Issue #17278: Fix a crash in heapq.heappush() and heapq.heappop() when the list is being resized concurrently.

Lib/test/test_heapq.py

@@ -319,6 +319,16 @@ def L(seqn):
     return chain(map(lambda x:x, R(Ig(G(seqn)))))
 
 
+class SideEffectLT:
+    def __init__(self, value, heap):
+        self.value = value
+        self.heap = heap
+
+    def __lt__(self, other):
+        self.heap[:] = []
+        return self.value < other.value
+
+
 class TestErrorHandling(TestCase):
     module = None
 
@@ -370,6 +380,22 @@ class TestErrorHandling(TestCase):
                 self.assertRaises(TypeError, f, 2, N(s))
                 self.assertRaises(ZeroDivisionError, f, 2, E(s))
 
+    # Issue #17278: the heap may change size while it's being walked.
+
+    def test_heappush_mutating_heap(self):
+        heap = []
+        heap.extend(SideEffectLT(i, heap) for i in range(200))
+        # Python version raises IndexError, C version RuntimeError
+        with self.assertRaises((IndexError, RuntimeError)):
+            self.module.heappush(heap, SideEffectLT(5, heap))
+
+    def test_heappop_mutating_heap(self):
+        heap = []
+        heap.extend(SideEffectLT(i, heap) for i in range(200))
+        # Python version raises IndexError, C version RuntimeError
+        with self.assertRaises((IndexError, RuntimeError)):
+            self.module.heappop(heap)
+
 
 class TestErrorHandlingPython(TestErrorHandling):
     module = py_heapq