Fast test suite-driven model-based fault localisation with application to pinpointing defects in student programs

Abstract

Fault localisation, i.e. the identification of program locations that cause errors, takes significant effort and cost. We describe a fast model-based fault localisation algorithm that, given a test suite, uses symbolic execution methods to fully automatically identify a small subset of program locations where genuine program repairs exist. Our algorithm iterates over failing test cases and collects locations where an assignment change can repair exhibited faulty behaviour. Our main contribution is an improved search through the test suite, reducing the effort for the symbolic execution of the models and leading to speed-ups of more than two orders of magnitude over the previously published implementation by Griesmayer et al. We implemented our algorithm for C programs, using the KLEE symbolic execution engine, and demonstrate its effectiveness on the Siemens TCAS variants. Its performance is in line with recent alternative model-based fault localisation techniques, but narrows the location set further without rejecting any genuine repair locations where faults can be fixed by changing a single assignment. We also show how our tool can be used in an educational context to improve self-guided learning and accelerate assessment. We apply our algorithm to a large selection of actual student coursework submissions, providing precise localisation within a sub-second response time. We show this using small test suites, already provided in the coursework management system, and on expanded test suites, demonstrating the scalability of our approach. We also show compliance with test suites does not reliably grade a class of “almost-correct” submissions, which our tool highlights, as being close to the correct answer. Finally, we show an extension to our tool that extends our fast localisation results to a selection of student submissions that contain two faults.