PyEval_GetLocals() leaks locals

[colesbury]

Bug report

PyEval_GetLocals() is documented as returning a borrowed reference. It now returns a new reference, which causes callers to leak the local variables:

cpython/Python/ceval.c

Lines 2478 to 2479 in 35c4361

	PyObject *locals = _PyEval_GetFrameLocals();
	return locals;

cc @gaogaotiantian @markshannon

[gaogaotiantian]

This is tricky, because after PEP 669, the reference PyEval_GetLocals() gets, is the only reference, for the function-level cases. We used to have a f_locals dict in the frame to host the dictionary, but we do not anymore. Each frame.f_locals call generates a new proxy and locals() (or PyEval_GetLocals()) turns that to a dict. That's the only reference, we can't return a borrowed reference.

So we either

• Change PyEval_GetLocals() to return a new reference, or
• Have a list in the frame object to host all the results from PyEval_GetLocals() call (because each call could result in a different dict, it's a snapshot), and release those when the frame is released.

Which is the lesser of two evils?

[encukou]

Per PEP 667:

PyEval_GetLocals() will be implemented roughly as follows:

PyObject *PyEval_GetLocals(void) {
    PyFrameObject * = ...; // Get the current frame.
    if (frame->_locals_cache == NULL) {
        frame->_locals_cache = PyEval_GetFrameLocals();
    }
    return frame->_locals_cache;
}

As with all functions that return a borrowed reference, care must be taken to ensure that the reference is not used beyond the lifetime of the object.

i.e. a proxy should be created on first acccess, and then returned.

Is that not an option any more?