SF #950057: itertools.chain doesn't "process" exceptions as they occur

Both cycle() and chain() were handling exceptions only when switching
input sources.  The patch makes the handle more immediate.

Will backport.

Lib/test/test_itertools.py

@@ -644,6 +644,36 @@ class RegressionTests(unittest.TestCase):
         self.assertEqual(first, second)
 
 
+    def test_sf_950057(self):
+        # Make sure that chain() and cycle() catch exceptions immediately
+        # rather than when shifting between input sources
+
+        def gen1():
+            hist.append(0)
+            yield 1
+            hist.append(1)
+            assert False
+            hist.append(2)
+
+        def gen2(x):
+            hist.append(3)
+            yield 2
+            hist.append(4)
+            if x:
+                raise StopIteration
+
+        hist = []
+        self.assertRaises(AssertionError, list, chain(gen1(), gen2(False)))
+        self.assertEqual(hist, [0,1])
+
+        hist = []
+        self.assertRaises(AssertionError, list, chain(gen1(), gen2(True)))
+        self.assertEqual(hist, [0,1])
+
+        hist = []
+        self.assertRaises(AssertionError, list, cycle(gen1()))
+        self.assertEqual(hist, [0,1])
+
 libreftest = """ Doctest for examples in the library reference: libitertools.tex
 
 

Modules/itertoolsmodule.c

@@ -699,6 +699,12 @@ cycle_next(cycleobject *lz)
 				PyList_Append(lz->saved, item);
 			return item;
 		}
+		if (PyErr_Occurred()) {
+			if (PyErr_ExceptionMatches(PyExc_StopIteration))
+				PyErr_Clear();
+			else
+				return NULL;
+		}
 		if (PyList_Size(lz->saved) == 0) 
 			return NULL;
 		it = PyObject_GetIter(lz->saved);
@@ -1658,6 +1664,12 @@ chain_next(chainobject *lz)
 		item = PyIter_Next(it);
 		if (item != NULL)
 			return item;
+		if (PyErr_Occurred()) {
+			if (PyErr_ExceptionMatches(PyExc_StopIteration))
+				PyErr_Clear();
+			else
+				return NULL;
+		}
 		lz->iternum++;
 	}
 	return NULL;