nilnil

Checks that there is no simultaneous return of nil error and an invalid value.

Installation & usage

$ go install github.com/Antonboom/nilnil@latest
$ nilnil ./...

Motivation

return nil, nil is not idiomatic for Go. The developers are used to the fact that if there is no error, then the return value is valid and can be used without additional checks:

user, err := getUser()
if err != nil {
    return err
}
if user != nil { // Ambiguous!
    // Use user.
}

In the worst case, code like this can lead to panic.

Rewrite the example for sentinel error:

user, err := getUser()
if errors.Is(err, errUserNotFound) {
    // Do something and return.
}
if err != nil {
    return err
}

// Use user.

What if I think it's bullshit?

I understand that each case needs to be analyzed separately, but I hope that the linter will make you think again - is it necessary to use an ambiguous API or is it better to do it using a sentinel error?

In any case, you can just not enable the linter.

Configuration

# command line (see help for full list of types)
$ nilnil --checked-types ptr,func ./...

# https://golangci-lint.run/usage/configuration/
nilnil:
  checked-types:
    - ptr
    - func
    - iface
    - map
    - chan

Examples

parsePublicKey from crypto/tls

// BEFORE

func parsePublicKey(algo PublicKeyAlgorithm, keyData *publicKeyInfo) (interface{}, error) {
    der := cryptobyte.String(keyData.PublicKey.RightAlign())
    switch algo {
    case RSA:
        // ...
        return pub, nil
    case ECDSA:
        // ...
        return pub, nil
    case Ed25519:
        // ...
        return ed25519.PublicKey(der), nil
    case DSA:
        // ...
        return pub, nil
    default:
        return nil, nil
    }
}

// AFTER

var errUnknownPublicKeyAlgo = errors.New("unknown public key algo")

func parsePublicKey(algo PublicKeyAlgorithm, keyData *publicKeyInfo) (interface{}, error) {
    der := cryptobyte.String(keyData.PublicKey.RightAlign())
    switch algo {
    case RSA:
        // ...
        return pub, nil
    case ECDSA:
        // ...
        return pub, nil
    case Ed25519:
        // ...
        return ed25519.PublicKey(der), nil
    case DSA:
        // ...
        return pub, nil
    default:
        return nil, fmt.Errorf("%w: %v", errUnknownPublicKeyAlgo, algo)
    }
}

http2clientConnReadLoop from net/http

// BEFORE

// As a special case, handleResponse may return (nil, nil) to skip the frame.
func (rl *http2clientConnReadLoop) handleResponse(/* ... */) (*Response, error) {
    if statusCode >= 100 && statusCode <= 199 {
        return nil, nil
    }
}

// ...
res, err := rl.handleResponse(cs, f)
if err != nil {
	return err
}
if res == nil {
    // (nil, nil) special case. See handleResponse docs.
    return nil
}

// AFTER

var errNeedSkipFrame = errors.New("need skip frame")

// As a special case, handleResponse may return errNeedSkipFrame to skip the frame.
func (rl *http2clientConnReadLoop) handleResponse(/* ... */) (*Response, error) {
    if statusCode >= 100 && statusCode <= 199 {
        return nil, errNeedSkipFrame
    }
}

// ...
res, err := rl.handleResponse(cs, f)
if errors.Is(err, errNeedSkipFrame) {
    return nil
}
if err != nil {
    return err
}

Not implemented

// BEFORE

func (s *Service) StartStream(ctx context.Context) (*Stream, error) {
    return nil, nil
}

// AFTER

func (s *Service) StartStream(ctx context.Context) (*Stream, error) {
    return nil, errors.New("not implemented")
}

nil-safe type

package ratelimiter

type RateLimiter struct {
    // ...
}

func New() (*RateLimiter, error) {
    // It's OK, RateLimiter is nil-safe.
    // But it's better not to do it anyway.
    return nil, nil
}

func (r *RateLimiter) Allow() bool {
    if r == nil {
        return true
    }
    return r.allow()
}

Assumptions

Click to expand

• Linter only checks funcs with two return arguments, the last of which has error type.
• Next types are checked:
  • pointers, functions & interfaces (panic: invalid memory address or nil pointer dereference);
  • maps (panic: assignment to entry in nil map);
  • channels (fatal error: all goroutines are asleep - deadlock!)
• uinptr & unsafe.Pointer are not checked as a special case.
• Supported only explicit return nil, nil.
• Types from external packages are not supported.

