fmt: support %w

When fmt.Errorf is provided with a %w verb with an error operand,
return an error implementing an Unwrap method returning that operand.

It is invalid to use %w with other formatting functions, to use %w
multiple times in a format string, or to use %w with a non-error
operand. When the Errorf format string contains an invalid use of %w,
the returned error does not implement Unwrap.

Change-Id: I534e20d3b163ab22c2b137b1c9095906dc243221
Reviewed-on: https://go-review.googlesource.com/c/go/+/176998
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>

src/fmt/errors.go

@@ -0,0 +1,43 @@
+// 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 fmt
+
+import "errors"
+
+// Errorf formats according to a format specifier and returns the string as a
+// value that satisfies error.
+//
+// If the format specifier includes a %w verb with an error operand,
+// the returned error will implement an Unwrap method returning the operand. It is
+// invalid to include more than one %w verb or to supply it with an operand
+// that does not implement the error innterface. The %w verb is otherwise
+// a synonym for %v.
+func Errorf(format string, a ...interface{}) error {
+ p := newPrinter()
+ p.wrapErrs = true
+ p.doPrintf(format, a)
+ s := string(p.buf)
+ var err error
+ if p.wrappedErr == nil {
+ err = errors.New(s)
+ } else {
+ err = &wrapError{s, p.wrappedErr}
+ }
+ p.free()
+ return err
+}
+
+type wrapError struct {
+ msg string
+ err error
+}
+
+func (e *wrapError) Error() string {
+ return e.msg
+}
+
+func (e *wrapError) Unwrap() error {
+ return e.err
+}

src/fmt/errors_test.go

@@ -0,0 +1,73 @@
+// 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 fmt_test
+
+import (
+ "errors"
+ "fmt"
+ "testing"
+)
+
+func TestErrorf(t *testing.T) {
+ wrapped := errors.New("inner error")
+ for _, test := range []struct {
+ err error
+ wantText string
+ wantUnwrap error
+ }{{
+ err: fmt.Errorf("%w", wrapped),
+ wantText: "inner error",
+ wantUnwrap: wrapped,
+ }, {
+ err: fmt.Errorf("added context: %w", wrapped),
+ wantText: "added context: inner error",
+ wantUnwrap: wrapped,
+ }, {
+ err: fmt.Errorf("%w with added context", wrapped),
+ wantText: "inner error with added context",
+ wantUnwrap: wrapped,
+ }, {
+ err: fmt.Errorf("%s %w %v", "prefix", wrapped, "suffix"),
+ wantText: "prefix inner error suffix",
+ wantUnwrap: wrapped,
+ }, {
+ err: fmt.Errorf("%[2]s: %[1]w", wrapped, "positional verb"),
+ wantText: "positional verb: inner error",
+ wantUnwrap: wrapped,
+ }, {
+ err: fmt.Errorf("%v", wrapped),
+ wantText: "inner error",
+ }, {
+ err: fmt.Errorf("added context: %v", wrapped),
+ wantText: "added context: inner error",
+ }, {
+ err: fmt.Errorf("%v with added context", wrapped),
+ wantText: "inner error with added context",
+ }, {
+ err: fmt.Errorf("%w is not an error", "not-an-error"),
+ wantText: "%!w(string=not-an-error) is not an error",
+ }, {
+ err: fmt.Errorf("wrapped two errors: %w %w", errString("1"), errString("2")),
+ wantText: "wrapped two errors: 1 %!w(fmt_test.errString=2)",
+ }, {
+ err: fmt.Errorf("wrapped three errors: %w %w %w", errString("1"), errString("2"), errString("3")),
+ wantText: "wrapped three errors: 1 %!w(fmt_test.errString=2) %!w(fmt_test.errString=3)",
+ }, {
+ err: fmt.Errorf("%w", nil),
+ wantText: "%!w(<nil>)",
+ wantUnwrap: nil, // still nil
+ }} {
+ if got, want := errors.Unwrap(test.err), test.wantUnwrap; got != want {
+ t.Errorf("Formatted error: %v\nerrors.Unwrap() = %v, want %v", test.err, got, want)
+ }
+ if got, want := test.err.Error(), test.wantText; got != want {
+ t.Errorf("err.Error() = %q, want %q", got, want)
+ }
+ }
+}
+
+type errString string
+
+func (e errString) Error() string { return string(e) }

src/fmt/print.go

@@ -5,7 +5,6 @@
 package fmt
 
 import (
- "errors"
  "internal/fmtsort"
  "io"
  "os"
@@ -123,6 +122,10 @@ type pp struct {
  panicking bool
  // erroring is set when printing an error string to guard against calling handleMethods.
  erroring bool
+ // wrapErrors is set when the format string may contain a %w verb.
+ wrapErrs bool
+ // wrappedErr records the target of the %w verb.
+ wrappedErr error
 }
 
 var ppFree = sync.Pool{
@@ -153,6 +156,7 @@ func (p *pp) free() {
  p.buf = p.buf[:0]
  p.arg = nil
  p.value = reflect.Value{}
+ p.wrappedErr = nil
  ppFree.Put(p)
 }
 
@@ -217,12 +221,6 @@ func Sprintf(format string, a ...interface{}) string {
  return s
 }
 
-// Errorf formats according to a format specifier and returns the string
-// as a value that satisfies error.
-func Errorf(format string, a ...interface{}) error {
- return errors.New(Sprintf(format, a...))
-}
-
 // These routines do not take a format string
 
 // Fprint formats using the default formats for its operands and writes to w.
@@ -576,6 +574,21 @@ func (p *pp) handleMethods(verb rune) (handled bool) {
  if p.erroring {
  return
  }
+ if verb == 'w' {
+ // It is invalid to use %w other than with Errorf, more than once,
+ // or with a non-error arg.
+ err, ok := p.arg.(error)
+ if !ok || !p.wrapErrs || p.wrappedErr != nil {
+ p.wrappedErr = nil
+ p.wrapErrs = false
+ p.badVerb(verb)
+ return true
+ }
+ p.wrappedErr = err
+ // If the arg is a Formatter, pass 'v' as the verb to it.
+ verb = 'v'
+ }
+
  // Is it a Formatter?
  if formatter, ok := p.arg.(Formatter); ok {
  handled = true

src/internal/oserror/errors_test.go

@@ -34,8 +34,7 @@ func TestIsTimeout(t *testing.T) {
  {true, ttError{timeout: true}},
  {true, isError{os.ErrTimeout}},
  {true, os.ErrTimeout},
- // TODO: restore when %w is reimplemented
- //{true, fmt.Errorf("wrap: %w", os.ErrTimeout)},
+ {true, fmt.Errorf("wrap: %w", os.ErrTimeout)},
  {false, ttError{timeout: false}},
  {false, errors.New("error")},
  } {
@@ -53,8 +52,7 @@ func TestIsTemporary(t *testing.T) {
  {true, ttError{temporary: true}},
  {true, isError{os.ErrTemporary}},
  {true, os.ErrTemporary},
- // TODO: restore when %w is reimplemented
- //{true, fmt.Errorf("wrap: %w", os.ErrTemporary)},
+ {true, fmt.Errorf("wrap: %w", os.ErrTemporary)},
  {false, ttError{temporary: false}},
  {false, errors.New("error")},
  } {