gh-101892: Fix `SystemError` when a callable iterator call exhausts the iterator (GH-101896)

(cherry picked from commit 705487c6557c3d8866622b4d32528bf7fc2e4204)

Co-authored-by: Raj <51259329+workingpayload@users.noreply.github.com>
Co-authored-by: Oleg Iarygin <oleg@arhadthedev.net>

1 files changed, 25 insertions, 0 deletions

diff --git a/Lib/test/test_iter.py b/Lib/test/test_iter.py
index 6ab9b7a..30aedb0 100644
--- a/Lib/test/test_iter.py
+++ b/Lib/test/test_iter.py
@@ -348,6 +348,31 @@ class TestCase(unittest.TestCase):
             return i
         self.check_iterator(iter(spam, 20), list(range(10)), pickle=False)
 
+    def test_iter_function_concealing_reentrant_exhaustion(self):
+        # gh-101892: Test two-argument iter() with a function that
+        # exhausts its associated iterator but forgets to either return
+        # a sentinel value or raise StopIteration.
+        HAS_MORE = 1
+        NO_MORE = 2
+
+        def exhaust(iterator):
+            """Exhaust an iterator without raising StopIteration."""
+            list(iterator)
+
+        def spam():
+            # Touching the iterator with exhaust() below will call
+            # spam() once again so protect against recursion.
+            if spam.is_recursive_call:
+                return NO_MORE
+            spam.is_recursive_call = True
+            exhaust(spam.iterator)
+            return HAS_MORE
+
+        spam.is_recursive_call = False
+        spam.iterator = iter(spam, NO_MORE)
+        with self.assertRaises(StopIteration):
+            next(spam.iterator)
+
     # Test exception propagation through function iterator
     def test_exception_function(self):
         def spam(state=[0]):