warnings.formatwarning(): catch exceptions

Issue #21925: warnings.formatwarning() now catches exceptions on
linecache.getline(...) to be able to log ResourceWarning emitted late during
the Python shutdown process.

Lib/test/test_warnings/__init__.py

@@ -953,6 +953,23 @@ a=A()
         # of the script
         self.assertEqual(err, b'__main__:7: UserWarning: test')
 
+    def test_late_resource_warning(self):
+        # Issue #21925: Emitting a ResourceWarning late during the Python
+        # shutdown must be logged.
+
+        expected = b"sys:1: ResourceWarning: unclosed file "
+
+        # don't import the warnings module
+        # (_warnings will try to import it)
+        code = "f = open(%a)" % __file__
+        rc, out, err = assert_python_ok("-c", code)
+        self.assertTrue(err.startswith(expected), ascii(err))
+
+        # import the warnings module
+        code = "import warnings; f = open(%a)" % __file__
+        rc, out, err = assert_python_ok("-c", code)
+        self.assertTrue(err.startswith(expected), ascii(err))
+
 
 def setUpModule():
     py_warnings.onceregistry.clear()

Lib/warnings.py

@@ -21,9 +21,15 @@ def showwarning(message, category, filename, lineno, file=None, line=None):
 
 def formatwarning(message, category, filename, lineno, line=None):
     """Function to format a warning the standard way."""
-    import linecache
     s =  "%s:%s: %s: %s\n" % (filename, lineno, category.__name__, message)
-    line = linecache.getline(filename, lineno) if line is None else line
+    if line is None:
+        try:
+            import linecache
+            line = linecache.getline(filename, lineno)
+        except Exception:
+            # When a warning is logged during Python shutdown, linecache
+            # and the improt machinery don't work anymore
+            line = None
     if line:
         line = line.strip()
         s += "  %s\n" % line

Misc/NEWS

@@ -94,6 +94,10 @@ Core and Builtins
 Library
 -------
 
+- Issue #21925: :func:`warnings.formatwarning` now catches exceptions on
+  ``linecache.getline(...)`` to be able to log :exc:`ResourceWarning` emitted
+  late during the Python shutdown process.
+
 - Issue #24266: Ctrl+C during Readline history search now cancels the search
   mode when compiled with Readline 7.