Move exc state to generator. Fixes bpo-25612 (#1773)

Move exception state information from frame objects to coroutine (generator/thread) object where it belongs.

13 files changed, 188 insertions, 164 deletions

diff --git a/Include/frameobject.h b/Include/frameobject.h
index dbe0a84..a95baf8 100644
--- a/Include/frameobject.h
+++ b/Include/frameobject.h
@@ -30,14 +30,6 @@ typedef struct _frame {
     char f_trace_lines;         /* Emit per-line trace events? */
     char f_trace_opcodes;       /* Emit per-opcode trace events? */
 
-    /* In a generator, we need to be able to swap between the exception
-       state inside the generator and the exception state of the calling
-       frame (which shouldn't be impacted when the generator "yields"
-       from an except handler).
-       These three fields exist exactly for that, and are unused for
-       non-generator frames. See the save_exc_state and swap_exc_state
-       functions in ceval.c for details of their use. */
-    PyObject *f_exc_type, *f_exc_value, *f_exc_traceback;
     /* Borrowed reference to a generator, or NULL */
     PyObject *f_gen;
 
diff --git a/Include/genobject.h b/Include/genobject.h
index b9db9f9..87fbe17 100644
--- a/Include/genobject.h
+++ b/Include/genobject.h
@@ -25,7 +25,8 @@ struct _frame; /* Avoid including frameobject.h */
     /* Name of the generator. */                                            \
     PyObject *prefix##_name;                                                \
     /* Qualified name of the generator. */                                  \
-    PyObject *prefix##_qualname;
+    PyObject *prefix##_qualname;                                            \
+    _PyErr_StackItem prefix##_exc_state;
 
 typedef struct {
     /* The gi_ prefix is intended to remind of generator-iterator. */
diff --git a/Include/pyerrors.h b/Include/pyerrors.h
index fcaba85..9418201 100644
--- a/Include/pyerrors.h
+++ b/Include/pyerrors.h
@@ -78,6 +78,7 @@ PyAPI_FUNC(void) PyErr_SetNone(PyObject *);
 PyAPI_FUNC(void) PyErr_SetObject(PyObject *, PyObject *);
 #ifndef Py_LIMITED_API
 PyAPI_FUNC(void) _PyErr_SetKeyError(PyObject *);
+_PyErr_StackItem *_PyErr_GetTopmostException(PyThreadState *tstate);
 #endif
 PyAPI_FUNC(void) PyErr_SetString(
     PyObject *exception,
diff --git a/Include/pystate.h b/Include/pystate.h
index 507a598..238008f 100644
--- a/Include/pystate.h
+++ b/Include/pystate.h
@@ -123,6 +123,21 @@ typedef int (*Py_tracefunc)(PyObject *, struct _frame *, int, PyObject *);
 #ifdef Py_LIMITED_API
 typedef struct _ts PyThreadState;
 #else
+
+typedef struct _err_stackitem {
+    /* This struct represents an entry on the exception stack, which is a 
+     * per-coroutine state. (Coroutine in the computer science sense, 
+     * including the thread and generators).
+     * This ensures that the exception state is not impacted by "yields"
+     * from an except handler.
+     */
+    PyObject *exc_type, *exc_value, *exc_traceback;
+
+    struct _err_stackitem *previous_item;
+
+} _PyErr_StackItem;
+
+
 typedef struct _ts {
     /* See Python/ceval.c for comments explaining most fields */
 
@@ -147,13 +162,19 @@ typedef struct _ts {
     PyObject *c_profileobj;
     PyObject *c_traceobj;
 
+    /* The exception currently being raised */
     PyObject *curexc_type;
     PyObject *curexc_value;
     PyObject *curexc_traceback;
 
-    PyObject *exc_type;
-    PyObject *exc_value;
-    PyObject *exc_traceback;
+    /* The exception currently being handled, if no coroutines/generators
+     * are present. Always last element on the stack referred to be exc_info.
+     */
+    _PyErr_StackItem exc_state;
+
+    /* Pointer to the top of the stack of the exceptions currently
+     * being handled */
+    _PyErr_StackItem *exc_info;
 
