bpo-30039: Don't run signal handlers while resuming a yield from stack

Lib/test/test_generators.py

@@ -9,6 +9,35 @@
 
 from test import support
 
+_testcapi = support.import_module('_testcapi')
+
+
+# This tests to make sure that if a SIGINT arrives just before we send into a
+# yield from chain, the KeyboardInterrupt is raised in the innermost
+# generator (see bpo-30039).
+class SignalAndYieldFromTest(unittest.TestCase):
+
+    def generator1(self):
+        return (yield from self.generator2())
+
+    def generator2(self):
+        try:
+            yield
+        except KeyboardInterrupt:
+            return "PASSED"
+        else:
+            return "FAILED"
+
+    def test_raise_and_yield_from(self):
+        gen = self.generator1()
+        gen.send(None)
+        try:
+            _testcapi.raise_SIGINT_then_send_None(gen)
+        except BaseException as _exc:
+            exc = _exc
+        self.assertIs(type(exc), StopIteration)
+        self.assertEqual(exc.value, "PASSED")
+
 
 class FinalizationTest(unittest.TestCase):
 

Misc/NEWS

@@ -10,6 +10,10 @@ What's New in Python 3.7.0 alpha 1?
 Core and Builtins
 -----------------
 
+- bpo-30039: If a KeyboardInterrupt happens when the interpreter is in
+  the middle of resuming a chain of nested 'yield from' or 'await'
+  calls, it's now correctly delivered to the innermost frame.
+
 - bpo-28974: ``object.__format__(x, '')`` is now equivalent to ``str(x)``
   rather than ``format(str(self), '')``.
 

Modules/_testcapimodule.c

@@ -4028,6 +4028,29 @@ dict_get_version(PyObject *self, PyObject *args)
 }
 
 
+static PyObject *
+raise_SIGINT_then_send_None(PyObject *self, PyObject *args)
+{
+    PyGenObject *gen;
+
+    if (!PyArg_ParseTuple(args, "O!", &PyGen_Type, &gen))
+        return NULL;
+
+    /* This is used in a test to check what happens if a signal arrives just
+       as we're in the process of entering a yield from chain (see
+       bpo-30039).
+
+       Needs to be done in C, because:
+       - we don't have a Python wrapper for raise()
+       - we need to make sure that the Python-level signal handler doesn't run
+         *before* we enter the generator frame, which is impossible in Python
+         because we check for signals before every bytecode operation.
+     */
+    raise(SIGINT);
+    return _PyGen_Send(gen, Py_None);
+}
+
+
 static PyMethodDef TestMethods[] = {
     {"raise_exception",         raise_exception,                 METH_VARARGS},
     {"raise_memoryerror",   (PyCFunction)raise_memoryerror,  METH_NOARGS},
@@ -4232,6 +4255,7 @@ static PyMethodDef TestMethods[] = {
     {"tracemalloc_untrack", tracemalloc_untrack, METH_VARARGS},
     {"tracemalloc_get_traceback", tracemalloc_get_traceback, METH_VARARGS},
     {"dict_get_version", dict_get_version, METH_VARARGS},
+    {"raise_SIGINT_then_send_None", raise_SIGINT_then_send_None, METH_VARARGS},
     {NULL, NULL} /* sentinel */
 };
 

Python/ceval.c

@@ -1064,9 +1064,20 @@ _PyEval_EvalFrameDefault(PyFrameObject *f, int throwflag)
            Py_MakePendingCalls() above. */
 
         if (_Py_atomic_load_relaxed(&eval_breaker)) {
-            if (_Py_OPCODE(*next_instr) == SETUP_FINALLY) {
-                /* Make the last opcode before
-                   a try: finally: block uninterruptible. */
+            if (_Py_OPCODE(*next_instr) == SETUP_FINALLY ||
+                _Py_OPCODE(*next_instr) == YIELD_FROM) {
+                /* Two cases where we skip running signal handlers and other
+                   pending calls:
+                   - If we're about to enter the try: of a try/finally (not
+                     *very* useful, but might help in some cases and it's
+                     traditional)
+                   - If we're resuming a chain of nested 'yield from' or
+                     'await' calls, then each frame is parked with YIELD_FROM
+                     as its next opcode. If the user hit control-C we want to
+                     wait until we've reached the innermost frame before
+                     running the signal handler and raising KeyboardInterrupt
+                     (see bpo-30039).
+                */
                 goto fast_next_opcode;
             }
             if (_Py_atomic_load_relaxed(&pendingcalls_to_do)) {