go-errorlint

go-errorlint is a source code linter for Go software that can be used to find code that will cause problems with the error wrapping scheme introduced in Go 1.13.

Error wrapping allows for extra context in errors without sacrificing type information about the error's cause.

For details on Go error wrapping, see: https://golang.org/pkg/errors/

Usage

go-errorlint accepts a set of package names similar to golint:

go-errorlint ./...

If there are one or more results, the exit status is set to 1.

Examples

fmt.Errorf wrapping verb

This lint is disabled by default. Use the -errorf flag to toggle.

// bad
fmt.Errorf("oh noes: %v", err)
// ^ non-wrapping format verb for fmt.Errorf. Use `%w` to format errors

// good
fmt.Errorf("oh noes: %w", err)

Caveats:

• When using the -errorf lint, keep in mind that any errors wrapped by fmt.Errorf implicitly become part of your API as according to Hyrum's Law.

Comparisons of errors

This lint is enabled by default. Use the -comparison flag to toggle.

// bad
err == ErrFoo
// ^ comparing with == will fail on wrapped errors. Use errors.Is to check for a specific error

// bad
switch err {
case ErrFoo:
}
// ^ switch on an error will fail on wrapped errors. Use errors.Is to check for specific errors

// good
errors.Is(err, ErrFoo)

Errors returned from standard library functions that explicitly document that an unwrapped error is returned are allowed by the linter. Notable cases are io.EOF and sql.ErrNoRows.

Caveats:

• Comparing the error returned from (io.Reader).Read to io.EOF without errors.Is is considered valid as this is explicitly documented behaviour. However, nothing stops 3rd party implementations from still wrapping io.EOF, causing this linter to not detect such cases.

Type assertions of errors

This lint is enabled by default. Use the -asserts flag to toggle.

// bad
myErr, ok := err.(*MyError)
// ^ type assertion on error will fail on wrapped errors. Use errors.As to check for specific errors

// bad
switch err.(type) {
case *MyError:
}
// ^ type switch on error will fail on wrapped errors. Use errors.As to check for specific errors

// good
var me MyError
ok := errors.As(err, &me)