Rseng Debugging

Debugging research software

Debugging is applied science: observe, hypothesize, run the decisive experiment, repeat. The anti-pattern is its opposite - changing code until the symptom moves. Research adds two hard twists: the worst bugs produce plausible wrong numbers instead of crashes (rseng-defensive-coding exists to make them crash), and the oracle problem - when the right answer is unknown, "wrong" itself needs evidence. The discipline below is tool-agnostic; the debuggers change, the method does not.

The loop

1. Reproduce first: a bug you cannot reproduce on demand cannot be diagnosed, only feared. Pin the environment (rseng-reproducible-environments), the data version (rseng-data-management), the seed (rseng-defensive-coding) and the exact command; record them in the issue (rseng-version-control-review).
2. Minimize: shrink input, code path and configuration until the smallest thing that still fails remains - minimization IS diagnosis (each removal that keeps the bug excludes a hypothesis), and the minimal reproducer becomes the regression test and, for dependency bugs, the upstream report.
3. Hypothesize before touching: state what would explain the evidence, predict what an experiment will show, THEN run it. One variable per experiment; write the trail down when the hunt exceeds a few steps (the notes are tomorrow's context and the postmortem's material).
4. Localize by bisection - the log-time weapon on three axes: commits (git bisect with the reproducer as the test, ideally scripted with git bisect run), data (which half of the input triggers it), and pipeline stages (diff intermediates against a known-good run - rseng-workflows' cached stages make this cheap).
5. Fix the cause, not the symptom, and prove it: the reproducer passes, the regression test is committed (rseng-testing), and the fix commit explains the WHY (rseng-version-control-review).

Scientific failure modes

• Wrong numbers, no crash: establish ground truth from analytic cases, conservation laws, invariants or a reference implementation (rseng-open-source-migration's parity discipline); then bisect the pipeline to the first stage whose intermediate diverges (rseng-numerical-accuracy decides what counts as divergence).
• "Results changed and I do not know why": diff environment lockfiles, data checksums and config before suspecting code; git bisect only after the inputs are proven identical (rseng-reproducibility's bookkeeping makes this a five-minute question).
• Nondeterministic bugs: suspect the usual three - unseeded or shared randomness (rseng-defensive-coding), parallel ordering and race conditions (run single-threaded to confirm), and iteration-order or filesystem-order dependence. Make it deterministic FIRST, then debug.
• Scale-dependent failures (fails on the cluster, passes locally): memory limits, walltime, node differences, thread counts, missing files on compute nodes - read the scheduler logs and seff before the code (rseng-hpc-computing); reproduce at the smallest failing scale.
• Heisenbugs that vanish under observation usually implicate timing or uninitialized state; prefer low-intrusion observation (sampling profilers, core dumps, logging) over stepping.

Service startup failures

When a delivered stack fails at launch (compose service exits, API returns 500s on first request), debug from the logs, not the code: docker compose logs <service> / the server's stderr almost always names the true failure - a missing env var, an unreachable database host, a port collision, a schema mismatch. Fix the FIRST error in the log (later ones cascade), restart, re-read; repeat until startup is clean, then re-run the request that failed. The same loop applies outside containers: read the traceback of the crashing entry point before touching application code.

Mechanics worth teaching

• Read the WHOLE error: the bottom-most frame of the traceback in YOUR code, the first error in a cascade (later ones are usually consequences), the actual message text - half of debugging is reading what the program already said.
• Debugger beats print for exploration (breakpoints, inspecting live state, post-mortem on the crash); print/logging beats debugger for production, parallel and long-running contexts - structured, greppable, left in place behind a verbosity flag (service-style logs when the code runs unattended).
• Rubber-duck honestly: explaining the bug out loud - to the agent - is a legitimate technique; the agent's role is to ask the hypothesis-forcing questions, not to guess along.
• Know when to stop: after real effort, write up the evidence trail and ask a colleague or file an issue - the write-up alone solves a good fraction (rseng-trainer's errors-are-curriculum: narrate the diagnosis as you go).

Working with this skill

This skill is source-independent: it encodes systematic debugging practice (hypothesis-driven diagnosis, delta debugging and bisection) as taught in the linked references, applied to research software.

Learn more (verified):

• https://www.debuggingbook.org - The Debugging Book (Zeller; systematic and automated debugging techniques)
• https://jvns.ca/blog/2022/12/08/a-debugging-manifesto/ - Julia Evans' debugging manifesto

Related skills

Check whether any of these applies before moving on:

• rseng-defensive-coding - make silent bugs crash first
• rseng-hpc-computing - scale-dependent cluster-only failures
• rseng-lessons-learned - postmortems from debugging trails
• rseng-numerical-accuracy - deciding whether divergence is real
• rseng-reproducible-environments - pin environment to reproduce the bug
• rseng-workflows - cached stages make pipeline bisection cheap