     PyObject *dict;  /* Stores per-thread state */
 
diff --git a/Lib/test/test_exceptions.py b/Lib/test/test_exceptions.py
index a25f3bf..ad4bc09 100644
--- a/Lib/test/test_exceptions.py
+++ b/Lib/test/test_exceptions.py
@@ -1097,6 +1097,62 @@ class ExceptionTests(unittest.TestCase):
                     self.assertIn("test message", report)
                 self.assertTrue(report.endswith("\n"))
 
+    def test_yield_in_nested_try_excepts(self):
+        #Issue #25612
+        class MainError(Exception):
+            pass
+
+        class SubError(Exception):
+            pass
+
+        def main():
+            try:
+                raise MainError()
+            except MainError:
+                try:
+                    yield
+                except SubError:
+                    pass
+                raise
+
+        coro = main()
+        coro.send(None)
+        with self.assertRaises(MainError):
+            coro.throw(SubError())
+
+    def test_generator_doesnt_retain_old_exc2(self):
+        #Issue 28884#msg282532
+        def g():
+            try:
+                raise ValueError
+            except ValueError:
+                yield 1
+            self.assertEqual(sys.exc_info(), (None, None, None))
+            yield 2
+
+        gen = g()
+
+        try:
+            raise IndexError
+        except IndexError:
+            self.assertEqual(next(gen), 1)
+        self.assertEqual(next(gen), 2)
+
+    def test_raise_in_generator(self):
+        #Issue 25612#msg304117
+        def g():
+            yield 1
+            raise
+            yield 2
+
+        with self.assertRaises(ZeroDivisionError):
+            i = g()
+            try:
+                1/0
+            except:
+                next(i)
+                next(i)
+
 
 class ImportErrorTests(unittest.TestCase):
 
diff --git a/Lib/test/test_sys.py b/Lib/test/test_sys.py
index fd45abe..6872629 100644
--- a/Lib/test/test_sys.py
+++ b/Lib/test/test_sys.py
@@ -971,7 +971,7 @@ class SizeofTest(unittest.TestCase):
         nfrees = len(x.f_code.co_freevars)
         extras = x.f_code.co_stacksize + x.f_code.co_nlocals +\
                   ncells + nfrees - 1
-        check(x, vsize('8P2c4P3ic' + CO_MAXBLOCKS*'3i' + 'P' + extras*'P'))
+        check(x, vsize('5P2c4P3ic' + CO_MAXBLOCKS*'3i' + 'P' + extras*'P'))
         # function
         def func(): pass
         check(func, size('12P'))
@@ -988,7 +988,7 @@ class SizeofTest(unittest.TestCase):
             check(bar, size('PP'))
         # generator
         def get_gen(): yield 1
-        check(get_gen(), size('Pb2PPP'))
+        check(get_gen(), size('Pb2PPP4P'))
         # iterator
         check(iter('abc'), size('lP'))
         # callable-iterator
diff --git a/Misc/NEWS.d/next/C API/2017-10-22-13-12-28.bpo-25612.1jnWKT.rst b/Misc/NEWS.d/next/C API/2017-10-22-13-12-28.bpo-25612.1jnWKT.rst
new file mode 100644
index 0000000..102c2e3
--- /dev/null
+++ b/Misc/NEWS.d/next/C API/2017-10-22-13-12-28.bpo-25612.1jnWKT.rst
@@ -0,0 +1,3 @@
+Move the current exception state from the frame object to the co-routine.
+This simplifies the interpreter and fixes a couple of obscure bugs caused by
+having swap exception state when entering or exiting a generator.
diff --git a/Objects/frameobject.c b/Objects/frameobject.c
index 8739596..6ab3a22 100644
--- a/Objects/frameobject.c
+++ b/Objects/frameobject.c
@@ -379,8 +379,7 @@ static PyGetSetDef frame_getsetlist[] = {
 
      * ob_type, ob_size, f_code, f_valuestack;
 
