bpo-29587: Enable implicit exception chaining with gen.throw() (GH-19811)

Before this commit, if an exception was active inside a generator
when calling gen.throw(), then that exception was lost (i.e. there
was no implicit exception chaining).  This commit fixes that.

Lib/test/test_generators.py

@@ -316,6 +316,23 @@ class ExceptionTest(unittest.TestCase):
         self.assertEqual(cm.exception.value.value, 2)
 
 
+class GeneratorThrowTest(unittest.TestCase):
+
+    def test_exception_context_set(self):
+        def f():
+            try:
+                raise KeyError('a')
+            except Exception:
+                yield
+
+        gen = f()
+        gen.send(None)
+        with self.assertRaises(ValueError) as cm:
+            gen.throw(ValueError)
+        context = cm.exception.__context__
+        self.assertEqual((type(context), context.args), (KeyError, ('a',)))
+
+
 class YieldFromTests(unittest.TestCase):
     def test_generator_gi_yieldfrom(self):
         def a():

Misc/NEWS.d/next/Core and Builtins/2020-04-30-00-50-25.bpo-29587.oEwSq.rst

@@ -0,0 +1 @@
+Enable implicit exception chaining when calling :meth:`generator.throw`.

Objects/genobject.c

@@ -512,6 +512,12 @@ throw_here:
     }
 
     PyErr_Restore(typ, val, tb);
+    if (gen->gi_exc_state.exc_type) {
+        Py_INCREF(gen->gi_exc_state.exc_type);
+        Py_XINCREF(gen->gi_exc_state.exc_value);
+        Py_XINCREF(gen->gi_exc_state.exc_traceback);
+        _PyErr_ChainExceptions(gen->gi_exc_state.exc_type, gen->gi_exc_state.exc_value, gen->gi_exc_state.exc_traceback);
+    }
     return gen_send_ex(gen, Py_None, 1, 0);
 
 failed_throw: