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Heisenbug

Category: Hazards · Status: stub

One line: A bug that disappears or changes when you look for it — adding a print, attaching a debugger or changing the optimizer shifts the timing it depends on.

Also called: flaky concurrency bug.

How it connects

flowchart LR
  n_deterministic_testing["Deterministic scheduling for tests"]
  n_heisenbug["Heisenbug"]
  n_nondeterminism["Nondeterminism"]
  n_race_condition["Race condition"]
  n_deterministic_testing -->|prevents| n_heisenbug
  n_nondeterminism -->|can cause| n_heisenbug
  n_race_condition -->|can cause| n_heisenbug
  classDef center stroke-width:3px
  class n_heisenbug center
  classDef outside stroke-dasharray: 4 3
  class n_deterministic_testing,n_nondeterminism,n_race_condition outside

In each language
Rust loom ↗ deterministically explores the possible execution permutations instead of relying on random runs
Go testing/synctest ↗, new in Go 1.25 ↗, runs a test in a bubble whose fake clock advances only when every goroutine in it is durably blocked
Elsewhere rr ↗ records a failing run once and replays it deterministically; its chaos mode makes intermittent bugs more reproducible

Where to read more