Go errors: combine multiple errors into one

errors.go

package util

import (
	"errors"
	"strings"
)

type combinedErr struct {
	errs []error
}

func (c combinedErr) Error() string {
	sb := &strings.Builder{}
	for i, err := range c.errs {
		if i > 0 {
			sb.WriteString(": ")
		}
		sb.WriteString(err.Error())
	}
	return sb.String()
}

func (c combinedErr) Unwrap() error {
	if len(c.errs) == 1 {
		return nil
	}
	return combinedErr{errs: c.errs[1:]}
}

func (c combinedErr) Is(target error) bool {
	for _, err := range c.errs {
		if errors.Is(err, target) {
			return true
		}
	}
	return false
}

func (c combinedErr) As(target interface{}) bool {
	for _, err := range c.errs {
		if errors.As(err, target) {
			return true
		}
	}
	return false
}

func CombineErrors(errs ...error) error {
	newErrs := make([]error, 0, len(errs))
	for _, err := range errs {
		if err != nil {
			newErrs = append(newErrs, err)
		}
	}
	if len(newErrs) == 0 {
		return nil
	}
	return combinedErr{errs: newErrs}
}

errors_test.go

package util

import (
	stderrors "errors"
	"fmt"
	"testing"
)

func TestCombineErrors(t *testing.T) {
	err1 := fmt.Errorf("error1")
	err2 := fmt.Errorf("error2")
	err3 := fmt.Errorf("error3")

	err12 := CombineErrors(err1, err2)

	if stderrors.Is(err12, err1) == false {
		t.Errorf("errors.Is(err12, err1) should be true")
	}
	if stderrors.Is(err12, err2) == false {
		t.Errorf("errors.Is(err12, err1) should be true")
	}
	if stderrors.Is(err12, err3) == true {
		t.Errorf("errors.Is(err12, err1) should be false")
	}
}