-     * f_locals, f_trace,
-       f_exc_type, f_exc_value, f_exc_traceback are NULL;
+     * f_locals, f_trace are NULL;
 
      * f_localsplus does not require re-allocation and
        the local variables in f_localsplus are NULL.
@@ -438,9 +437,6 @@ frame_dealloc(PyFrameObject *f)
     Py_DECREF(f->f_globals);
     Py_CLEAR(f->f_locals);
     Py_CLEAR(f->f_trace);
-    Py_CLEAR(f->f_exc_type);
-    Py_CLEAR(f->f_exc_value);
-    Py_CLEAR(f->f_exc_traceback);
 
     co = f->f_code;
     if (co->co_zombieframe == NULL)
@@ -469,9 +465,6 @@ frame_traverse(PyFrameObject *f, visitproc visit, void *arg)
     Py_VISIT(f->f_globals);
     Py_VISIT(f->f_locals);
     Py_VISIT(f->f_trace);
-    Py_VISIT(f->f_exc_type);
-    Py_VISIT(f->f_exc_value);
-    Py_VISIT(f->f_exc_traceback);
 
     /* locals */
     slots = f->f_code->co_nlocals + PyTuple_GET_SIZE(f->f_code->co_cellvars) + PyTuple_GET_SIZE(f->f_code->co_freevars);
@@ -502,9 +495,6 @@ frame_tp_clear(PyFrameObject *f)
     f->f_stacktop = NULL;
     f->f_executing = 0;
 
-    Py_CLEAR(f->f_exc_type);
-    Py_CLEAR(f->f_exc_value);
-    Py_CLEAR(f->f_exc_traceback);
     Py_CLEAR(f->f_trace);
 
     /* locals */
@@ -698,7 +688,6 @@ _PyFrame_New_NoTrack(PyThreadState *tstate, PyCodeObject *code,
             f->f_localsplus[i] = NULL;
         f->f_locals = NULL;
         f->f_trace = NULL;
-        f->f_exc_type = f->f_exc_value = f->f_exc_traceback = NULL;
     }
     f->f_stacktop = f->f_valuestack;
     f->f_builtins = builtins;
diff --git a/Objects/genobject.c b/Objects/genobject.c
index 5d5798c..7793a54 100644
--- a/Objects/genobject.c
+++ b/Objects/genobject.c
@@ -16,6 +16,15 @@ static char *NON_INIT_CORO_MSG = "can't send non-None value to a "
 static char *ASYNC_GEN_IGNORED_EXIT_MSG =
                                  "async generator ignored GeneratorExit";
 
+static inline int
+exc_state_traverse(_PyErr_StackItem *exc_state, visitproc visit, void *arg)
+{
+    Py_VISIT(exc_state->exc_type);
+    Py_VISIT(exc_state->exc_value);
+    Py_VISIT(exc_state->exc_traceback);
+    return 0;
+}
+
 static int
 gen_traverse(PyGenObject *gen, visitproc visit, void *arg)
 {
@@ -23,7 +32,7 @@ gen_traverse(PyGenObject *gen, visitproc visit, void *arg)
     Py_VISIT(gen->gi_code);
     Py_VISIT(gen->gi_name);
     Py_VISIT(gen->gi_qualname);
-    return 0;
+    return exc_state_traverse(&gen->gi_exc_state, visit, arg);
 }
 
 void
@@ -87,6 +96,21 @@ _PyGen_Finalize(PyObject *self)
     PyErr_Restore(error_type, error_value, error_traceback);
 }
 
+static inline void
+exc_state_clear(_PyErr_StackItem *exc_state)
+{
+    PyObject *t, *v, *tb;
+    t = exc_state->exc_type;
+    v = exc_state->exc_value;
+    tb = exc_state->exc_traceback;
+    exc_state->exc_type = NULL;
+    exc_state->exc_value = NULL;
+    exc_state->exc_traceback = NULL;
+    Py_XDECREF(t);
+    Py_XDECREF(v);
+    Py_XDECREF(tb);
+}
+
 static void
 gen_dealloc(PyGenObject *gen)
 {
@@ -116,6 +140,7 @@ gen_dealloc(PyGenObject *gen)
     Py_CLEAR(gen->gi_code);
     Py_CLEAR(gen->gi_name);
     Py_CLEAR(gen->gi_qualname);
+    exc_state_clear(&gen->gi_exc_state);
     PyObject_GC_Del(gen);
 }
 
