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>

3 files changed, 136 insertions, 7 deletions

diff --git a/src/fmt/errors.go b/src/fmt/errors.go
new file mode 100644
index 0000000000..6ae6c47fd9
--- /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
index 0000000000..0c774bc28b
--- /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 e597639429..3253e8042e 100644
--- 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