errors: add Unwrap, Is, and As

Unwrap, Is and As are as defined in proposal
Issue #29934.

Also add Opaque for enforcing an error cannot
be unwrapped.

Change-Id: I4f3feaa42e3ee7477b588164ac622ba4d5e77cad
Reviewed-on: https://go-review.googlesource.com/c/163558
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>

3 files changed, 379 insertions, 0 deletions

diff --git a/src/errors/example_test.go b/src/errors/example_test.go
index 5dc8841237..7724c16cdf 100644
--- a/src/errors/example_test.go
+++ b/src/errors/example_test.go
@@ -5,7 +5,9 @@
 package errors_test
 
 import (
+	"errors"
 	"fmt"
+	"os"
 	"time"
 )
 
@@ -32,3 +34,16 @@ func Example() {
 	}
 	// Output: 1989-03-15 22:30:00 +0000 UTC: the file system has gone away
 }
+
+func ExampleAs() {
+	_, err := os.Open("non-existing")
+	if err != nil {
+		var pathError *os.PathError
+		if errors.As(err, &pathError) {
+			fmt.Println("Failed at path:", pathError.Path)
+		}
+	}
+
+	// Output:
+	// Failed at path: non-existing
+}
diff --git a/src/errors/wrap.go b/src/errors/wrap.go
new file mode 100644
index 0000000000..fc7bf71f8a
--- /dev/null
+++ b/src/errors/wrap.go
@@ -0,0 +1,106 @@
+// Copyright 2018 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
+
+import (
+	"internal/reflectlite"
+)
+
+// A Wrapper provides context around another error.
+type Wrapper interface {
+	// Unwrap returns the next error in the error chain.
+	// If there is no next error, Unwrap returns nil.
+	Unwrap() error
+}
+
+// Opaque returns an error with the same error formatting as err
+// but that does not match err and cannot be unwrapped.
+func Opaque(err error) error {
+	return noWrapper{err}
+}
+
+type noWrapper struct {
+	error
+}
+
+func (e noWrapper) FormatError(p Printer) (next error) {
+	if f, ok := e.error.(Formatter); ok {
+		return f.FormatError(p)
+	}
+	p.Print(e.error)
+	return nil
+}
+
+// Unwrap returns the result of calling the Unwrap method on err, if err
+// implements Unwrap. Otherwise, Unwrap returns nil.
+func Unwrap(err error) error {
+	u, ok := err.(Wrapper)
+	if !ok {
+		return nil
+	}
+	return u.Unwrap()
+}
+
+// Is reports whether any error in err's chain matches target.
+//
+// An error is considered to match a target if it is equal to that target or if
+// it implements a method Is(error) bool such that Is(target) returns true.
+func Is(err, target error) bool {
+	if target == nil {
+		return err == target
+	}
+	for {
+		if err == target {
+			return true
+		}
+		if x, ok := err.(interface{ Is(error) bool }); ok && x.Is(target) {
+			return true
+		}
+		// TODO: consider supporing target.Is(err). This would allow
+		// user-definable predicates, but also may allow for coping with sloppy
+		// APIs, thereby making it easier to get away with them.
+		if err = Unwrap(err); err == nil {
+			return false
+		}
+	}
+}
+
+// As finds the first error in err's chain that matches the type to which target
+// points, and if so, sets the target to its value and returns true. An error
+// matches a type if it is assignable to the target type, or if it has a method
+// As(interface{}) bool such that As(target) returns true. As will panic if
+// target is not a non-nil pointer to a type which implements error or is of
+// interface type.
+//
+// The As method should set the target to its value and return true if err
+// matches the type to which target points.
+func As(err error, target interface{}) bool {
+	if target == nil {
+		panic("errors: target cannot be nil")
+	}
+	val := reflectlite.ValueOf(target)
+	typ := val.Type()
+	if typ.Kind() != reflectlite.Ptr || val.IsNil() {
+		panic("errors: target must be a non-nil pointer")
+	}
+	if e := typ.Elem(); e.Kind() != reflectlite.Interface && !e.Implements(errorType) {
+		panic("errors: *target must be interface or implement error")
+	}
+	targetType := typ.Elem()
+	for {
+		if reflectlite.TypeOf(err).AssignableTo(targetType) {
+			val.Elem().Set(reflectlite.ValueOf(err))
+			return true
+		}
+		if x, ok := err.(interface{ As(interface{}) bool }); ok && x.As(target) {
+			return true
+		}
+		if err = Unwrap(err); err == nil {
+			return false
+		}
+	}
+}
+
+var errorType = reflectlite.TypeOf((*error)(nil)).Elem()
diff --git a/src/errors/wrap_test.go b/src/errors/wrap_test.go
new file mode 100644
index 0000000000..657890c1a6
--- /dev/null
+++ b/src/errors/wrap_test.go
@@ -0,0 +1,258 @@
+// Copyright 2018 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 (
+	"bytes"
+	"errors"
+	"fmt"
+	"os"
+	"testing"
+)
+
+func TestIs(t *testing.T) {
+	err1 := errors.New("1")
+	erra := wrapped{"wrap 2", err1}
+	errb := wrapped{"wrap 3", erra}
+	erro := errors.Opaque(err1)
+	errco := wrapped{"opaque", erro}
+
+	err3 := errors.New("3")
+
