util/multierr: new package

github.com/go-multierror/multierror served us well.
But we need a few feature from it (implement Is),
and it's not worth maintaining a fork of such a small module.

Instead, I did a clean room implementation inspired by its API.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

cmd/tailscaled/depaware.txt

@@ -59,7 +59,6 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
    L    github.com/aws/smithy-go/transport/http/internal/io          from github.com/aws/smithy-go/transport/http
    L    github.com/aws/smithy-go/waiter                              from github.com/aws/aws-sdk-go-v2/service/ssm
    L    github.com/coreos/go-iptables/iptables                       from tailscale.com/wgengine/router
-        github.com/go-multierror/multierror                          from tailscale.com/cmd/tailscaled+
    W 💣 github.com/go-ole/go-ole                                     from github.com/go-ole/go-ole/oleutil+
    W 💣 github.com/go-ole/go-ole/oleutil                             from tailscale.com/wgengine/winnet
    L 💣 github.com/godbus/dbus/v5                                    from tailscale.com/net/dns
@@ -228,6 +227,7 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
   LW    tailscale.com/util/endian                                    from tailscale.com/net/dns+
         tailscale.com/util/groupmember                               from tailscale.com/ipn/ipnserver
         tailscale.com/util/lineread                                  from tailscale.com/hostinfo+
+        tailscale.com/util/multierr                                  from tailscale.com/cmd/tailscaled+
         tailscale.com/util/osshare                                   from tailscale.com/cmd/tailscaled+
         tailscale.com/util/pidowner                                  from tailscale.com/ipn/ipnserver
         tailscale.com/util/racebuild                                 from tailscale.com/logpolicy

cmd/tailscaled/tailscaled.go

@@ -27,7 +27,6 @@
 	"syscall"
 	"time"
 
-	"github.com/go-multierror/multierror"
 	"tailscale.com/ipn"
 	"tailscale.com/ipn/ipnserver"
 	"tailscale.com/logpolicy"
@@ -38,6 +37,7 @@
 	"tailscale.com/paths"
 	"tailscale.com/types/flagtype"
 	"tailscale.com/types/logger"
+	"tailscale.com/util/multierr"
 	"tailscale.com/util/osshare"
 	"tailscale.com/version"
 	"tailscale.com/version/distro"
@@ -361,7 +361,7 @@ func createEngine(logf logger.Logf, linkMon *monitor.Mon) (e wgengine.Engine, us
 		logf("wgengine.NewUserspaceEngine(tun %q) error: %v", name, err)
 		errs = append(errs, err)
 	}
-	return nil, false, multierror.New(errs)
+	return nil, false, multierr.New(errs...)
 }
 
 var wrapNetstack = shouldWrapNetstack()

go.mod

@@ -15,7 +15,6 @@ require (
 	github.com/dave/jennifer v1.4.1
 	github.com/frankban/quicktest v1.13.1
 	github.com/gliderlabs/ssh v0.3.3
-	github.com/go-multierror/multierror v1.0.2
 	github.com/go-ole/go-ole v1.2.6-0.20210915003542-8b1f7f90f6b1
 	github.com/godbus/dbus/v5 v5.0.5
 	github.com/google/go-cmp v0.5.6

health/health.go

@@ -16,8 +16,8 @@
 	"sync/atomic"
 	"time"
 
-	"github.com/go-multierror/multierror"
 	"tailscale.com/tailcfg"
+	"tailscale.com/util/multierr"
 )
 
 var (
@@ -268,7 +268,7 @@ func selfCheckLocked() {
 // OverallError returns a summary of the health state.
 //
 // If there are multiple problems, the error will be of type
-// multierror.MultipleErrors.
+// multierr.Error.
 func OverallError() error {
 	mu.Lock()
 	defer mu.Unlock()
@@ -337,7 +337,7 @@ func overallErrorLocked() error {
 		// Not super efficient (stringifying these in a sort), but probably max 2 or 3 items.
 		return errs[i].Error() < errs[j].Error()
 	})
-	return multierror.New(errs)
+	return multierr.New(errs...)
 }
 
 var (

ipn/ipnlocal/local.go

@@ -25,7 +25,6 @@
 	"syscall"
 	"time"
 
