Backout c89febab4648 following private feedback by Guido.

(Issue #17807: Generators can now be finalized even when they are part of a reference cycle)

1 files changed, 219 insertions, 32 deletions

diff --git a/Objects/genobject.c b/Objects/genobject.c
index 34ecf2c..016bfa2 100644
--- a/Objects/genobject.c
+++ b/Objects/genobject.c
@@ -19,50 +19,112 @@ static void
 gen_dealloc(PyGenObject *gen)
 {
     PyObject *self = (PyObject *) gen;
-    PyFrameObject *f = gen->gi_frame;
 
     _PyObject_GC_UNTRACK(gen);
 
     if (gen->gi_weakreflist != NULL)
         PyObject_ClearWeakRefs(self);
 
-    gen->gi_frame = NULL;
-    if (f) {
-        /* Close the generator by finalizing the frame */
-        PyObject *retval, *t, *v, *tb;
-        PyErr_Fetch(&t, &v, &tb);
-        f->f_gen = NULL;
-        retval = _PyFrame_Finalize(f);
-        if (retval)
-            Py_DECREF(retval);
-        else if (PyErr_Occurred())
-            PyErr_WriteUnraisable((PyObject *) gen);
-        Py_DECREF(f);
-        PyErr_Restore(t, v, tb);
+    _PyObject_GC_TRACK(self);
+
+    if (gen->gi_frame != NULL && gen->gi_frame->f_stacktop != NULL) {
+        /* Generator is paused, so we need to close */
+        Py_TYPE(gen)->tp_del(self);
+        if (self->ob_refcnt > 0)
+            return;                     /* resurrected.  :( */
     }
+
+    _PyObject_GC_UNTRACK(self);
+    Py_CLEAR(gen->gi_frame);
     Py_CLEAR(gen->gi_code);
     PyObject_GC_Del(gen);
 }
 
+
 static PyObject *
 gen_send_ex(PyGenObject *gen, PyObject *arg, int exc)
 {
+    PyThreadState *tstate = PyThreadState_GET();
     PyFrameObject *f = gen->gi_frame;
+    PyObject *result;
 
-    /* For compatibility, we check gi_running before f == NULL */
     if (gen->gi_running) {
         PyErr_SetString(PyExc_ValueError,
                         "generator already executing");
         return NULL;
     }
-    if (f == NULL) {
-        /* Only set exception if send() called, not throw() or next() */
+    if (f == NULL || f->f_stacktop == NULL) {
+        /* Only set exception if called from send() */
         if (arg && !exc)
             PyErr_SetNone(PyExc_StopIteration);
         return NULL;
     }
 
-    return _PyFrame_GeneratorSend(f, arg, exc);
+    if (f->f_lasti == -1) {
+        if (arg && arg != Py_None) {
+            PyErr_SetString(PyExc_TypeError,
+                            "can't send non-None value to a "
+                            "just-started generator");
+            return NULL;
+        }
+    } else {
+        /* Push arg onto the frame's value stack */
+        result = arg ? arg : Py_None;
+        Py_INCREF(result);
+        *(f->f_stacktop++) = result;
+    }
+
+    /* Generators always return to their most recent caller, not
+     * necessarily their creator. */
+    Py_XINCREF(tstate->frame);
+    assert(f->f_back == NULL);
+    f->f_back = tstate->frame;
+
+    gen->gi_running = 1;
+    result = PyEval_EvalFrameEx(f, exc);
+    gen->gi_running = 0;
+
+    /* Don't keep the reference to f_back any longer than necessary.  It
+     * may keep a chain of frames alive or it could create a reference
+     * cycle. */
+    assert(f->f_back == tstate->frame);
+    Py_CLEAR(f->f_back);
+
+    /* If the generator just returned (as opposed to yielding), signal
+     * that the generator is exhausted. */
+    if (result && f->f_stacktop == NULL) {
+        if (result == Py_None) {
+            /* Delay exception instantiation if we can */
+            PyErr_SetNone(PyExc_StopIteration);
+        } else {
+            PyObject *e = PyObject_CallFunctionObjArgs(
+                               PyExc_StopIteration, result, NULL);
+            if (e != NULL) {
+                PyErr_SetObject(PyExc_StopIteration, e);
+                Py_DECREF(e);
+            }
+        }
+        Py_CLEAR(result);
+    }
+
+    if (!result || f->f_stacktop == NULL) {
+        /* generator can't be rerun, so release the frame */
+        /* first clean reference cycle through stored exception traceback */
+        PyObject *t, *v, *tb;
+        t = f->f_exc_type;
+        v = f->f_exc_value;
+        tb = f->f_exc_traceback;
+        f->f_exc_type = NULL;
+        f->f_exc_value = NULL;
+        f->f_exc_traceback = NULL;
+        Py_XDECREF(t);
+        Py_XDECREF(v);
+        Py_XDECREF(tb);
+        gen->gi_frame = NULL;
+        Py_DECREF(f);
+    }
+
+    return result;
 }
 
