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>

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)
+	}
+}