The Problem

There are many challenges when dealing with a C FFI in Rust, including lifetimes, ownership, error handling and non-synchronous code. A less discussed but prominent issue with DOCA is callback hell. C interfaces are often designed to be used with user callbacks, which often obscure program flow and type safety. DOCA is one such library, centered around a task based event system that uses callbacks. At first glance, there is no easy way to handle program logic except within these callbacks. This means submitting tasks for callbacks within callbacks, and so on and so forth, making it challenging to find an opening for Rust to integrate, as well as allow for arbitrary user logic in between. Our Solution

Following the wisdom of message passing, our design was able to completely decouple callbacks from regular program logic using messages and static dispatch (with generics). Essentially, the callbacks themselves only need to construct a message. Then, the message can be delivered to a wrapper structure for each of the various DOCA libraries. As a result, virtually all remaining logic can be written in safe Rust. This allows the wrapper library to be easily used with both synchronous and asynchronous Rust. During the talk, we’ll answer further questions about the design, including how to reason about lifetimes for task submission into the DOCA black box, how should the user interface with the wrapper structures, and how should errors be propagated to the user.