 PyDoc_STRVAR(send_doc,
@@ -83,33 +145,146 @@ PyDoc_STRVAR(close_doc,
  *   close a subiterator being delegated to by yield-from.
  */
 
+static int
+gen_close_iter(PyObject *yf)
+{
+    PyObject *retval = NULL;
+    _Py_IDENTIFIER(close);
+
+    if (PyGen_CheckExact(yf)) {
+        retval = gen_close((PyGenObject *)yf, NULL);
+        if (retval == NULL)
+            return -1;
+    } else {
+        PyObject *meth = _PyObject_GetAttrId(yf, &PyId_close);
+        if (meth == NULL) {
+            if (!PyErr_ExceptionMatches(PyExc_AttributeError))
+                PyErr_WriteUnraisable(yf);
+            PyErr_Clear();
+        } else {
+            retval = PyObject_CallFunction(meth, "");
+            Py_DECREF(meth);
+            if (retval == NULL)
+                return -1;
+        }
+    }
+    Py_XDECREF(retval);
+    return 0;
+}
+
 static PyObject *
 gen_yf(PyGenObject *gen)
 {
+    PyObject *yf = NULL;
     PyFrameObject *f = gen->gi_frame;
-    if (f)
-        return _PyFrame_YieldingFrom(f);
-    else
-        return NULL;
+
+    if (f && f->f_stacktop) {
+        PyObject *bytecode = f->f_code->co_code;
+        unsigned char *code = (unsigned char *)PyBytes_AS_STRING(bytecode);
+
+        if (code[f->f_lasti + 1] != YIELD_FROM)
+            return NULL;
+        yf = f->f_stacktop[-1];
+        Py_INCREF(yf);
+    }
+
+    return yf;
 }
 
 static PyObject *
 gen_close(PyGenObject *gen, PyObject *args)
 {
-    PyFrameObject *f = gen->gi_frame;
+    PyObject *retval;
+    PyObject *yf = gen_yf(gen);
+    int err = 0;
 
-    /* For compatibility, we check gi_running before f == NULL */
-    if (gen->gi_running) {
-        PyErr_SetString(PyExc_ValueError,
-                        "generator already executing");
+    if (yf) {
+        gen->gi_running = 1;
+        err = gen_close_iter(yf);
+        gen->gi_running = 0;
+        Py_DECREF(yf);
+    }
+    if (err == 0)
+        PyErr_SetNone(PyExc_GeneratorExit);
+    retval = gen_send_ex(gen, Py_None, 1);
+    if (retval) {
+        Py_DECREF(retval);
+        PyErr_SetString(PyExc_RuntimeError,
+                        "generator ignored GeneratorExit");
         return NULL;
     }
-    if (f == NULL)
-        Py_RETURN_NONE;
+    if (PyErr_ExceptionMatches(PyExc_StopIteration)
+        || PyErr_ExceptionMatches(PyExc_GeneratorExit)) {
+        PyErr_Clear();          /* ignore these errors */
+        Py_INCREF(Py_None);
+        return Py_None;
+    }
+    return NULL;
+}
 
