First, there’s some essential reading in the excellent Google SWE Book: Large-Scale Changes chapter. A key point being brought up by Adam Bender: Today it is common for a double-digit percentage (10% to 20%) of the changes in a project to be the result of LSCs, meaning a substantial amount of code is changed in projects by people whose full-time job is unrelated to those projects. Without good tests, such work would be impossible, and Google’s codebase would quickly atrophy under its own weight.

A “modify after write” error refers to a situation where a program or process attempts to modify data after it has been written or committed to a particular location or state. In Go terms, this can mean that a field may be modified after the struct has been written to the database. When debugging a login issue, we found a field was not updated in the database. On investigation, I found that a field in the user data was being modified after it had been saved to the database.