Check Golang source code

Click to expand

$ cd $GOROOT/src
$ nilnil ./...
/usr/local/go/src/net/sockopt_posix.go:48:3: return both the `nil` error and invalid value: use a sentinel error instead
/usr/local/go/src/crypto/x509/parser.go:321:3: return both the `nil` error and invalid value: use a sentinel error instead
/usr/local/go/src/crypto/tls/key_agreement.go:45:2: return both the `nil` error and invalid value: use a sentinel error instead
/usr/local/go/src/database/sql/driver/types.go:157:3: return both the `nil` error and invalid value: use a sentinel error instead
/usr/local/go/src/database/sql/driver/types.go:231:3: return both the `nil` error and invalid value: use a sentinel error instead
/usr/local/go/src/database/sql/driver/types.go:262:4: return both the `nil` error and invalid value: use a sentinel error instead
/usr/local/go/src/debug/dwarf/entry.go:882:3: return both the `nil` error and invalid value: use a sentinel error instead
/usr/local/go/src/debug/dwarf/line.go:146:3: return both the `nil` error and invalid value: use a sentinel error instead
/usr/local/go/src/debug/dwarf/line.go:153:3: return both the `nil` error and invalid value: use a sentinel error instead
/usr/local/go/src/debug/dwarf/typeunit.go:138:3: return both the `nil` error and invalid value: use a sentinel error instead
/usr/local/go/src/debug/pe/file.go:450:3: return both the `nil` error and invalid value: use a sentinel error instead
/usr/local/go/src/net/http/h2_bundle.go:8644:3: return both the `nil` error and invalid value: use a sentinel error instead
/usr/local/go/src/net/http/transfer.go:768:3: return both the `nil` error and invalid value: use a sentinel error instead
/usr/local/go/src/net/http/transfer.go:778:3: return both the `nil` error and invalid value: use a sentinel error instead
/usr/local/go/src/net/http/transfer.go:801:3: return both the `nil` error and invalid value: use a sentinel error instead
/usr/local/go/src/go/build/build.go:1404:3: return both the `nil` error and invalid value: use a sentinel error instead
/usr/local/go/src/go/build/build.go:1414:3: return both the `nil` error and invalid value: use a sentinel error instead
/usr/local/go/src/go/build/build.go:1419:3: return both the `nil` error and invalid value: use a sentinel error instead
/usr/local/go/src/go/build/build.go:1453:3: return both the `nil` error and invalid value: use a sentinel error instead
/usr/local/go/src/internal/profile/legacy_profile.go:1087:4: return both the `nil` error and invalid value: use a sentinel error instead
/usr/local/go/src/net/internal/socktest/switch.go:142:3: return both the `nil` error and invalid value: use a sentinel error instead
/usr/local/go/src/crypto/tls/handshake_server_test.go:411:4: return both the `nil` error and invalid value: use a sentinel error instead
/usr/local/go/src/crypto/tls/handshake_server_test.go:1012:3: return both the `nil` error and invalid value: use a sentinel error instead
/usr/local/go/src/crypto/tls/handshake_server_test.go:1470:4: return both the `nil` error and invalid value: use a sentinel error instead
/usr/local/go/src/crypto/tls/tls_test.go:747:4: return both the `nil` error and invalid value: use a sentinel error instead
/usr/local/go/src/crypto/tls/tls_test.go:751:4: return both the `nil` error and invalid value: use a sentinel error instead
/usr/local/go/src/crypto/tls/tls_test.go:755:4: return both the `nil` error and invalid value: use a sentinel error instead
/usr/local/go/src/encoding/xml/xml_test.go:92:4: return both the `nil` error and invalid value: use a sentinel error instead