-	"github.com/go-multierror/multierror"
 	"inet.af/netaddr"
 	"tailscale.com/client/tailscale/apitype"
 	"tailscale.com/control/controlclient"
@@ -49,6 +48,7 @@
 	"tailscale.com/types/preftype"
 	"tailscale.com/util/deephash"
 	"tailscale.com/util/dnsname"
+	"tailscale.com/util/multierr"
 	"tailscale.com/util/osshare"
 	"tailscale.com/util/systemd"
 	"tailscale.com/version"
@@ -334,8 +334,8 @@ func (b *LocalBackend) updateStatus(sb *ipnstate.StatusBuilder, extraLocked func
 
 		if err := health.OverallError(); err != nil {
 			switch e := err.(type) {
-			case multierror.MultipleErrors:
-				for _, err := range e {
+			case multierr.Error:
+				for _, err := range e.Errors() {
 					s.Health = append(s.Health, err.Error())
 				}
 			default:

tstest/integration/tailscaled_deps_test_darwin.go

@@ -17,7 +17,6 @@
 	_ "errors"
 	_ "flag"
 	_ "fmt"
-	_ "github.com/go-multierror/multierror"
 	_ "inet.af/netaddr"
 	_ "io"
 	_ "io/ioutil"
@@ -54,6 +53,7 @@
 	_ "tailscale.com/types/flagtype"
 	_ "tailscale.com/types/key"
 	_ "tailscale.com/types/logger"
+	_ "tailscale.com/util/multierr"
 	_ "tailscale.com/util/osshare"
 	_ "tailscale.com/version"
 	_ "tailscale.com/version/distro"

tstest/integration/tailscaled_deps_test_freebsd.go

@@ -17,7 +17,6 @@
 	_ "errors"
 	_ "flag"
 	_ "fmt"
-	_ "github.com/go-multierror/multierror"
 	_ "inet.af/netaddr"
 	_ "io"
 	_ "io/ioutil"
@@ -52,6 +51,7 @@
 	_ "tailscale.com/types/flagtype"
 	_ "tailscale.com/types/key"
 	_ "tailscale.com/types/logger"
+	_ "tailscale.com/util/multierr"
 	_ "tailscale.com/util/osshare"
 	_ "tailscale.com/version"
 	_ "tailscale.com/version/distro"

tstest/integration/tailscaled_deps_test_linux.go

@@ -17,7 +17,6 @@
 	_ "errors"
 	_ "flag"
 	_ "fmt"
-	_ "github.com/go-multierror/multierror"
 	_ "inet.af/netaddr"
 	_ "io"
 	_ "io/ioutil"
@@ -52,6 +51,7 @@
 	_ "tailscale.com/types/flagtype"
 	_ "tailscale.com/types/key"
 	_ "tailscale.com/types/logger"
+	_ "tailscale.com/util/multierr"
 	_ "tailscale.com/util/osshare"
 	_ "tailscale.com/version"
 	_ "tailscale.com/version/distro"

tstest/integration/tailscaled_deps_test_openbsd.go

@@ -17,7 +17,6 @@
 	_ "errors"
 	_ "flag"
 	_ "fmt"
-	_ "github.com/go-multierror/multierror"
 	_ "inet.af/netaddr"
 	_ "io"
 	_ "io/ioutil"
@@ -52,6 +51,7 @@
 	_ "tailscale.com/types/flagtype"
 	_ "tailscale.com/types/key"
 	_ "tailscale.com/types/logger"
+	_ "tailscale.com/util/multierr"
 	_ "tailscale.com/util/osshare"
 	_ "tailscale.com/version"
 	_ "tailscale.com/version/distro"

