Go error handling

Go Error Handling

Go's explicit error handling is a feature, not a limitation. These patterns ensure errors are informative, actionable, and properly propagated.

1. Error Decision Tree

When creating or returning an error, follow this tree:

1. Simple, no extra context needed? → errors.New("message")
2. Need to add context to existing error? → fmt.Errorf("doing X: %w", err)
3. Caller needs to detect this error? → Sentinel var or custom type
4. Error carries structured data? → Custom type implementing error
5. Propagating from downstream? → Wrap with %w and add context

2. Sentinel Errors

Use package-level var for errors that callers need to check:

// ✅ Good — exported sentinel error
var (
    ErrNotFound     = errors.New("user: not found")
    ErrUnauthorized = errors.New("user: unauthorized")
)

// Naming convention: Err + Description
// Prefix with package context in the message

Callers check with errors.Is:

if errors.Is(err, user.ErrNotFound) {
    // handle not found
}

NEVER compare errors with ==. Always use errors.Is().

3. Custom Error Types

When errors need to carry structured information:

type ValidationError struct {
    Field   string
    Message string
}

func (e *ValidationError) Error() string {
    return fmt.Sprintf("validation: field %s: %s", e.Field, e.Message)
}

// Callers extract with errors.As:
var valErr *ValidationError
if errors.As(err, &valErr) {
    log.Printf("invalid field: %s", valErr.Field)
}

4. Error Wrapping

ALWAYS add context when propagating errors up the stack. Use %w to preserve the error chain:

// ✅ Good — context added, chain preserved
func getUser(id int64) (*User, error) {
    row, err := db.QueryRow(ctx, query, id)
    if err != nil {
        return nil, fmt.Errorf("get user %d: %w", id, err)
    }
    // ...
}

// ❌ Bad — no context
return nil, err

// ❌ Bad — chain broken, callers can't errors.Is/As
return nil, fmt.Errorf("failed: %v", err)

When NOT to use %w

Use %v instead of %w when you explicitly want to break the error chain, preventing callers from depending on internal implementation errors:

// Intentionally hiding internal DB error from public API
return nil, fmt.Errorf("user lookup failed: %v", err)

5. Handle Errors Exactly Once

An error should be either logged OR returned, never both:

// ✅ Good — return the error, let caller decide
func loadConfig(path string) (*Config, error) {
    data, err := os.ReadFile(path)
    if err != nil {
        return nil, fmt.Errorf("load config %s: %w", path, err)
    }
    // ...
}

// ❌ Bad — log AND return (error handled twice)
func loadConfig(path string) (*Config, error) {
    data, err := os.ReadFile(path)
    if err != nil {
        log.Printf("failed to read config: %v", err) // handled once
        return nil, err                                 // handled again
    }
    // ...
}

The rule: the component that decides what to do about the error is the one that logs/metrics it. Everyone else wraps and returns.

6. Error Naming Conventions

// Sentinel errors: Err prefix
var ErrNotFound = errors.New("not found")

// Error types: Error suffix
type NotFoundError struct { ... }
type ValidationError struct { ... }

// Error messages: lowercase, no punctuation, no "failed to" prefix
// Include context: "package: action: detail"
errors.New("auth: token expired")
fmt.Errorf("user: get by id %d: %w", id, err)

7. Panic Rules

Panic is NOT error handling. Use panic only when:

• Program initialization fails and cannot continue (template.Must, flag parsing)
• Programmer error that should never happen (violated invariant)
• Nil dereference that indicates a bug, not a runtime condition

In tests, use t.Fatal / t.FailNow, never panic.

In HTTP handlers and middleware, recover from panics at the boundary to prevent one request from crashing the server.

8. Error Checking Patterns

// Inline error check — preferred for simple cases
if err := doSomething(); err != nil {
    return fmt.Errorf("do something: %w", err)
}

// Multi-return with named result — acceptable for complex functions
func process() (result string, err error) {
    defer func() {
        if err != nil {
            err = fmt.Errorf("process: %w", err)
        }
    }()
    // ...
}

// errors.Join for multiple errors (Go 1.20+)
var errs []error
for _, item := range items {
    if err := validate(item); err != nil {
        errs = append(errs, err)
    }
}
return errors.Join(errs...)

Verification Checklist

1. No _ discarding errors (unless explicitly justified with comment)
2. Every fmt.Errorf wrapping uses %w (or %v with documented reason)
3. Sentinel errors use var Err... naming
4. Custom error types implement error interface
5. Callers use errors.Is / errors.As, never == or type assertion
6. No log-and-return patterns
7. Error messages are lowercase, contextual, chain-friendly