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.
}

At best this leads to comments like

// ...
// If this compilation unit has no line table, it returns nil, nil.
func (d *Data) LineReader(cu *Entry) (*LineReader, error) {

which is not a true constraint. And in the worst case such code can lead to panic.

Rewrite the example above with sentinel error:

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

// Use user.

Opposite situation

Sometimes people consider the opposite situation (returning a non-nil error and a valid value at the same time) to be an anti-pattern too, since it can lead to hard-to-find bugs.

For example, this kubernetes code

func (fh *fakeHistory) UpdateControllerRevision(revision *apps.ControllerRevision, newRevision int64) (*apps.ControllerRevision, error) {
    clone := revision.DeepCopy()
    clone.Revision = newRevision
    return clone, fh.indexer.Update(clone)
}

can be rewritten as follows

func (fh *fakeHistory) UpdateControllerRevision(revision *apps.ControllerRevision, newRevision int64) (*apps.ControllerRevision, error) {
    clone := revision.DeepCopy()
    clone.Revision = newRevision
    if err := fh.indexer.Update(clone); err != nil {
        return nil, fmt.Errorf("update index: %w", err)
    }
    return clone, nil
}

Use the --detect-opposite flag to cover such cases too.

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

CLI

$ nilnil --checked-types chan,func,iface,map,ptr,uintptr,unsafeptr ./...
$ nilnil --detect-opposite ./...
$ nilnil --detect-opposite --checked-types ptr ./..
$ nilnil --only-two=false ./...
$ nilnil --detect-opposite --only-two=false --checked-types iface,map ./..

golangci-lint

https://golangci-lint.run/docs/linters/configuration/#nilnil

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

• Linter only checks functions with two return arguments, the last of which implements error. To disable this behavior and enable linter for N returned values, use --only-two=false flag.
• Next types are checked:
• pointers (including uinptr and unsafe.Pointer), functions and 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!)
• Only explicit nils (e.g. return nil, nil or return nil, nil, ..., nil) are supported.

Check Go 1.22.2 source code

Click to expand

$ cd $GOROOT/src
$ nilnil ./... 2>&1 | grep -v "_test.go"  
/usr/local/go/src/internal/bisect/bisect.go:196:3: return both a `nil` error and an invalid value: use a sentinel error instead
/usr/local/go/src/net/fd_unix.go:71:3: return both a `nil` error and an invalid value: use a sentinel error instead
/usr/local/go/src/net/fd_unix.go:79:4: return both a `nil` error and an invalid value: use a sentinel error instead
/usr/local/go/src/net/fd_unix.go:156:4: return both a `nil` error and an invalid value: use a sentinel error instead
/usr/local/go/src/net/iprawsock_posix.go:36:3: return both a `nil` error and an invalid value: use a sentinel error instead
/usr/local/go/src/net/tcpsock_posix.go:38:3: return both a `nil` error and an invalid value: use a sentinel error instead
/usr/local/go/src/net/udpsock_posix.go:37:3: return both a `nil` error and an invalid value: use a sentinel error instead
/usr/local/go/src/net/unixsock_posix.go:92:3: return both a `nil` error and an invalid value: use a sentinel error instead
/usr/local/go/src/crypto/tls/key_agreement.go:46:2: return both a `nil` error and an invalid value: use a sentinel error instead
/usr/local/go/src/crypto/tls/ticket.go:355:3: return both a `nil` error and an invalid value: use a sentinel error instead
/usr/local/go/src/crypto/tls/ticket.go:359:3: return both a `nil` error and an invalid value: use a sentinel error instead
/usr/local/go/src/database/sql/driver/types.go:157:3: return both a `nil` error and an invalid value: use a sentinel error instead
/usr/local/go/src/database/sql/driver/types.go:232:3: return both a `nil` error and an invalid value: use a sentinel error instead
/usr/local/go/src/database/sql/driver/types.go:263:4: return both a `nil` error and an invalid value: use a sentinel error instead
/usr/local/go/src/database/sql/convert.go:548:3: return both a `nil` error and an invalid value: use a sentinel error instead
/usr/local/go/src/database/sql/sql.go:205:3: return both a `nil` error and an invalid value: use a sentinel error instead
/usr/local/go/src/database/sql/sql.go:231:3: return both a `nil` error and an invalid value: use a sentinel error instead
/usr/local/go/src/database/sql/sql.go:257:3: return both a `nil` error and an invalid value: use a sentinel error instead
/usr/local/go/src/database/sql/sql.go:284:3: return both a `nil` error and an invalid value: use a sentinel error instead
/usr/local/go/src/database/sql/sql.go:311:3: return both a `nil` error and an invalid value: use a sentinel error instead
/usr/local/go/src/database/sql/sql.go:337:3: return both a `nil` error and an invalid value: use a sentinel error instead
/usr/local/go/src/database/sql/sql.go:363:3: return both a `nil` error and an invalid value: use a sentinel error instead
/usr/local/go/src/database/sql/sql.go:389:3: return both a `nil` error and an invalid value: use a sentinel error instead
/usr/local/go/src/database/sql/sql.go:422:3: return both a `nil` error and an invalid value: use a sentinel error instead
/usr/local/go/src/debug/dwarf/entry.go:884:3: return both a `nil` error and an invalid value: use a sentinel error instead
/usr/local/go/src/debug/dwarf/line.go:146:3: return both a `nil` error and an invalid value: use a sentinel error instead
/usr/local/go/src/debug/dwarf/line.go:153:3: return both a `nil` error and an invalid value: use a sentinel error instead
/usr/local/go/src/debug/dwarf/typeunit.go:138:3: return both a `nil` error and an invalid value: use a sentinel error instead
/usr/local/go/src/debug/pe/file.go:470:3: return both a `nil` error and an invalid value: use a sentinel error instead
/usr/local/go/src/net/http/h2_bundle.go:9530:3: return both a `nil` error and an invalid value: use a sentinel error instead
/usr/local/go/src/net/http/transfer.go:765:3: return both a `nil` error and an invalid value: use a sentinel error instead
/usr/local/go/src/net/http/transfer.go:775:3: return both a `nil` error and an invalid value: use a sentinel error instead
/usr/local/go/src/net/http/transfer.go:798:3: return both a `nil` error and an invalid value: use a sentinel error instead
/usr/local/go/src/go/build/build.go:1442:3: return both a `nil` error and an invalid value: use a sentinel error instead
/usr/local/go/src/go/build/build.go:1453:3: return both a `nil` error and an invalid value: use a sentinel error instead
/usr/local/go/src/go/build/build.go:1457:3: return both a `nil` error and an invalid value: use a sentinel error instead
/usr/local/go/src/go/build/build.go:1491:3: return both a `nil` error and an invalid value: use a sentinel error instead
/usr/local/go/src/go/internal/gccgoimporter/ar.go:125:3: return both a `nil` error and an invalid value: use a sentinel error instead
/usr/local/go/src/image/jpeg/reader.go:622:5: return both a `nil` error and an invalid value: use a sentinel error instead
/usr/local/go/src/image/png/reader.go:434:4: return both a `nil` error and an invalid value: use a sentinel error instead
/usr/local/go/src/internal/profile/legacy_profile.go:1089:4: return both a `nil` error and an invalid value: use a sentinel error instead
/usr/local/go/src/net/internal/socktest/switch.go:142:3: return both a `nil` error and an invalid value: use a sentinel error instead