gh-118934: Make PyEval_GetLocals return borrowed reference (#119769)

Co-authored-by: Alyssa Coghlan <ncoghlan@gmail.com>

Include/internal/pycore_frame.h

@@ -27,6 +27,10 @@ struct _frame {
     char f_trace_lines;         /* Emit per-line trace events? */
     char f_trace_opcodes;       /* Emit per-opcode trace events? */
     PyObject *f_extra_locals;   /* Dict for locals set by users using f_locals, could be NULL */
+    /* This is purely for backwards compatibility for PyEval_GetLocals.
+       PyEval_GetLocals requires a borrowed reference so the actual reference
+       is stored here */
+    PyObject *f_locals_cache;
     /* The frame data, if this frame object owns the frame */
     PyObject *_f_frame_data[1];
 };

Lib/test/test_sys.py

@@ -1603,7 +1603,7 @@ class SizeofTest(unittest.TestCase):
         def func():
             return sys._getframe()
         x = func()
-        check(x, size('3Pi2cP7P2ic??2P'))
+        check(x, size('3Pi2c2P7P2ic??2P'))
         # function
         def func(): pass
         check(func, size('16Pi'))

Misc/NEWS.d/next/Core and Builtins/2024-05-30-04-11-36.gh-issue-118934.fbDqve.rst

@@ -0,0 +1 @@
+Make ``PyEval_GetLocals`` return borrowed reference

Objects/frameobject.c

@@ -1627,6 +1627,7 @@ frame_dealloc(PyFrameObject *f)
     Py_CLEAR(f->f_back);
     Py_CLEAR(f->f_trace);
     Py_CLEAR(f->f_extra_locals);
+    Py_CLEAR(f->f_locals_cache);
     PyObject_GC_Del(f);
     Py_XDECREF(co);
     Py_TRASHCAN_END;
@@ -1638,6 +1639,7 @@ frame_traverse(PyFrameObject *f, visitproc visit, void *arg)
     Py_VISIT(f->f_back);
     Py_VISIT(f->f_trace);
     Py_VISIT(f->f_extra_locals);
+    Py_VISIT(f->f_locals_cache);
     if (f->f_frame->owner != FRAME_OWNED_BY_FRAME_OBJECT) {
         return 0;
     }
@@ -1650,6 +1652,7 @@ frame_tp_clear(PyFrameObject *f)
 {
     Py_CLEAR(f->f_trace);
     Py_CLEAR(f->f_extra_locals);
+    Py_CLEAR(f->f_locals_cache);
 
     /* locals and stack */
     _PyStackRef *locals = _PyFrame_GetLocalsArray(f->f_frame);
@@ -1787,6 +1790,7 @@ _PyFrame_New_NoTrack(PyCodeObject *code)
     f->f_trace_opcodes = 0;
     f->f_lineno = 0;
     f->f_extra_locals = NULL;
+    f->f_locals_cache = NULL;
     return f;
 }
 

Python/ceval.c

@@ -2525,6 +2525,7 @@ _PyEval_GetBuiltinId(_Py_Identifier *name)
 PyObject *
 PyEval_GetLocals(void)
 {
+    // We need to return a borrowed reference here, so some tricks are needed
     PyThreadState *tstate = _PyThreadState_GET();
      _PyInterpreterFrame *current_frame = _PyThreadState_GetFrame(tstate);
     if (current_frame == NULL) {
@@ -2532,7 +2533,37 @@ PyEval_GetLocals(void)
         return NULL;
     }
 
-    PyObject *locals = _PyEval_GetFrameLocals();
+    // Be aware that this returns a new reference
+    PyObject *locals = _PyFrame_GetLocals(current_frame);
+
+    if (locals == NULL) {
+        return NULL;
+    }
+
+    if (PyFrameLocalsProxy_Check(locals)) {
+        PyFrameObject *f = _PyFrame_GetFrameObject(current_frame);
+        PyObject *ret = f->f_locals_cache;
+        if (ret == NULL) {
+            PyObject *ret = PyDict_New();
+            if (ret == NULL) {
+                Py_DECREF(locals);
+                return NULL;
+            }
+            f->f_locals_cache = ret;
+        }
+        if (PyDict_Update(ret, locals) < 0) {
+            // At this point, if the cache dict is broken, it will stay broken, as
+            // trying to clean it up or replace it will just cause other problems
+            ret = NULL;
+        }
+        Py_DECREF(locals);
+        return ret;
+    }
+
+    assert(PyMapping_Check(locals));
+    assert(Py_REFCNT(locals) > 1);
+    Py_DECREF(locals);
+
     return locals;
 }