@@ -187,7 +212,11 @@ gen_send_ex(PyGenObject *gen, PyObject *arg, int exc, int closing)
     f->f_back = tstate->frame;
 
     gen->gi_running = 1;
+    gen->gi_exc_state.previous_item = tstate->exc_info;
+    tstate->exc_info = &gen->gi_exc_state;
     result = PyEval_EvalFrameEx(f, exc);
+    tstate->exc_info = gen->gi_exc_state.previous_item;
+    gen->gi_exc_state.previous_item = NULL;
     gen->gi_running = 0;
 
     /* Don't keep the reference to f_back any longer than necessary.  It
@@ -281,16 +310,7 @@ gen_send_ex(PyGenObject *gen, PyObject *arg, int exc, int closing)
     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);
+        exc_state_clear(&gen->gi_exc_state);
         gen->gi_frame->f_gen = NULL;
         gen->gi_frame = NULL;
         Py_DECREF(f);
@@ -810,6 +830,10 @@ gen_new_with_qualname(PyTypeObject *type, PyFrameObject *f,
     gen->gi_code = (PyObject *)(f->f_code);
     gen->gi_running = 0;
     gen->gi_weakreflist = NULL;
+    gen->gi_exc_state.exc_type = NULL;
+    gen->gi_exc_state.exc_value = NULL;
+    gen->gi_exc_state.exc_traceback = NULL;
+    gen->gi_exc_state.previous_item = NULL;
     if (name != NULL)
         gen->gi_name = name;
     else
diff --git a/Python/ceval.c b/Python/ceval.c
index 0f7a40c..f9a798c 100644
--- a/Python/ceval.c
+++ b/Python/ceval.c
@@ -511,9 +511,6 @@ enum why_code {
         WHY_SILENCED =  0x0080  /* Exception silenced by 'with' */
 };
 
-static void save_exc_state(PyThreadState *, PyFrameObject *);
-static void swap_exc_state(PyThreadState *, PyFrameObject *);
-static void restore_and_clear_exc_state(PyThreadState *, PyFrameObject *);
 static int do_raise(PyObject *, PyObject *);
 static int unpack_iterable(PyObject *, int, int, PyObject **);
 
@@ -813,17 +810,19 @@ _PyEval_EvalFrameDefault(PyFrameObject *f, int throwflag)
 #define UNWIND_EXCEPT_HANDLER(b) \
     do { \
         PyObject *type, *value, *traceback; \
+        _PyErr_StackItem *exc_info; \
         assert(STACK_LEVEL() >= (b)->b_level + 3); \
         while (STACK_LEVEL() > (b)->b_level + 3) { \
             value = POP(); \
             Py_XDECREF(value); \
         } \
-        type = tstate->exc_type; \
-        value = tstate->exc_value; \
-        traceback = tstate->exc_traceback; \
-        tstate->exc_type = POP(); \
-        tstate->exc_value = POP(); \
-        tstate->exc_traceback = POP(); \
+        exc_info = tstate->exc_info; \
+        type = exc_info->exc_type; \
+        value = exc_info->exc_value; \
+        traceback = exc_info->exc_traceback; \
+        exc_info->exc_type = POP(); \
+        exc_info->exc_value = POP(); \
+        exc_info->exc_traceback = POP(); \
         Py_XDECREF(type); \
         Py_XDECREF(value); \
         Py_XDECREF(traceback); \
@@ -910,16 +909,6 @@ _PyEval_EvalFrameDefault(PyFrameObject *f, int throwflag)
     f->f_stacktop = NULL;       /* remains NULL unless yield suspends frame */
     f->f_executing = 1;
 
