What is a Heisenbug?

Heisenbug is a bug that disappears or changes its behavior when you try to investigate or detect it. Its name derives from Heisenberg's uncertainty principle in quantum mechanics.

Causes of Heisenbugs are often related to:

• Timing factors: Errors dependent on precise execution time or order of operations.
• System state: Changes in environment or configuration during debugging affect the bug's manifestation.
• Debugging tools: Connecting a debugger or adding logs can alter program behavior and hide the bug (e.g., affecting timing).
• Multithreading/parallelism: Race conditions or incorrect synchronization manifest only under certain execution sequences, which are disrupted when trying to reproduce the problem.

To identify Heisenbugs, the following are often used:

• Careful logging without interfering with timing (asynchronous logging).
• Post-mortem analysis of memory dumps or logs.
• Testing in environments as close to production as possible.
• Specialized tools for analyzing thread execution and race conditions.
• Pair programming / code review to identify potentially dangerous code sections.