Testing Dafny (Experience Paper)

Verification toolchains are widely used to prove the correctness of critical software systems. To build confidence in their results, it is important to develop testing frameworks that help detect bugs in these toolchains. Inspired by the success of fuzzing in finding bugs in compilers and SMT solvers, we have built the first fuzzing and differential testing framework for Dafny, a high-level programming language with a Floyd-Hoare-style program verifier and compilers to C#, Java, Go, and Javascript. This paper presents our experience building and using XDsmith, a testing framework that targets the entire Dafny toolchain, from verification to compilation. XDsmith randomly generates annotated programs in a subset of Dafny that is free of loops and heap-mutating operations. The generated programs include preconditions, postconditions, and assertions, and they have a known verification outcome. These programs are used to test the soundness and precision of the Dafny verifier, and to perform differential testing on the four Dafny compilers. Using XDsmith, we uncovered 31 bugs across the Dafny verifier and compilers, each of which has been confirmed by the Dafny developers. Moreover, 8 of these bugs have been fixed in the mainline release of Dafny.