97 Things Every Programmer Should Know

Chapter 86. Two Wrongs Can Make a Right (and Are Difficult to Fix)

Allan Kelly

CODE NEVER LIES, BUT IT CAN CONTRADICT ITSELF. Some contradictions lead to those “How can that possibly work?” moments.

In an interview,^[12] the principal designer of the Apollo 11 Lunar Module software, Allan Klumpp, disclosed that the software controlling the engines contained a bug that should have made the lander unstable. However, another bug compensated for the first, and the software was used for both Apollo 11 and 12 Moon landings before either bug was found or fixed.

Consider a function that returns a completion status. Imagine that it returns false when it should return true. Now imagine that the calling function neglects to check the return value. Everything works fine until one day someone notices the missing check and inserts it.

Or consider an application that stores state as an XML document. Imagine that one of the nodes is incorrectly written as TimeToLive instead of TimeToDie, as the documentation says it should. Everything appears fine while the writer code and the reader code both contain the same error. But fix one, or add a new application reading the same document, and the symmetry is broken, as well as the code.

When two defects in the code create one visible fault, the methodical approach to fixing faults can itself break down. The developer gets a bug report, finds the defect, fixes ...