Test-case reduction via test-case generation: Insights from the Hypothesis reducer

David R. MacIver, Alastair F. Donaldson
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Read: 09 July 2020

34th European Conference on Object-Oriented Programming (ECOOP 2020)
Schloss Dagstuhl, Leibniz-Zentrum für Informatik
Pages 13:1–13:28
Note(s): fuzz testing, test-case reduction, unit tests, property-based testing
Papers: regehr:pldi:2012

This paper proposes a general purpose way of getting test-case reduction “for free”. The paper shows that it “provides adequate reduction for any generated test case, while avoiding the test-case validity problem.”

There are three main ideas to achieve this.

The key to avoiding creation of invalid values is “internal reduction”: the reduction is integrated into the random generator instead of being performed by an external tool as in C-reduce regehr:pldi:2012.

The key to achieving this is to view a random generator as a “parser of choice sequences” and to recover the structure of this parser (i.e., the generator) by instrumenting the API. This allows the reducer to perform optimizations such as removing/replacing contiguous sequences of choices as seen in hierarchial delta debugging.

The final ingredient is to have a generic notion of what makes one test-case better than another. This is defined as an ordering over the choice sequence (not over some application-dependent rendering of the sequence). They use “shortlex order” where one sequence is ordered before another if it is shorter or it is the same length but lexicographically earlier.

The “shortlex” order generally works well but for very carefully encoded data types like binary floating point, you need to write a special generator to arrange that a number like “1.0” will be preferred to a number like “5e-324”.

The evaluation shows that this general purpose approach works about as well as custom approaches on a broad range of cases. (But the comparision with C-reduce should be read carefully: my interpretation of the results is that if it really, really matters it may be worth putting in considerably more effort to write a custom reducer.)

Perhaps the most important thing shown by the evaluation though is the ability to apply the approach to many cases.

This is an important selling point for internal reduction: it works at the level of choice sequences, and any randomized generator can be relatively easily adapted to consume a choice sequence instead of using a pseudo-random number generator, thus internal reduction has wide applicability.
– Section 4

The evaluation also reveals a scaling problem. Their tool is fine generating inputs up to about 8kB but, because the chance of triggering a bug is quite low, CSmith is generally better with inputs of around 80kB. It is not completely clear whether this is fundamental to the way that Hypothesis works.

Test-case reduction