Go Defensive

Go Defensive Programming Patterns

Defensive Checklist Priority

When hardening code at API boundaries, check in this order:

Reviewing an API boundary?
├─ 1. Error handling     → Return errors; don't panic
├─ 2. Input validation   → Copy slices/maps received from callers
├─ 3. Output safety      → Copy slices/maps before returning to callers
├─ 4. Resource cleanup   → Use defer for Close/Unlock/Cancel
├─ 5. Interface checks   → var _ Interface = (*Type)(nil) for compile-time verification
├─ 6. Time correctness   → Use time.Time and time.Duration, not int/float
├─ 7. Enum safety        → Start iota at 1 so zero-value is invalid
└─ 8. Crypto safety      → crypto/rand for keys, never math/rand

Quick Reference

Copy Slices and Maps at Boundaries

Slices and maps contain pointers to underlying data. Copy at API boundaries to prevent unintended modifications.

// Receiving: copy incoming slice
d.trips = make([]Trip, len(trips))
copy(d.trips, trips)

// Returning: copy map before returning
result := make(map[string]int, len(s.counters))
for k, v := range s.counters { result[k] = v }

Read references/BOUNDARY-COPYING.md when copying slices or maps at API boundaries, or deciding when defensive copies are necessary vs. when they can be skipped.

Defer to Clean Up

Use defer to clean up resources (files, locks). Avoids missed cleanup on multiple return paths.

p.Lock()
defer p.Unlock()

if p.count < 10 {
  return p.count
}
p.count++
return p.count

Defer overhead is negligible. Place defer f.Close() immediately after os.Open for clarity. Arguments to deferred functions are evaluated when defer executes, not when the function runs. Multiple defers execute in LIFO order.

Struct Field Tags

Advisory: Always add explicit field tags to structs that are marshaled or unmarshaled.

type User struct {
    Name  string `json:"name"  yaml:"name"`
    Email string `json:"email" yaml:"email"`
}

Field tags are a serialization contract — renaming a struct field without updating the tag silently breaks wire compatibility. Treat tags as part of the public API for any type that crosses a serialization boundary.

Start Enums at One

Start enums at non-zero to distinguish uninitialized from valid values.

const (
  Add Operation = iota + 1  // Add=1, zero value = uninitialized
  Subtract
  Multiply
)

Exception: When zero is the sensible default (e.g., LogToStdout = iota).

Time, Struct Tags, and Embedding

Read references/TIME-ENUMS-TAGS.md when using time.Time/time.Duration instead of raw ints, adding field tags to marshaled structs, or deciding whether to embed types in public structs.

Avoid Mutable Globals

Inject dependencies instead of mutating package-level variables.

type signer struct {
  now func() time.Time  // injected; tests replace with fixed time
}

func newSigner() *signer {
  return &signer{now: time.Now}
}

Read references/GLOBAL-STATE.md when designing the New() + Default() package state pattern or replacing mutable globals with dependency injection.

Crypto Rand

Do not use math/rand or math/rand/v2 to generate keys — this is a security concern. Time-seeded generators have predictable output.

import "crypto/rand"

func Key() string { return rand.Text() }

Panic and Recover

Use panic only for truly unrecoverable situations. Library functions should avoid panic.

func safelyDo(work *Work) {
    defer func() {
        if err := recover(); err != nil {
            log.Println("work failed:", err)
        }
    }()
    do(work)
}

Key rules:

• Never expose panics across package boundaries — always convert to errors
• Acceptable to panic in init() if a library truly cannot set itself up
• Use recover to isolate panics in server goroutine handlers

Read references/PANIC-RECOVER.md when writing panic recovery in HTTP servers, using panic as an internal control flow mechanism in parsers, or deciding between log.Fatal and panic.

Must Functions

Must functions panic on error — use them only during program initialization where failure means the program cannot run.

var validID = regexp.MustCompile(`^[a-z][a-z0-9-]{0,62}$`)
var tmpl = template.Must(template.ParseFiles("index.html"))

Read references/MUST-FUNCTIONS.md when writing custom Must functions, deciding whether Must is appropriate for a given call site, or wrapping fallible initialization in a panicking helper.