Johannes Bader

Barry Jay, Johannes Bader

This paper introduces a simple type system for combinatory logic in which combinators have at most one type, whose polymorphism is revealed by application. The combinatory types exactly describe the structure of their values, which may be hidden by abstract types, such as list types and function types. Even without any quantified types, it supports polymorphism beyond that of the Hindley-Milner type system that underpins functional programming, and an effective type inference algorithm. Also, the simplicity of the formalism should make other static program analyses easier.

Moritz Beller, Chu-Pan Wong, Johannes Bader et al.

Traditionally, mutation testing generates an abundance of small deviations of a program, called mutants. At industrial systems the scale and size of Facebook's, doing this is infeasible. We should not create mutants that the test suite would likely fail on or that give no actionable signal to developers. To tackle this problem, in this paper, we semi-automatically learn error-inducing patterns from a corpus of common Java coding errors and from changes that caused operational anomalies at Facebook specifically. We combine the mutations with instrumentation that measures which tests exactly visited the mutated piece of code. Results on more than 15,000 generated mutants show that more than half of the generated mutants survive Facebook's rigorous test suite of unit, integration, and system tests. Moreover, in a case study with 26 developers, all but two found information of automatically detected test holes interesting in principle. As such, almost half of the 26 would actually act on the mutant presented to them by adapting an existing or creating a new test. The others did not for a variety of reasons often outside the scope of mutation testing. It remains a practical challenge how we can include such external information to increase the true actionability rate on mutants.

Johannes Bader, Andrew Scott, Michael Pradel et al.

Static analyzers help find bugs early by warning about recurring bug categories. While fixing these bugs still remains a mostly manual task in practice, we observe that fixes for a specific bug category often are repetitive. This paper addresses the problem of automatically fixing instances of common bugs by learning from past fixes. We present Getafix, an approach that produces human-like fixes while being fast enough to suggest fixes in time proportional to the amount of time needed to obtain static analysis results in the first place. Getafix is based on a novel hierarchical clustering algorithm that summarizes fix patterns into a hierarchy ranging from general to specific patterns. Instead of a computationally expensive exploration of a potentially large space of candidate fixes, Getafix uses a simple yet effective ranking technique that uses the context of a code change to select the most appropriate fix for a given bug. Our evaluation applies Getafix to 1,268 bug fixes for six bug categories reported by popular static analyzers for Java, including null dereferences, incorrect API calls, and misuses of particular language constructs. The approach predicts exactly the human-written fix as the top-most suggestion between 12% and 91% of the time, depending on the bug category. The top-5 suggestions contain fixes for 526 of the 1,268 bugs. Moreover, we report on deploying the approach within Facebook, where it contributes to the reliability of software used by billions of people. To the best of our knowledge, Getafix is the first industrially-deployed automated bug-fixing tool that learns fix patterns from past, human-written fixes to produce human-like fixes.