+	poser := &poser{"either 1 or 3", func(err error) bool {
+		return err == err1 || err == err3
+	}}
+
+	testCases := []struct {
+		err    error
+		target error
+		match  bool
+	}{
+		{nil, nil, true},
+		{err1, nil, false},
+		{err1, err1, true},
+		{erra, err1, true},
+		{errb, err1, true},
+		{errco, erro, true},
+		{errco, err1, false},
+		{erro, erro, true},
+		{err1, err3, false},
+		{erra, err3, false},
+		{errb, err3, false},
+		{poser, err1, true},
+		{poser, err3, true},
+		{poser, erra, false},
+		{poser, errb, false},
+		{poser, erro, false},
+		{poser, errco, false},
+	}
+	for _, tc := range testCases {
+		t.Run("", func(t *testing.T) {
+			if got := errors.Is(tc.err, tc.target); got != tc.match {
+				t.Errorf("Is(%v, %v) = %v, want %v", tc.err, tc.target, got, tc.match)
+			}
+		})
+	}
+}
+
+type poser struct {
+	msg string
+	f   func(error) bool
+}
+
+func (p *poser) Error() string     { return p.msg }
+func (p *poser) Is(err error) bool { return p.f(err) }
+func (p *poser) As(err interface{}) bool {
+	switch x := err.(type) {
+	case **poser:
+		*x = p
+	case *errorT:
+		*x = errorT{}
+	case **os.PathError:
+		*x = &os.PathError{}
+	default:
+		return false
+	}
+	return true
+}
+
+func TestAs(t *testing.T) {
+	var errT errorT
+	var errP *os.PathError
+	var timeout interface{ Timeout() bool }
+	var p *poser
+	_, errF := os.Open("non-existing")
+
+	testCases := []struct {
+		err    error
+		target interface{}
+		match  bool
+	}{{
+		wrapped{"pittied the fool", errorT{}},
+		&errT,
+		true,
+	}, {
+		errF,
+		&errP,
+		true,
+	}, {
+		errors.Opaque(errT),
+		&errT,
+		false,
+	}, {
+		errorT{},
+		&errP,
+		false,
+	}, {
+		wrapped{"wrapped", nil},
+		&errT,
+		false,
+	}, {
+		&poser{"error", nil},
+		&errT,
+		true,
+	}, {
+		&poser{"path", nil},
+		&errP,
+		true,
+	}, {
+		&poser{"oh no", nil},
+		&p,
+		true,
+	}, {
+		errors.New("err"),
+		&timeout,
+		false,
+	}, {
+		errF,
+		&timeout,
+		true,
+	}, {
+		wrapped{"path error", errF},
+		&timeout,
+		true,
+	}}
+	for i, tc := range testCases {
+		name := fmt.Sprintf("%d:As(Errorf(..., %v), %v)", i, tc.err, tc.target)
+		t.Run(name, func(t *testing.T) {
+			match := errors.As(tc.err, tc.target)
+			if match != tc.match {
+				t.Fatalf("match: got %v; want %v", match, tc.match)
+			}
+			if !match {
+				return
+			}
+			if tc.target == nil {
+				t.Fatalf("non-nil result after match")
+			}
+		})
+	}
+}
+
+func TestAsValidation(t *testing.T) {
+	var s string
+	testCases := []interface{}{
+		nil,
+		(*int)(nil),
+		"error",
+		&s,
+	}
+	err := errors.New("error")
+	for _, tc := range testCases {
+		t.Run(fmt.Sprintf("%T(%v)", tc, tc), func(t *testing.T) {
+			defer func() {
+				recover()
+			}()
+			if errors.As(err, tc) {
+				t.Errorf("As(err, %T(%v)) = true, want false", tc, tc)
+				return
+			}
+			t.Errorf("As(err, %T(%v)) did not panic", tc, tc)
+		})
+	}
+}
+
+func TestUnwrap(t *testing.T) {
+	err1 := errors.New("1")
+	erra := wrapped{"wrap 2", err1}
+	erro := errors.Opaque(err1)
+
+	testCases := []struct {
+		err  error
+		want error
+	}{
+		{nil, nil},
+		{wrapped{"wrapped", nil}, nil},
+		{err1, nil},
+		{erra, err1},
+		{wrapped{"wrap 3", erra}, erra},
+
+		{erro, nil},
+		{wrapped{"opaque", erro}, erro},
+	}
+	for _, tc := range testCases {
+		if got := errors.Unwrap(tc.err); got != tc.want {
+			t.Errorf("Unwrap(%v) = %v, want %v", tc.err, got, tc.want)
+		}
+	}
+}
+
+func TestOpaque(t *testing.T) {
+	someError := errors.New("some error")
+	testCases := []struct {
+		err  error
+		next error
+	}{
+		{errorT{}, nil},
+		{wrapped{"b", nil}, nil},
+		{wrapped{"c", someError}, someError},
+	}
+	for _, tc := range testCases {
+		t.Run("", func(t *testing.T) {
+			opaque := errors.Opaque(tc.err)
+
+			f, ok := opaque.(errors.Formatter)
+			if !ok {
+				t.Fatal("Opaque error does not implement Formatter")
+			}
+			var p printer
+			next := f.FormatError(&p)
+			if next != tc.next {
+				t.Errorf("next was %v; want %v", next, tc.next)
+			}
+			if got, want := p.buf.String(), tc.err.Error(); got != want {
+				t.Errorf("error was %q; want %q", got, want)
+			}
+			if got := errors.Unwrap(opaque); got != nil {
+				t.Errorf("Unwrap returned non-nil error (%v)", got)
+			}
+		})
+	}
+}
+
+type errorT struct{}
+
+func (errorT) Error() string { return "errorT" }
+
+type wrapped struct {
+	msg string
+	err error
+}
+
+func (e wrapped) Error() string { return e.msg }
+
+func (e wrapped) Unwrap() error { return e.err }
+
+func (e wrapped) FormatError(p errors.Printer) error {
+	p.Print(e.msg)
+	return e.err
+}
+
+type printer struct {
+	errors.Printer
+	buf bytes.Buffer
+}
+
+func (p *printer) Print(args ...interface{}) { fmt.Fprint(&p.buf, args...) }