TigerBeetle's Hidden Bug: How Sophisticated Fuzzing Failed

The TigerBeetle team discovered a bug in their query engine using Jepsen, surprisingly in a component previously fuzzed extensively by four separate fuzzers. The investigation revealed a blind spot in the fuzzer's input generation strategy, leaving certain query combinations uncovered. This stemmed from the fuzzer pre-registering queries during initialization—a simplification that inadvertently constrained input space diversity. By improving the fuzzer to generate more random inputs and perform more precise verification, the bug was successfully reproduced and fixed. This case highlights how even sophisticated fuzzing strategies can have blind spots, necessitating a combination of testing approaches for comprehensive software quality assurance.