Tweak handling of f_stackdepth.

Author: markshannon

Objects/frameobject.c

@@ -300,6 +300,7 @@ first_line_not_before(int *lines, int len, int line)
 static void
 frame_stack_pop(PyFrameObject *f)
 {
+    assert(f->f_stackdepth >= 0);
     f->f_stackdepth--;
     PyObject *v = f->f_valuestack[f->f_stackdepth];
     Py_DECREF(v);
@@ -308,6 +309,7 @@ frame_stack_pop(PyFrameObject *f)
 static void
 frame_block_unwind(PyFrameObject *f)
 {
+    assert(f->f_stackdepth >= 0);
     assert(f->f_iblock > 0);
     f->f_iblock--;
     PyTryBlock *b = &f->f_blockstack[f->f_iblock];

Python/ceval.c

@@ -1350,8 +1350,12 @@ _PyEval_EvalFrameDefault(PyThreadState *tstate, PyFrameObject *f, int throwflag)
         next_instr += f->f_lasti / sizeof(_Py_CODEUNIT) + 1;
     }
     stack_pointer = f->f_valuestack + f->f_stackdepth;
-    /* To help with debugging, set f->f_stackdepth to -1.
-     * Update when returning or calling trace function */
+    /* Set f->f_stackdepth to -1.
+     * Update when returning or calling trace function.
+       Having f_stackdepth <= 0 ensures that invalid
+       values are not visible to the cycle GC.
+       We choose -1 rather than 0 to assist debugging.
+     */
     f->f_stackdepth = -1;
     f->f_state = FRAME_EXECUTING;
 
@@ -1450,6 +1454,7 @@ _PyEval_EvalFrameDefault(PyThreadState *tstate, PyFrameObject *f, int throwflag)
             /* Reload possibly changed frame fields */
             JUMPTO(f->f_lasti);
             stack_pointer = f->f_valuestack+f->f_stackdepth;
+            f->f_stackdepth = -1;
             if (err)
                 /* trace function raised an exception */
                 goto error;
@@ -2075,6 +2080,7 @@ _PyEval_EvalFrameDefault(PyThreadState *tstate, PyFrameObject *f, int throwflag)
             assert(f->f_iblock == 0);
             assert(EMPTY());
             f->f_state = FRAME_RETURNED;
+            f->f_stackdepth = 0;
             goto exiting;
         }
 
@@ -2245,6 +2251,7 @@ _PyEval_EvalFrameDefault(PyThreadState *tstate, PyFrameObject *f, int throwflag)
             assert(f->f_lasti >= (int)sizeof(_Py_CODEUNIT));
             f->f_lasti -= sizeof(_Py_CODEUNIT);
             f->f_state = FRAME_SUSPENDED;
+            f->f_stackdepth = stack_pointer-f->f_valuestack;
             goto exiting;
         }
 
@@ -2261,6 +2268,7 @@ _PyEval_EvalFrameDefault(PyThreadState *tstate, PyFrameObject *f, int throwflag)
                 retval = w;
             }
             f->f_state = FRAME_SUSPENDED;
+            f->f_stackdepth = stack_pointer-f->f_valuestack;
             goto exiting;
         }
 
@@ -3843,10 +3851,9 @@ _PyEval_EvalFrameDefault(PyThreadState *tstate, PyFrameObject *f, int throwflag)
         PyObject *o = POP();
         Py_XDECREF(o);
     }
-
+    f->f_stackdepth = 0;
     f->f_state = FRAME_RAISED;
 exiting:
-    f->f_stackdepth = stack_pointer-f->f_valuestack;
     if (tstate->use_tracing) {
         if (tstate->c_tracefunc) {
             if (call_trace_protected(tstate->c_tracefunc, tstate->c_traceobj,