/golang-safety

name: golang-safety
description: "Defensive Golang coding against accidental bugs — nil panics, typed-nil interfaces, `append` backing-array aliasing, silent int64-to-int32 truncation, float `==` comparison, `defer` inside loops, defensive copies of slices and maps, and usable zero values. Use when a Go program panics on a nil map write or nil pointer dereference, when reviewing code for nil-safety, numeric conversion overflow, or resource lifecycle, or when designing a type whose zero value must be safe. Not for designing concurrent access with goroutines, channels, or sync primitives (→ See `samber/cc-skills-golang@golang-concurrency` skill), not for exploitable vulnerabilities such as injection, weak crypto, or leaked secrets (→ See `samber/cc-skills-golang@golang-security` skill), and not for debugging an already-failing program (→ See `samber/cc-skills-golang@golang-troubleshooting` skill)."
user-invocable: true
license: MIT
compatibility: Designed for Claude Code, Codex or similar harness, and for projects using Golang.
metadata:
  author: samber
  version: "1.3.2"
  openclaw:
    emoji: "🛡"
    homepage: https://github.com/samber/cc-skills-golang
    requires:
      bins:
        - go
    install: []
allowed-tools: Read Edit Write Glob Grep Bash(go:*) Bash(golangci-lint:*) Bash(git:*) Agent
paths:
  - "**/*.go"

**Persona:** You are a defensive Go engineer. You treat every untested assumption about nil, capacity, and numeric range as a latent crash waiting to happen.

Go Safety: Correctness & Defensive Coding

Prevents programmer mistakes — bugs, panics, and silent data corruption in normal (non-adversarial) code. Security handles attackers; safety handles ourselves.

Best Practices Summary

1. **Prefer generics over `any`** when the type set is known — compiler catches mismatches instead of runtime panics 2. **Always use safe type assertions** — for normal interfaces use comma-ok (`v, ok := x.(T)`); for reflection in Go 1.25+ prefer `reflect.TypeAssert[T](value)` over `value.Interface().(T)`. 3. **Typed nil pointer in an interface is not `== nil`** — the type descriptor makes it non-nil 4. **Writing to a nil map panics** — always initialize before use 5. **`append` may reuse the backing array** — both slices share memory if capacity allows, silently corrupting each other 6. **Return defensive copies** from exported functions — otherwise callers mutate your internals 7. **`defer` runs at function exit, not loop iteration** — extract loop body to a function 8. **Integer conversions truncate silently** — `int64` to `int32` wraps without error 9. **Float arithmetic is not exact** — use epsilon comparison or `math/big` 10. **Design useful zero values** — nil map fields panic on first write; use lazy init 11. **Use `sync.Once` for lazy init** — guarantees exactly-once even under concurrency

Nil Safety

Nil-related panics are the most common crash in Go.

The nil interface trap

Interfaces store (type, value). An interface is `nil` only when both are nil. Returning a typed nil pointer sets the type descriptor, making it non-nil:

// ✗ Dangerous — interface{type: *MyHandler, value: nil} is not == nil
func getHandler() http.Handler {
    var h *MyHandler // nil pointer
    if !enabled {
        return h // interface{type: *MyHandler, value: nil} != nil
    }
    return h
}

// ✓ Good — return nil explicitly
func getHandler() http.Handler {
    if !enabled {
        return nil // interface{type: nil, value: nil} == nil
    }
    return &MyHandler{}
}

Nil map, slice, and channel behavior

| Type | Index into nil | Write to nil | Len/Cap of nil | Range over nil | | ------- | -------------- | -------------- | -------------- | -------------- | | Map | Zero value | **panic** | 0 | 0 iterations | | Slice | **panic** | **panic** | 0 | 0 iterations | | Channel | Blocks forever | Blocks forever | 0 | Blocks forever |

// ✗ Bad — nil map panics on write
var m map[string]int
m["key"] = 1

// ✓ Good — initialize or lazy-init in methods
m := make(map[string]int)

func (r *Registry) Add(name string, val int) {
    if r.items == nil { r.items = make(map[string]int) }
    r.items[name] = val
}

See **[Nil Safety Deep Dive](./references/nil-safety.md)** for nil receivers, nil in generics, and nil interface performance.

Slice & Map Safety

Slice aliasing — the append trap

`append` reuses the backing array if capacity allows. Both slices then share memory:

// ✗ Dangerous — a and b share backing array
a := make([]int, 3, 5)
b := append(a, 4)
b[0] = 99 // also modifies a[0]

// ✓ Good — full slice expression forces new allocation
b := append(a[:len(a):len(a)], 4)

Map concurrent access

Maps MUST NOT be accessed concurrently — → see `samber/cc-skills-golang@golang-concurrency` for sync primitives.

See **[Slice and Map Deep Dive](./references/slice-map-safety.md)** for range pitfalls, subslice memory retention, and `slices.Clone`/`maps.Clone`.

Numeric Safety

Implicit type conversions truncate silently

// ✗ Bad — silently wraps around if val > math.MaxInt32 (3B becomes -1.29B)
var val int64 = 3_000_000_000
i32 := int32(val) // -1294967296 (silent wraparound)

// ✓ Good — check before converting
if val > math.MaxInt32 || val < math.MinInt32 {
    return fmt.Errorf("value %d overflows int32", val)
}
i32 := int32(val)

Float comparison

// ✗ Bad — floating point arithmetic is not exact
var a, b, c float64 = 0.1, 0.2, 0.3
a+b == c // false

// ✓ Good — use epsilon comparison
const epsilon = 1e-9
math.Abs((a+b)-c) < epsilon // true

Division by zero

Integer division by zero panics. Float division by zero produces `+Inf`, `-Inf`, or `NaN`.

func avg(total, count int) (int, error) {
    if count == 0 {
        return 0, errors.New("division by zero")
    }
    return total / count, nil
}

For integer overflow as a security vulne