-    if (co->co_flags & (CO_GENERATOR | CO_COROUTINE | CO_ASYNC_GENERATOR)) {
-        if (!throwflag && f->f_exc_type != NULL && f->f_exc_type != Py_None) {
-            /* We were in an except handler when we left,
-               restore the exception state which was put aside
-               (see YIELD_VALUE). */
-            swap_exc_state(tstate, f);
-        }
-        else
-            save_exc_state(tstate, f);
-    }
 
 #ifdef LLTRACE
     lltrace = _PyDict_GetItemId(f->f_globals, &PyId___ltrace__) != NULL;
@@ -3447,12 +3436,13 @@ fast_block_end:
                 || b->b_type == SETUP_FINALLY)) {
                 PyObject *exc, *val, *tb;
                 int handler = b->b_handler;
+                _PyErr_StackItem *exc_info = tstate->exc_info;
                 /* Beware, this invalidates all b->b_* fields */
                 PyFrame_BlockSetup(f, EXCEPT_HANDLER, -1, STACK_LEVEL());
-                PUSH(tstate->exc_traceback);
-                PUSH(tstate->exc_value);
-                if (tstate->exc_type != NULL) {
-                    PUSH(tstate->exc_type);
+                PUSH(exc_info->exc_traceback);
+                PUSH(exc_info->exc_value);
+                if (exc_info->exc_type != NULL) {
+                    PUSH(exc_info->exc_type);
                 }
                 else {
                     Py_INCREF(Py_None);
@@ -3470,10 +3460,10 @@ fast_block_end:
                 else
                     PyException_SetTraceback(val, Py_None);
                 Py_INCREF(exc);
-                tstate->exc_type = exc;
+                exc_info->exc_type = exc;
                 Py_INCREF(val);
-                tstate->exc_value = val;
-                tstate->exc_traceback = tb;
+                exc_info->exc_value = val;
+                exc_info->exc_traceback = tb;
                 if (tb == NULL)
                     tb = Py_None;
                 Py_INCREF(tb);
@@ -3516,28 +3506,6 @@ fast_block_end:
     assert((retval != NULL) ^ (PyErr_Occurred() != NULL));
 
 fast_yield:
-    if (co->co_flags & (CO_GENERATOR | CO_COROUTINE | CO_ASYNC_GENERATOR)) {
-
-        /* The purpose of this block is to put aside the generator's exception
-           state and restore that of the calling frame. If the current
-           exception state is from the caller, we clear the exception values
-           on the generator frame, so they are not swapped back in latter. The
-           origin of the current exception state is determined by checking for
-           except handler blocks, which we must be in iff a new exception
-           state came into existence in this frame. (An uncaught exception
-           would have why == WHY_EXCEPTION, and we wouldn't be here). */
-        int i;
-        for (i = 0; i < f->f_iblock; i++) {
-            if (f->f_blockstack[i].b_type == EXCEPT_HANDLER) {
-                break;
-            }
-        }
-        if (i == f->f_iblock)
-            /* We did not create this exception. */
-            restore_and_clear_exc_state(tstate, f);
-        else
-            swap_exc_state(tstate, f);
-    }
 
     if (tstate->use_tracing) {
         if (tstate->c_tracefunc) {
@@ -4057,60 +4025,6 @@ special_lookup(PyObject *o, _Py_Identifier *id)
 }
 
