go-defensive

# Go Defensive Programming Hardening Go code is not paranoia — it is the discipline of making your boundaries honest. Copy what crosses them, clean up what you opened, model time and randomness honestly, and never let a panic escape a package. ## Core Rules 1. **Copy slices and maps at API boundaries.** They are reference types — leaking the backing array leaks mutation. 2. **`defer` the cleanup right after the acquire.** `f, err := os.Open(...); defer f.Close()`. 3. **Verify interface compliance at compile time:** `var _ I = (*T)(nil)`. 4. **Model time and durations with `time.Time` and `time.Duration`,** never raw ints. 5. **Inject `now func() time.Time`** instead of calling `time.Now()` directly in production code. 6. **Enums start at `iota + 1`** so the zero value is invalid. 7. **`crypto/rand` for secrets, never `math/rand`.** 8. **Panics never cross package boundaries.** Convert to errors at the edge. 9. **Avoid mutable package-level state.** Inject dependencies instead. ## Boundary Hardening Checklist When you touch an exported function or method, walk this list in order: | # | Check | |---|---| | 1 | Return errors, don't panic across boundaries | | 2 | Copy slices/maps you'll retain | | 3 | Copy slices/maps you'll return if internal state aliases them | | 4 | `defer` Close / Unlock / cancel right after the acquire | | 5 | Compile-time interface satisfaction check | | 6 | `time.Time` / `time.Duration` types, injected clock | | 7 | Enum zero = invalid (`iota + 1`)...