ConFuzz: Coverage-Guided Property Fuzzing for Event-Driven Programs

Bug-free concurrent programs are hard to write due to non-determinism arising out of concurrency and program inputs. Since concurrency bugs typically manifest under specific inputs and thread schedules, conventional testing methodologies for concurrent programs like stress testing and random testing, which explore random schedules, have a strong chance of missing buggy schedules.In this paper, we introduce a novel technique that combines property-based testing with mutation-based, grey box fuzzer, applied to event-driven OCaml programs. We have implemented this technique in , a directed concurrency bug-finding tool for event-driven OCaml programs. Using , programmers specify high-level program properties as assertions in the concurrent program. uses the popular greybox fuzzer AFL to generate inputs as well as concurrent schedules to maximise the likelihood of finding new schedules and paths in the program so as to make the assertion fail. does not require any modification to the concurrent program, which is free to perform arbitrary I/O operations. Our experimental results show that is easy-to-use, effective, detects concurrency bugs faster than Node.Fz - a random fuzzer for event-driven JavaScript programs, and is able to reproduce known concurrency bugs in widely used OCaml libraries.

Putting Gradual Types to Work

In this paper, we describe our experience incorporating gradual types in a statically typed functional language with Hindley-Milner style type inference. Where most gradually typed systems aim to improve static checking in a dynamically typed language, we approach it from the opposite perspective and promote dynamic checking in a statically typed language. Our approach provides a glimpse into how languages like SML and OCaml might handle gradual typing. We discuss our implementation and challenges faced—specifically how gradual typing rules apply to our representation of composite and recursive types. We review the various implementations that add dynamic typing to a statically typed language in order to highlight the different ways of mixing static and dynamic typing and examine possible inspirations while maintaining the gradual nature of our type system. This paper also discusses our motivation for adding gradual types to our language, and the practical benefits of doing so in our industrial setting.