 
-/* These 3 functions deal with the exception state of generators. */
-
-static void
-save_exc_state(PyThreadState *tstate, PyFrameObject *f)
-{
-    PyObject *type, *value, *traceback;
-    Py_XINCREF(tstate->exc_type);
-    Py_XINCREF(tstate->exc_value);
-    Py_XINCREF(tstate->exc_traceback);
-    type = f->f_exc_type;
-    value = f->f_exc_value;
-    traceback = f->f_exc_traceback;
-    f->f_exc_type = tstate->exc_type;
-    f->f_exc_value = tstate->exc_value;
-    f->f_exc_traceback = tstate->exc_traceback;
-    Py_XDECREF(type);
-    Py_XDECREF(value);
-    Py_XDECREF(traceback);
-}
-
-static void
-swap_exc_state(PyThreadState *tstate, PyFrameObject *f)
-{
-    PyObject *tmp;
-    tmp = tstate->exc_type;
-    tstate->exc_type = f->f_exc_type;
-    f->f_exc_type = tmp;
-    tmp = tstate->exc_value;
-    tstate->exc_value = f->f_exc_value;
-    f->f_exc_value = tmp;
-    tmp = tstate->exc_traceback;
-    tstate->exc_traceback = f->f_exc_traceback;
-    f->f_exc_traceback = tmp;
-}
-
-static void
-restore_and_clear_exc_state(PyThreadState *tstate, PyFrameObject *f)
-{
-    PyObject *type, *value, *tb;
-    type = tstate->exc_type;
-    value = tstate->exc_value;
-    tb = tstate->exc_traceback;
-    tstate->exc_type = f->f_exc_type;
-    tstate->exc_value = f->f_exc_value;
-    tstate->exc_traceback = f->f_exc_traceback;
-    f->f_exc_type = NULL;
-    f->f_exc_value = NULL;
-    f->f_exc_traceback = NULL;
-    Py_XDECREF(type);
-    Py_XDECREF(value);
-    Py_XDECREF(tb);
-}
-
-
 /* Logic for the raise statement (too complicated for inlining).
    This *consumes* a reference count to each of its arguments. */
 static int
@@ -4121,10 +4035,11 @@ do_raise(PyObject *exc, PyObject *cause)
     if (exc == NULL) {
         /* Reraise */
         PyThreadState *tstate = PyThreadState_GET();
+        _PyErr_StackItem *exc_info = _PyErr_GetTopmostException(tstate);
         PyObject *tb;
-        type = tstate->exc_type;
-        value = tstate->exc_value;
-        tb = tstate->exc_traceback;
+        type = exc_info->exc_type;
+        value = exc_info->exc_value;
+        tb = exc_info->exc_traceback;
         if (type == Py_None || type == NULL) {
             PyErr_SetString(PyExc_RuntimeError,
                             "No active exception to reraise");
diff --git a/Python/errors.c b/Python/errors.c
index 5709fdd..51fc791 100644
--- a/Python/errors.c
+++ b/Python/errors.c
@@ -53,6 +53,18 @@ PyErr_Restore(PyObject *type, PyObject *value, PyObject *traceback)
     Py_XDECREF(oldtraceback);
 }
 
+_PyErr_StackItem *
+_PyErr_GetTopmostException(PyThreadState *tstate)
+{
+    _PyErr_StackItem *exc_info = tstate->exc_info;
+    while ((exc_info->exc_type == NULL || exc_info->exc_type == Py_None) &&
+           exc_info->previous_item != NULL)
+    {
+        exc_info = exc_info->previous_item;
+    }
+    return exc_info;
+}
+
 static PyObject*
 _PyErr_CreateException(PyObject *exception, PyObject *value)
 {
@@ -83,7 +95,7 @@ PyErr_SetObject(PyObject *exception, PyObject *value)
     }
 
     Py_XINCREF(value);
-    exc_value = tstate->exc_value;
+    exc_value = _PyErr_GetTopmostException(tstate)->exc_value;
     if (exc_value != NULL && exc_value != Py_None) {
         /* Implicit exception chaining */
         Py_INCREF(exc_value);
@@ -335,9 +347,11 @@ PyErr_GetExcInfo(PyObject **p_type, PyObject **p_value, PyObject **p_traceback)
 {
     PyThreadState *tstate = PyThreadState_GET();
 
-    *p_type = tstate->exc_type;
-    *p_value = tstate->exc_value;
-    *p_traceback = tstate->exc_traceback;
+    _PyErr_StackItem *exc_info = _PyErr_GetTopmostException(tstate);
+    *p_type = exc_info->exc_type;
+    *p_value = exc_info->exc_value;
+    *p_traceback = exc_info->exc_traceback;
+
 
