        }
         *lineno = 0;
     }
     else {
@@ -1043,8 +1052,6 @@ setup_context(Py_ssize_t stack_level,
         Py_DECREF(f);
     }
 
-    *module = NULL;
-
     /* Setup registry. */
     assert(globals != NULL);
     assert(PyAnyDict_Check(globals));
@@ -1084,7 +1091,7 @@ setup_context(Py_ssize_t stack_level,
  handle_error:
     Py_XDECREF(*registry);
     Py_XDECREF(*module);
-    Py_DECREF(*filename);
+    Py_XDECREF(*filename);
     return 0;
 }
 

Original file line number	Diff line number	Diff line change
`@@ -0,0 +1,3 @@`
	`1`	+Fix a crash in :func:`warnings.warn` when the ``"<sys>"`` fallback filename
	`2`	+could not be allocated under memory pressure: :c:func:`!setup_context` left
	`3`	+the filename ``NULL`` and it was later passed to :c:func:`Py_DECREF`.