errors, fmt: add support for wrapping multiple errors

An error which implements an "Unwrap() []error" method wraps all the
non-nil errors in the returned []error.

We replace the concept of the "error chain" inspected by errors.Is
and errors.As with the "error tree". Is and As perform a pre-order,
depth-first traversal of an error's tree. As returns the first
matching result, if any.

The new errors.Join function returns an error wrapping a list of errors.

The fmt.Errorf function now supports multiple instances of the %w verb.

For #53435.

Change-Id: Ib7402e70b68e28af8f201d2b66bd8e87ccfb5283
Reviewed-on: https://go-review.googlesource.com/c/go/+/432898
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Rob Pike <r@golang.org>
Run-TryBot: Damien Neil <dneil@google.com>
Reviewed-by: Joseph Tsai <joetsai@digital-static.net>

1 files changed, 49 insertions, 0 deletions

diff --git a/src/errors/join_test.go b/src/errors/join_test.go
new file mode 100644
index 0000000000..ee69314529
--- /dev/null
+++ b/src/errors/join_test.go
@@ -0,0 +1,49 @@
+// Copyright 2022 The Go 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 errors_test
+
+import (
+	"errors"
+	"reflect"
+	"testing"
+)
+
+func TestJoinReturnsNil(t *testing.T) {
+	if err := errors.Join(); err != nil {
+		t.Errorf("errors.Join() = %v, want nil", err)
+	}
+	if err := errors.Join(nil); err != nil {
+		t.Errorf("errors.Join(nil) = %v, want nil", err)
+	}
+	if err := errors.Join(nil, nil); err != nil {
+		t.Errorf("errors.Join(nil, nil) = %v, want nil", err)
+	}
+}
+
+func TestJoin(t *testing.T) {
+	err1 := errors.New("err1")
+	err2 := errors.New("err2")
+	for _, test := range []struct {
+		errs []error
+		want []error
+	}{{
+		errs: []error{err1},
+		want: []error{err1},
+	}, {
+		errs: []error{err1, err2},
+		want: []error{err1, err2},
+	}, {
+		errs: []error{err1, nil, err2},
+		want: []error{err1, err2},
+	}} {
+		got := errors.Join(test.errs...).(interface{ Unwrap() []error }).Unwrap()
+		if !reflect.DeepEqual(got, test.want) {
+			t.Errorf("Join(%v) = %v; want %v", test.errs, got, test.want)
+		}
+		if len(got) != cap(got) {
+			t.Errorf("Join(%v) returns errors with len=%v, cap=%v; want len==cap", test.errs, len(got), cap(got))
+		}
+	}
+}