     Py_XINCREF(*p_type);
     Py_XINCREF(*p_value);
@@ -350,13 +364,13 @@ PyErr_SetExcInfo(PyObject *p_type, PyObject *p_value, PyObject *p_traceback)
     PyObject *oldtype, *oldvalue, *oldtraceback;
     PyThreadState *tstate = PyThreadState_GET();
 
-    oldtype = tstate->exc_type;
-    oldvalue = tstate->exc_value;
-    oldtraceback = tstate->exc_traceback;
+    oldtype = tstate->exc_info->exc_type;
+    oldvalue = tstate->exc_info->exc_value;
+    oldtraceback = tstate->exc_info->exc_traceback;
 
-    tstate->exc_type = p_type;
-    tstate->exc_value = p_value;
-    tstate->exc_traceback = p_traceback;
+    tstate->exc_info->exc_type = p_type;
+    tstate->exc_info->exc_value = p_value;
+    tstate->exc_info->exc_traceback = p_traceback;
 
     Py_XDECREF(oldtype);
     Py_XDECREF(oldvalue);
diff --git a/Python/pystate.c b/Python/pystate.c
index 3feae34..d85d604 100644
--- a/Python/pystate.c
+++ b/Python/pystate.c
@@ -257,9 +257,11 @@ new_threadstate(PyInterpreterState *interp, int init)
         tstate->curexc_value = NULL;
         tstate->curexc_traceback = NULL;
 
-        tstate->exc_type = NULL;
-        tstate->exc_value = NULL;
-        tstate->exc_traceback = NULL;
+        tstate->exc_state.exc_type = NULL;
+        tstate->exc_state.exc_value = NULL;
+        tstate->exc_state.exc_traceback = NULL;
+        tstate->exc_state.previous_item = NULL;
+        tstate->exc_info = &tstate->exc_state;
 
         tstate->c_profilefunc = NULL;
         tstate->c_tracefunc = NULL;
@@ -444,9 +446,16 @@ PyThreadState_Clear(PyThreadState *tstate)
     Py_CLEAR(tstate->curexc_value);
     Py_CLEAR(tstate->curexc_traceback);
 
-    Py_CLEAR(tstate->exc_type);
-    Py_CLEAR(tstate->exc_value);
-    Py_CLEAR(tstate->exc_traceback);
+    Py_CLEAR(tstate->exc_state.exc_type);
+    Py_CLEAR(tstate->exc_state.exc_value);
+    Py_CLEAR(tstate->exc_state.exc_traceback);
+    
+    /* The stack of exception states should contain just this thread. */
+    assert(tstate->exc_info->previous_item == NULL);
+    if (Py_VerboseFlag && tstate->exc_info != &tstate->exc_state) {
+        fprintf(stderr,
+          "PyThreadState_Clear: warning: thread still has a generator\n");
+    }
 
     tstate->c_profilefunc = NULL;
     tstate->c_tracefunc = NULL;
diff --git a/Python/sysmodule.c b/Python/sysmodule.c
index e38a200..6dc8e08 100644
--- a/Python/sysmodule.c
+++ b/Python/sysmodule.c
@@ -311,14 +311,13 @@ PyDoc_STRVAR(excepthook_doc,
 static PyObject *
 sys_exc_info(PyObject *self, PyObject *noargs)
 {
-    PyThreadState *tstate;
-    tstate = PyThreadState_GET();
+    _PyErr_StackItem *err_info = _PyErr_GetTopmostException(PyThreadState_GET());
     return Py_BuildValue(
         "(OOO)",
-        tstate->exc_type != NULL ? tstate->exc_type : Py_None,
-        tstate->exc_value != NULL ? tstate->exc_value : Py_None,
-        tstate->exc_traceback != NULL ?
-            tstate->exc_traceback : Py_None);
+        err_info->exc_type != NULL ? err_info->exc_type : Py_None,
+        err_info->exc_value != NULL ? err_info->exc_value : Py_None,
+        err_info->exc_traceback != NULL ?
+            err_info->exc_traceback : Py_None);
 }
 
 PyDoc_STRVAR(exc_info_doc,