[3.6] bpo-30347: Stop crashes when concurrently iterate over itertools.groupby() iterators. (GH-1557) (#3770)

(cherry picked from commit c740e4fe8a)

Lib/test/test_itertools.py

@@ -1984,6 +1984,30 @@ class RegressionTests(unittest.TestCase):
         with self.assertRaises(StopIteration):
             next(it)
 
+    def test_issue30347_1(self):
+        def f(n):
+            if n == 5:
+                list(b)
+            return n != 6
+        for (k, b) in groupby(range(10), f):
+            list(b)  # shouldn't crash
+
+    def test_issue30347_2(self):
+        class K:
+            def __init__(self, v):
+                pass
+            def __eq__(self, other):
+                nonlocal i
+                i += 1
+                if i == 1:
+                    next(g, None)
+                return True
+        i = 0
+        g = next(groupby(range(10), K))[1]
+        for j in range(2):
+            next(g, None)  # shouldn't crash
+
+
 class SubclassWithKwargsTest(unittest.TestCase):
     def test_keywords_in_subclass(self):
         # count is not subclassable...