Legacy tracing & PY_UNWIND: arg of legacy tracer call should be NULL

[jpe]

Bug report

Bug description:

When using a legacy tracer with PyEval_SetTrace and a function exits with an exception, the arg of the tracer event PyTrace_RETURN should be NULL docs. Currently in 3.12 (& 3.13), the arg is set to the exception value because the PY_UNWIND handler is forwarding its arg to the legacy tracer function.

CPython versions tested on:

3.12

Operating systems tested on:

No response

Linked PRs

• gh-110892: Return NULL for PyTrace_RETURN events caused by an exception #110909

[gaogaotiantian]

I'm confused. Why do you think the exception is passed to the legacy tracer? Could you share an example code?

In _PyEval_SetTrace, the callback of PY_MONITORING_EVENT_PY_UNWIND is sys_trace_func3 which unconditionally passes Py_None as the arg to the trace function. That is the case for both 3.12 and 3.13.

[jpe]

The arg needs to be NULL and not Py_None when there's an exception. For the legacy tracer 'r' event, the arg is the value returned so arg=Py_None indicates None is returned without an exception.

I found this while debugging Wing's debugger which uses a C implemented trace function and I saw the arg was not NULL and didn't look at what the value was or look closely up the stack; sorry. It may be that the trampoline that called a Python implemented trace function passed Py_None when arg was NULL, but it's been years since I worked with a Python implemented trace function.

[gaogaotiantian]

Okay I think I understood what you were talking about.

[gaogaotiantian]

PR is made to fix this.

[jpe]

I've verified that the trace function gets a NULL on an UNWIND event with the PR. Thanks!

Shouldn't the profile function also get a NULL on an UNWIND event?

[gaogaotiantian]

I've verified that the trace function gets a NULL on an UNWIND event with the PR. Thanks!

Shouldn't the profile function also get a NULL on an UNWIND event?

Yes, sys_profile_unwind is changed to return NULL in the same PR.

[jpe]

I see the profile change now; LGTM.