profiling.sampling fails when a native thread has an empty Python stack #157605

Bug description

A native thread can retain a Python thread state while waiting in C code, with no Python functions executing. Its frame chain contains only the base_frame sentinel.

The sampler skips this sentinel, then raises:

RuntimeError: Failed to parse initial frame in chain

This fails the entire sampling call, so valid Python stacks from other threads are also lost.

Reproduction

Create a pthread that registers and detaches a Python thread state:

PyGILState_STATE state = PyGILState_Ensure();
PyThreadState *saved = PyEval_SaveThread();

/* Wait on a pthread condition variable while the parent samples. */

/* Cleanup after sampling. */
PyEval_RestoreThread(saved);
PyGILState_Release(state);

Keep the main thread inside a Python function and sample the child process from its parent:

unwinder = _remote_debugging.RemoteUnwinder(
    pid, all_threads=True, cache_frames=False,
)
unwinder.get_stack_trace()

• Without registering the native thread's Python state: 40/40 samples succeed.
• With the retained state: 40/40 samples fail.
• Reproduces with frame caching both enabled and disabled.

Expected behavior

Accept the native thread's empty Python stack and return valid stacks from the other threads.

Suspected cause

The check frame == NULL && PyList_GET_SIZE(ctx->frame_info) == 0 treats the valid empty-stack sentinel as a broken frame chain.

Environment

Linux, unmodified CPython 3.15.0rc2+dev free-threaded build.
Current main has not been tested.

tachyon-empty-native-repro.zip

Linked PRs

• gh-157605: Accept empty Python frame stack in sampling frame chain parser #157657