-    return _PyFrame_Finalize(f);
+static void
+gen_del(PyObject *self)
+{
+    PyObject *res;
+    PyObject *error_type, *error_value, *error_traceback;
+    PyGenObject *gen = (PyGenObject *)self;
+
+    if (gen->gi_frame == NULL || gen->gi_frame->f_stacktop == NULL)
+        /* Generator isn't paused, so no need to close */
+        return;
+
+    /* Temporarily resurrect the object. */
+    assert(self->ob_refcnt == 0);
+    self->ob_refcnt = 1;
+
+    /* Save the current exception, if any. */
+    PyErr_Fetch(&error_type, &error_value, &error_traceback);
+
+    res = gen_close(gen, NULL);
+
+    if (res == NULL)
+        PyErr_WriteUnraisable(self);
+    else
+        Py_DECREF(res);
+
+    /* Restore the saved exception. */
+    PyErr_Restore(error_type, error_value, error_traceback);
+
+    /* Undo the temporary resurrection; can't use DECREF here, it would
+     * cause a recursive call.
+     */
+    assert(self->ob_refcnt > 0);
+    if (--self->ob_refcnt == 0)
+        return; /* this is the normal path out */
+
+    /* close() resurrected it!  Make it look like the original Py_DECREF
+     * never happened.
+     */
+    {
+        Py_ssize_t refcnt = self->ob_refcnt;
+        _Py_NewReference(self);
+        self->ob_refcnt = refcnt;
+    }
+    assert(PyType_IS_GC(Py_TYPE(self)) &&
+           _Py_AS_GC(self)->gc.gc_refs != _PyGC_REFS_UNTRACKED);
+
+    /* If Py_REF_DEBUG, _Py_NewReference bumped _Py_RefTotal, so
+     * we need to undo that. */
+    _Py_DEC_REFTOTAL;
+    /* If Py_TRACE_REFS, _Py_NewReference re-added self to the object
+     * chain, so no more to do there.
+     * If COUNT_ALLOCS, the original decref bumped tp_frees, and
+     * _Py_NewReference bumped tp_allocs:  both of those need to be
+     * undone.
+     */
+#ifdef COUNT_ALLOCS
+    --(Py_TYPE(self)->tp_frees);
+    --(Py_TYPE(self)->tp_allocs);
+#endif
 }
 
+
+
 PyDoc_STRVAR(throw_doc,
 "throw(typ[,val[,tb]]) -> raise exception in generator,\n\
 return next yielded value or raise StopIteration.");
@@ -131,7 +306,7 @@ gen_throw(PyGenObject *gen, PyObject *args)
         int err;
         if (PyErr_GivenExceptionMatches(typ, PyExc_GeneratorExit)) {
             gen->gi_running = 1;
-            err = _PyFrame_CloseIterator(yf);
+            err = gen_close_iter(yf);
             gen->gi_running = 0;
             Py_DECREF(yf);
             if (err < 0)
@@ -369,6 +544,7 @@ PyTypeObject PyGen_Type = {
     0,                                          /* tp_cache */
     0,                                          /* tp_subclasses */
     0,                                          /* tp_weaklist */
+    gen_del,                                    /* tp_del */
 };
 
 PyObject *
@@ -380,7 +556,6 @@ PyGen_New(PyFrameObject *f)
         return NULL;
     }
     gen->gi_frame = f;
-    f->f_gen = (PyObject *) gen;
     Py_INCREF(f->f_code);
     gen->gi_code = (PyObject *)(f->f_code);
     gen->gi_running = 0;
@@ -392,5 +567,17 @@ PyGen_New(PyFrameObject *f)
 int
 PyGen_NeedsFinalizing(PyGenObject *gen)
 {
+    int i;
+    PyFrameObject *f = gen->gi_frame;
+
+    if (f == NULL || f->f_stacktop == NULL)
+        return 0; /* no frame or empty blockstack == no finalization */
+
+    /* Any block type besides a loop requires cleanup. */
+    for (i = 0; i < f->f_iblock; i++)
+        if (f->f_blockstack[i].b_type != SETUP_LOOP)
+            return 1;
+
+    /* No blocks except loops, it's safe to skip finalization. */
     return 0;
 }