tstest/integration/tailscaled_deps_test_windows.go

@@ -17,7 +17,6 @@
 	_ "errors"
 	_ "flag"
 	_ "fmt"
-	_ "github.com/go-multierror/multierror"
 	_ "golang.org/x/sys/windows"
 	_ "golang.org/x/sys/windows/svc"
 	_ "golang.org/x/sys/windows/svc/mgr"
@@ -56,6 +55,7 @@
 	_ "tailscale.com/types/flagtype"
 	_ "tailscale.com/types/key"
 	_ "tailscale.com/types/logger"
+	_ "tailscale.com/util/multierr"
 	_ "tailscale.com/util/osshare"
 	_ "tailscale.com/util/winutil"
 	_ "tailscale.com/version"

util/multierr/multierr.go

@@ -0,0 +1,88 @@
+// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package multierr provides a simple multiple-error type.
+// It was inspired by github.com/go-multierror/multierror.
+package multierr
+
+import (
+	"errors"
+	"strings"
+)
+
+// An Error represents multiple errors.
+type Error struct {
+	errs []error
+}
+
+// Error implements the error interface.
+func (e Error) Error() string {
+	s := new(strings.Builder)
+	s.WriteString("multiple errors:")
+	for _, err := range e.errs {
+		s.WriteString("\n\t")
+		s.WriteString(err.Error())
+	}
+	return s.String()
+}
+
+// Errors returns a slice containing all errors in e.
+func (e Error) Errors() []error {
+	return append(e.errs[:0:0], e.errs...)
+}
+
+// New returns an error composed from errs.
+// Some errors in errs get special treatment:
+//   * nil errors are discarded
+//   * errors of type Error are expanded into the top level
+// If the resulting slice has length 0, New returns nil.
+// If the resulting slice has length 1, New returns that error.
+// If the resulting slice has length > 1, New returns that slice as an Error.
+func New(errs ...error) error {
+	dst := make([]error, 0, len(errs))
+	for _, e := range errs {
+		switch e := e.(type) {
+		case nil:
+			continue
+		case Error:
+			dst = append(dst, e.errs...)
+		default:
+			dst = append(dst, e)
+		}
+	}
+	// dst has been filtered and splatted.
+	switch len(dst) {
+	case 0:
+		return nil
+	case 1:
+		return dst[0]
+	}
+	// Zero any remaining elements of dst left over from errs, for GC.
+	tail := dst[len(dst):cap(dst)]
+	for i := range tail {
+		tail[i] = nil
+	}
+	return Error{errs: dst}
+}
+
+// Is reports whether any error in e matches target.
+func (e Error) Is(target error) bool {
+	for _, err := range e.errs {
+		if errors.Is(err, target) {
+			return true
+		}
+	}
+	return false
+}
+
+// As finds the first error in e that matches target, and if any is found,
+// sets target to that error value and returns true. Otherwise, it returns false.
+func (e Error) As(target interface{}) bool {
+	for _, err := range e.errs {
+		if ok := errors.As(err, target); ok {
+			return true
+		}
+	}
+	return false
+}

