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>

1 files changed, 43 insertions, 0 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
+}