gh-157605: Accept empty native-thread stacks when sampling

Author: pablogsal

Lib/test/test_external_inspection.py

@@ -1666,6 +1666,54 @@ def test_self_trace(self):
         self.assertEqual(this_thread_stack[1].funcname, "TestGetStackTrace.test_self_trace")
         self.assertTrue(this_thread_stack[1].filename.endswith("test_external_inspection.py"))
 
+    @skip_if_not_supported
+    @unittest.skipIf(
+        sys.platform == "linux" and not PROCESS_VM_READV_SUPPORTED,
+        "Test only runs on Linux with process_vm_readv support",
+    )
+    def test_empty_native_thread_stack(self):
+        import_module("_testcapi")
+        script = textwrap.dedent("""\
+            import _testcapi
+            import socket
+            import threading
+            import time
+
+            lock = threading.Lock()
+            lock.acquire()
+            # A built-in callback leaves the C thread's Python stack empty.
+            _testcapi.call_in_temporary_c_thread(lock.acquire, False)
+
+            def main_work():
+                with socket.create_connection(('localhost', {port})) as sock:
+                    sock.sendall(b'ready')
+                    time.sleep(10_000)
+
+            main_work()
+            """)
+
+        def check_trace(pid):
+            for cache_frames in (False, True):
+                with self.subTest(cache_frames=cache_frames):
+                    unwinder = RemoteUnwinder(
+                        pid, all_threads=True, cache_frames=cache_frames,
+                    )
+                    # Wait until the C thread has registered its thread state.
+                    trace = _get_stack_trace_with_retry(
+                        unwinder, condition=lambda t: len(t[0].threads) == 2,
+                    )
+                    for _ in range(3):
+                        self.assertEqual(len(trace[0].threads), 2)
+                        self.assertEqual(
+                            sum(not t.frame_info for t in trace[0].threads), 1,
+                        )
+                        self.assertIsNotNone(self._find_frame_in_trace(
+                            trace, lambda f: f.funcname == "main_work",
+                        ))
+                        trace = unwinder.get_stack_trace()
+
+        self._run_script_and_get_trace(script, check_trace, [b"ready"])
+
     @skip_if_not_supported
     @unittest.skipIf(
         sys.platform == "linux" and not PROCESS_VM_READV_SUPPORTED,

Misc/NEWS.d/next/Library/2026-09-19-12-00-00.gh-issue-157605.Qh92xZ.rst

@@ -0,0 +1,2 @@
+Fix :mod:`profiling.sampling` failing when a native thread has an empty Python
+stack.

Modules/_remote_debugging/frames.c

@@ -358,11 +358,6 @@ process_frame_chain(
             continue;
         }
 
-        if (frame == NULL && PyList_GET_SIZE(ctx->frame_info) == 0) {
-            const char *e = "Failed to parse initial frame in chain";
-            PyErr_SetString(PyExc_RuntimeError, e);
-            return -1;
-        }
         PyObject *extra_frame = NULL;
         if (unwinder->gc && frame_addr == ctx->gc_frame) {
             _Py_DECLARE_STR(gc, "<GC>");