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

Author: pablogsal

Lib/test/test_external_inspection.py

@@ -1684,6 +1684,41 @@ 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):
+        _testcapi = import_module("_testcapi")
+        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)
+        try:
+            for cache_frames, native in ((False, False), (False, True),
+                                         (True, False), (True, True)):
+                with self.subTest(cache_frames=cache_frames, native=native):
+                    unwinder = RemoteUnwinder(
+                        os.getpid(), all_threads=True, cache_frames=cache_frames,
+                        native=native,
+                    )
+                    _get_stack_trace_with_retry(
+                        unwinder, condition=lambda trace: len(trace[0].threads) == 2,
+                    )
+                    threads = unwinder.get_stack_trace()[0].threads
+                    native_stack, python_stack = sorted(
+                        (thread.frame_info for thread in threads), key=len,
+                    )
+                    self.assertEqual(native_stack, [])
+                    self.assertEqual(
+                        python_stack[0].funcname,
+                        "TestGetStackTrace.test_empty_native_thread_stack",
+                    )
+        finally:
+            lock.release()
+            _testcapi.join_temporary_c_thread()
+
     @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

@@ -358,19 +358,18 @@ 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>");
             extra_frame = &_Py_STR(gc);
         }
+        // A leading frame without Python code marks no transition between
+        // Python frames: it is a frame being popped or C code the thread is
+        // returning into.
         else if (unwinder->native &&
                  frame == NULL &&
                  next_frame_addr &&
+                 PyList_GET_SIZE(ctx->frame_info) > 0 &&
                  !(unwinder->gc && next_frame_addr == ctx->gc_frame))
         {
             _Py_DECLARE_STR(native, "<native>");