util/multierr: add Error.Unwrap and Range

Have Error implement the new interface for Unwrap coming in Go 1.20,
where Unwrap can either return an error or an []error.
See golang/go#53435.

Errors in Go are no longer viewed as a linear chain, but a tree.

Also add a Range function that iterates through an error
in a pre-order, depth-first order.
This matches the iteration order of errors.As in Go 1.20.

Signed-off-by: Joe Tsai <joetsai@digital-static.net>

util/multierr/multierr.go

@@ -9,6 +9,8 @@ package multierr
 import (
 	"errors"
 	"strings"
+
+	"golang.org/x/exp/slices"
 )
 
 // An Error represents multiple errors.
@@ -29,7 +31,14 @@ func (e Error) Error() string {
 
 // Errors returns a slice containing all errors in e.
 func (e Error) Errors() []error {
-	return append(e.errs[:0:0], e.errs...)
+	return slices.Clone(e.errs)
+}
+
+// Unwrap returns the underlying errors as is.
+func (e Error) Unwrap() []error {
+	// Do not clone since Unwrap requires callers to not mutate the slice.
+	// See the documentation in the Go "errors" package.
+	return e.errs
 }
 
 // New returns an error composed from errs.
@@ -87,3 +96,31 @@ func (e Error) As(target any) bool {
 	}
 	return false
 }
+
+// Range performs a pre-order, depth-first iteration of the error tree
+// by successively unwrapping all error values.
+// For each iteration it calls fn with the current error value and
+// stops iteration if it ever reports false.
+func Range(err error, fn func(error) bool) bool {
+	if err == nil {
+		return true
+	}
+	if !fn(err) {
+		return false
+	}
+	switch err := err.(type) {
+	case interface{ Unwrap() error }:
+		if err := err.Unwrap(); err != nil {
+			if !Range(err, fn) {
+				return false
+			}
+		}
+	case interface{ Unwrap() []error }:
+		for _, err := range err.Unwrap() {
+			if !Range(err, fn) {
+				return false
+			}
+		}
+	}
+	return true
+}

util/multierr/multierr_test.go

@@ -6,9 +6,11 @@ package multierr_test
 
 import (
 	"errors"
+	"fmt"
 	"testing"
 
 	qt "github.com/frankban/quicktest"
+	"github.com/google/go-cmp/cmp"
 	"github.com/google/go-cmp/cmp/cmpopts"
 	"tailscale.com/util/multierr"
 )
@@ -78,3 +80,29 @@ func TestAll(t *testing.T) {
 		C.Assert(ee.Is(x), qt.IsFalse)
 	}
 }
+
+func TestRange(t *testing.T) {
+	C := qt.New(t)
+
+	errA := errors.New("A")
+	errB := errors.New("B")
+	errC := errors.New("C")
+	errD := errors.New("D")
+	errCD := multierr.New(errC, errD)
+	errCD1 := fmt.Errorf("1:%w", errCD)
+	errE := errors.New("E")
+	errE1 := fmt.Errorf("1:%w", errE)
+	errE2 := fmt.Errorf("2:%w", errE1)
+	errF := errors.New("F")
+	root := multierr.New(errA, errB, errCD1, errE2, errF)
+
+	var got []error
+	want := []error{root, errA, errB, errCD1, errCD, errC, errD, errE2, errE1, errE, errF}
+	multierr.Range(root, func(err error) bool {
+		got = append(got, err)
+		return true
+	})
+	C.Assert(got, qt.CmpEquals(cmp.Comparer(func(x, y error) bool {
+		return x.Error() == y.Error()
+	})), want)
+}