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>

diff --git a/src/fmt/errors.go b/src/fmt/errors.go
new file mode 100644 (file)
index 0000000..6ae6c47
--- /dev/null
+++ b/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
+}

diff --git a/src/fmt/errors_test.go b/src/fmt/errors_test.go
new file mode 100644 (file)
index 0000000..0c774bc
--- /dev/null
+++ b/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) }

diff --git a/src/fmt/print.go b/src/fmt/print.go
index e597639429189e6966ba96d2cbff009e191c8536..3253e8042e22086badd48280ec15b5e34c066c76 100644 (file)
--- a/src/fmt/print.go
+++ b/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

diff --git a/src/internal/oserror/errors_test.go b/src/internal/oserror/errors_test.go
index 50dd4678d4ab391be388cbc1893463284fdb6524..6d6a56a0c74719f63fe4ce192db9c9942bb63b17 100644 (file)
--- a/src/internal/oserror/errors_test.go
+++ b/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")},
        } {