chore: checkpoint before Python removal

vendor/tower/src/buffer/message.rs

@@ -0,0 +1,16 @@
+use super::error::ServiceError;
+use tokio::sync::oneshot;
+
+/// Message sent over buffer
+#[derive(Debug)]
+pub(crate) struct Message<Request, Fut> {
+    pub(crate) request: Request,
+    pub(crate) tx: Tx<Fut>,
+    pub(crate) span: tracing::Span,
+}
+
+/// Response sender
+pub(crate) type Tx<Fut> = oneshot::Sender<Result<Fut, ServiceError>>;
+
+/// Response receiver
+pub(crate) type Rx<Fut> = oneshot::Receiver<Result<Fut, ServiceError>>;