GH-50591: [Python] Fix reference in ConvertToSequenceAndInferSize when an iterator raises on array conversion

python/pyarrow/src/arrow/python/python_to_arrow.cc

@@ -1259,6 +1259,7 @@ Status ConvertToSequenceAndInferSize(PyObject* obj, PyObject** seq, int64_t* siz
     OwnedRef iter_ref(iter);
     PyObject* lst = PyList_New(n);
     RETURN_IF_PYERROR();
+    OwnedRef lst_ref(lst);
     for (i = 0; i < n; i++) {
       PyObject* item = PyIter_Next(iter);
       if (!item) {
@@ -1270,10 +1271,9 @@ Status ConvertToSequenceAndInferSize(PyObject* obj, PyObject** seq, int64_t* siz
     }
     // Shrink list if len(iterator) < size
     if (i < n && PyList_SetSlice(lst, i, n, NULL)) {
-      Py_DECREF(lst);
       RETURN_IF_PYERROR();
     }
-    *seq = lst;
+    *seq = lst_ref.detach();
     *size = std::min<int64_t>(i, *size);
   }
   return Status::OK();

python/pyarrow/tests/test_convert_builtin.py

@@ -128,6 +128,29 @@ def test_failing_iterator():
         pa.array((1 // 0 for x in range(10)), size=10)
 
 
+def test_failing_iterator_does_not_leak():
+    # GH-50591
+    import gc
+    import sys
+
+    # Create an arbitrary long int that is hopefully not cached by the interpreter
+    value = 10**20
+
+    def raising_iter():
+        for _ in range(5):
+            yield value
+        raise ValueError("boom")
+
+    gc.collect()
+    original_refcount = sys.getrefcount(value)
+
+    with pytest.raises(ValueError, match="boom"):
+        pa.array(raising_iter(), size=100)
+
+    gc.collect()
+    assert sys.getrefcount(value) == original_refcount
+
+
 class ObjectWithOnlyGetitem:
     def __getitem__(self, key):
         return 3