Protect from scenarios where all frames should be ignored

src/structlog/_frames.py

@@ -39,6 +39,8 @@ def _find_first_app_frame_and_name(additional_ignores=None):
     f = sys._getframe()
     name = f.f_globals.get("__name__") or "?"
     while any(name.startswith(i) for i in ignores):
+        if f.f_back is None:
+            break
         f = f.f_back
         name = f.f_globals.get("__name__") or "?"
     return f, name

tests/test_frames.py

@@ -70,6 +70,15 @@ def test_tolerates_name_explicitly_None_manyframe(self, monkeypatch):
         f, n = _find_first_app_frame_and_name()
         assert ((f1, "?") == (f, n))
 
+    def test_tolerates_f_back_is_None(self, monkeypatch):
+        """
+        Use ``?`` if all frames are in ignored frames.
+        """
+        f1 = stub(f_globals={'__name__': 'hello_world'}, f_back=None)
+        monkeypatch.setattr(structlog._frames.sys, "_getframe", lambda: f1)
+        f, n = _find_first_app_frame_and_name()
+        assert ((f1, "?") == (f, n))
+
 
 @pytest.fixture
 def exc_info():