util/multierr/multierr_test.go

@@ -0,0 +1,80 @@
+// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package multierr_test
+
+import (
+	"errors"
+	"testing"
+
+	qt "github.com/frankban/quicktest"
+	"github.com/google/go-cmp/cmp/cmpopts"
+	"tailscale.com/util/multierr"
+)
+
+func TestAll(t *testing.T) {
+	C := qt.New(t)
+	eqErr := qt.CmpEquals(cmpopts.EquateErrors())
+
+	type E = []error
+	N := multierr.New
+
+	a := errors.New("a")
+	b := errors.New("b")
+	c := errors.New("c")
+	d := errors.New("d")
+	x := errors.New("x")
+	abcd := E{a, b, c, d}
+
+	tests := []struct {
+		In         E       // input to New
+		WantNil    bool    // want nil returned?
+		WantSingle error   // if non-nil, want this single error returned
+		WantErrors []error // if non-nil, want an Error composed of these errors returned
+	}{
+		{In: nil, WantNil: true},
+
+		{In: E{nil}, WantNil: true},
+		{In: E{nil, nil}, WantNil: true},
+
+		{In: E{a}, WantSingle: a},
+		{In: E{a, nil}, WantSingle: a},
+		{In: E{nil, a}, WantSingle: a},
+		{In: E{nil, a, nil}, WantSingle: a},
+
+		{In: E{a, b}, WantErrors: E{a, b}},
+		{In: E{nil, a, nil, b, nil}, WantErrors: E{a, b}},
+
+		{In: E{a, b, N(c, d)}, WantErrors: E{a, b, c, d}},
+		{In: E{a, N(b, c), d}, WantErrors: E{a, b, c, d}},
+		{In: E{N(a, b), c, d}, WantErrors: E{a, b, c, d}},
+		{In: E{N(a, b), N(c, d)}, WantErrors: E{a, b, c, d}},
+		{In: E{nil, N(a, nil, b), nil, N(c, d)}, WantErrors: E{a, b, c, d}},
+
+		{In: E{N(a, N(b, N(c, N(d))))}, WantErrors: E{a, b, c, d}},
+		{In: E{N(N(N(N(a), b), c), d)}, WantErrors: E{a, b, c, d}},
+
+		{In: E{N(abcd...)}, WantErrors: E{a, b, c, d}},
+		{In: E{N(abcd...), N(abcd...)}, WantErrors: E{a, b, c, d, a, b, c, d}},
+	}
+
+	for _, test := range tests {
+		got := multierr.New(test.In...)
+		if test.WantNil {
+			C.Assert(got, qt.IsNil)
+			continue
+		}
+		if test.WantSingle != nil {
+			C.Assert(got, eqErr, test.WantSingle)
+			continue
+		}
+		ee, _ := got.(multierr.Error)
+		C.Assert(ee.Errors(), eqErr, test.WantErrors)
+
+		for _, e := range test.WantErrors {
+			C.Assert(ee.Is(e), qt.IsTrue)
+		}
+		C.Assert(ee.Is(x), qt.IsFalse)
+	}
+}

wgengine/router/ifconfig_windows.go

@@ -15,7 +15,6 @@
 	"sort"
 	"time"
 
-	"github.com/go-multierror/multierror"
 	ole "github.com/go-ole/go-ole"
 	"golang.org/x/sys/windows"
 	"golang.zx2c4.com/wireguard/tun"
@@ -24,6 +23,7 @@
 	"tailscale.com/health"
 	"tailscale.com/net/interfaces"
 	"tailscale.com/net/tsaddr"
+	"tailscale.com/util/multierr"
 	"tailscale.com/wgengine/winnet"
 )
 
@@ -809,5 +809,5 @@ func syncRoutes(ifc *winipcfg.IPAdapterAddresses, want []*winipcfg.RouteData, do
 		}
 	}
 
-	return multierror.New(errs)
+	return multierr.New(errs...)
 }

wgengine/router/router_linux.go

@@ -17,7 +17,6 @@
 	"time"
 
 	"github.com/coreos/go-iptables/iptables"
-	"github.com/go-multierror/multierror"
 	"github.com/tailscale/netlink"
 	"golang.org/x/sys/unix"
 	"golang.org/x/time/rate"
@@ -27,6 +26,7 @@
 	"tailscale.com/syncs"
 	"tailscale.com/types/logger"
 	"tailscale.com/types/preftype"
+	"tailscale.com/util/multierr"
 	"tailscale.com/version/distro"
 	"tailscale.com/wgengine/monitor"
 )
@@ -320,7 +320,7 @@ func (r *linuxRouter) Set(cfg *Config) error {
 	}
 	r.snatSubnetRoutes = cfg.SNATSubnetRoutes
 
-	return multierror.New(errs)
+	return multierr.New(errs...)
 }
 
 // setNetfilterMode switches the router to the given netfilter