gh-104371: Fix calls to __release_buffer__ while an exception is active

Lib/test/test_buffer.py

@@ -4749,6 +4749,19 @@ def __buffer__(self, flags):
         c.clear()
         self.assertIs(c.buffer, None)
 
+    def test_release_buffer_with_exception_set(self):
+        class A:
+            def __buffer__(self, flags):
+                return memoryview(bytes(8))
+            def __release_buffer__(self, view):
+                pass
+
+        b = bytearray(8)
+        with memoryview(b):
+            # now b.extend will raise an exception due to exports
+            with self.assertRaises(BufferError):
+                b.extend(A())
+
 
 if __name__ == "__main__":
     unittest.main()

Objects/typeobject.c

@@ -9115,8 +9115,9 @@ releasebuffer_maybe_call_super(PyObject *self, Py_buffer *buffer)
 }
 
 static void
-releasebuffer_call_python(PyObject *self, Py_buffer *buffer)
+releasebuffer_call_python_inner(PyObject *self, Py_buffer *buffer)
 {
+    assert(!PyErr_Occurred());
     PyObject *mv;
     bool is_buffer_wrapper = Py_TYPE(buffer->obj) == &_PyBufferWrapper_Type;
     if (is_buffer_wrapper) {
@@ -9157,6 +9158,22 @@ releasebuffer_call_python(PyObject *self, Py_buffer *buffer)
     Py_DECREF(mv);
 }
 
+static void
+releasebuffer_call_python(PyObject *self, Py_buffer *buffer)
+{
+    // bf_releasebuffer may be called while an exception is already active.
+    // We have no way to report additional errors up the stack, because
+    // this slot returns void, so we simply stash away the active exception
+    // and restore it after the call to Python returns.
+    PyObject *type, *value, *traceback;
+    PyErr_Fetch(&type, &value, &traceback);
+
+    releasebuffer_call_python_inner(self, buffer);
+    assert(!PyErr_Occurred());
+
+    PyErr_Restore(type, value, traceback);
+}
+
 /*
  * bf_releasebuffer is very delicate, because we need to ensure that
  * C bf_releasebuffer slots are called correctly (or we'll leak memory),