MEPX
Chapter 7 of 10All chapters
  1. 1Why test
  2. 2Kinds of test
  3. 3What a good test looks like
  4. 4Fakes, stubs and mocks
  5. 5Coverage and flakiness
  6. 6Reproducing a bug
  7. 7Narrowing it down
  8. 8Reading the evidence
  9. 9Debuggers and tools
  10. 10The hard ones

Chapter 7 of 10

Narrowing it down

Halving the search space.

Bisecting

If it worked last month, find the commit that broke it. git bisect turns a thousand commits into about ten checks, and the answer usually explains itself.

  • Bisect the data too: remove half the input and see if the failure survives.
  • Comment out half the code, or disable half the config, on the same principle.

One change at a time

Changing three things and seeing the bug disappear teaches you nothing about which one mattered. Revert everything that did not help before moving on.