Support multierr.Every

[kdeberk]

Hello,

There seems to be a correctness issue the implementation of multiError.Is. I say "seems" because I cannot find any consensus on what errors.Is(err, ErrIgnorable) should actually be doing when err is a multiError containing both ErrIgnorable and an important error (that you do want to handle).

Perhaps Is should only return true if all contained errors match the target, so instead

func (merr *multiError) Is(target error) bool {
        if len(merr.Errors()) == 0 {
                return false
        }
	for _, err := range merr.Errors() {
		if !errors.Is(err, target) {
			return false
		}
	}
	return true
}

an alternative would be that Is is not implemented at all, and then errors.Is should return false.

Again, I don't know if the golang developers ever specified some semantics for errors.Is, but I think that from both an intuitive and practical sense, errors.Is(err, ErrIgnorable) should return false. Intuitive, because a multiError is not equal to a single error, and practical because if the important error is not represented by a value, e.g. val ErrImportant = errors.New(...), then there is no way to easily ignore only ignorable error without calling Errors() and iterating over the individual errors..

[tchung1118]

We agree that this would be a good convenience feature to add to multierr, but we're still considering if this should work with errors.Is or should be provided as a separate function from this package.

[abhinav]

FWIW, the standard library's errors.Join matches multierr's behavior.
errors.Is will return true if any of the errors in a errors.Join-ed error matches.
Despite the name, errors.Is is not an equality check but an approximate match.

Perhaps we need an "all" variant of Is (multierr.AllAre or multierr.Every?), where every error in the multierr list gets an errors.Is call individually, but not at the top-level of the multierr.

[r-hang]

Internal ref: GO-1932