Safe Sessions of Channel Actions in Clojure: A Tour of the Discourje Project

Abstract

To simplify shared-memory concurrent programming, in addition to low-level synchronisation primitives, several modern programming languages have started to offer core support for higher-level communication primitives as well, in the guise of message passing through channels. Yet, a growing body of evidence suggests that channel-based programming abstractions for shared memory also have their issues.

The Discourje project aims to help programmers cope with message-passing concurrency bugs in Clojure programs, based on run-time verification and dynamic monitoring. The idea is that programmers write not only implementations, but also specifications (of sessions of channel actions). Discourje then offers a library to ensure that implementations run safely relative to specifications (= “bad” channel actions never happen).

This paper gives a tour of the current state of Discourje, by example; it is intended to serve both as a general overview for readers who are unfamiliar with previous work on Discourje, and as an introduction to new features for readers who are familiar.

A Clojure

Standard library :

• : first evaluates e to v; then binds x to v in the global environment.

• : first evaluates $e_1$; if , evaluates $e_2$; else, evaluates $e_3$.

• : first evaluates $e_1$ to $v_1$; then evaluates $e_2$ to $v_2$ with $x_1$ bound to $v_1$; ...; then evaluates $e_n$ to $v_n$ with $x_1$, ..., $x_{n-1}$ bound to $v_1$, ..., $v_{n-1}$; then evaluates e with $x_1$, ..., $x_n$ bound to $v_1$, ..., $v_n$.

• : evaluates to a function with parameters $x_1$, ..., $x_n$ and creates a recursion point; then, when applied to arguments $v_1$, ..., $v_n$, sequentially evaluates $e_1$, ..., $e_m$ with $x_1$, ..., $x_n$ bound to $v_1$, ..., $v_n$.

• : same as , but also creates a recursion point.

• : first evaluates $e_1$, ..., $e_n$ to $v_1$, ..., $v_n$; then evaluates the nearest recursion point with $x_1$, ..., $x_n$ bound to $v_1$, ..., $v_n$.

Standard library :

• : starts a new thread that evaluates e.

• : evaluates to a new unbuffered channel.

• : first evaluates e to channel c; then closes c.

• : first evaluates $e_1$ to channel c; then evaluates $e_2$ to v; then sends v through c.

• : first evaluates e to channel c; then receives a value through c.

• : for every $a_i$ of the form (send) or (receive), evaluates $e_{i,1}$ and $e_i$ to channel $c_i$, and then, evaluates $e_{i,2}$ to v; then, waits until one of these channel actions can be performed; then, performs a channel action that can be performed (non-deterministically selected).