Order types: static reasoning about message races in asynchronous message passing concurrency

Asynchronous message passing concurrency with higher level concurrency constructs including activities, asynchronous method invocations and future return values is gaining increased popularity, as an alternative to shared memory concurrency with lower level threads and locks. However, similar to data races in shared memory concurrency, message races in asynchronous message passing concurrency can make a program incorrect and cause bugs that are hard to find and fix. This paper presents order types, a novel type system for static reasoning about message races in asynchronous message passing concurrency. Order types are local, causal and polymorphic types with the following features. First, order types encode both communication and flow behaviors and their happens-before relations. Second, order types are designed for an imperative calculus with concurrent activities, asynchronous method invocations, future return values, wait-by-necessity synchronizations on futures, first-class activities and futures, recursion and dynamic creation of activities. Third, order types are polymorphic and introduce universally quantified type variables to encode unknown values of variables. Fourth, the order type of a module can be inferred modularly using only its implementation and independent of implementations of other modules in the program. Order types complement previous work on static reasoning about races in asynchronous message passing concurrency.