GoKawiilDeveloper builds Nix-based deterministic debugger for reproducing race conditions
A developer has extended Rewind VM, a deterministic virtual machine that replays Nix builds including thread scheduling, adding a source panel, stack frames, bookmarks, a comparison view, expanded gdb support, thread lanes showing CPU ownership, and a 'Check from here' feature to pinpoint race conditions. The author demonstrates the tool on a classic bank-account race condition, showing deposits lost in 396 of 1,000 runs on a multi-core machine versus none when pinned to a single core.
GoKawiil's interpretation of the reporting above, not reported fact.
The author suggests that Nix's existing infrastructure for pinning exact dependencies, debug symbols and sources inadvertently solves much of the hard work needed to build a reproducible debugger, which could make this approach useful for other developers hunting concurrency bugs. This implies Nix's reproducibility properties may have underexplored applications beyond build reliability, into debugging and diagnostics tooling.
- Rewind VM replays Nix builds deterministically, including thread scheduling, to reproduce race conditions.
- New debugger features like thread lanes and 'Check from here' leverage Nix's existing dependency and symbol infrastructure.
- A demonstrated bank-account race condition lost money in nearly 40% of multi-core runs but none on a single core.
Source: fzakaria.com, 2026-10-09
Published there as: “Nix wrote half of my debugger”
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