errors: add Frame and Formatter/Printer interfaces

errors.New now implements Formatter and includes Frame
information that is reported when detail is requested.

Partly implements proposal Issue #29934.

Change-Id: Id76888d246d7d862595b5e92d517b9c03f23a7a6
Reviewed-on: https://go-review.googlesource.com/c/163557
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>

4 files changed, 145 insertions, 2 deletions

diff --git a/src/errors/errors.go b/src/errors/errors.go
index b8a46921be..f23a96c43e 100644
--- a/src/errors/errors.go
+++ b/src/errors/errors.go
@@ -6,15 +6,25 @@
 package errors
 
 // New returns an error that formats as the given text.
+//
+// The returned error contains a Frame set to the caller's location and
+// implements Formatter to show this information when printed with details.
 func New(text string) error {
-	return &errorString{text}
+	return &errorString{text, Caller(1)}
 }
 
 // errorString is a trivial implementation of error.
 type errorString struct {
-	s string
+	s     string
+	frame Frame
 }
 
 func (e *errorString) Error() string {
 	return e.s
 }
+
+func (e *errorString) FormatError(p Printer) (next error) {
+	p.Print(e.s)
+	e.frame.Format(p)
+	return nil
+}
diff --git a/src/errors/format.go b/src/errors/format.go
new file mode 100644
index 0000000000..12deed3cf7
--- /dev/null
+++ b/src/errors/format.go
@@ -0,0 +1,34 @@
+// 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
+
+// A Formatter formats error messages.
+type Formatter interface {
+	error
+
+	// FormatError prints the receiver's first error and returns the next error in
+	// the error chain, if any.
+	FormatError(p Printer) (next error)
+}
+
+// A Printer formats error messages.
+//
+// The most common implementation of Printer is the one provided by package fmt
+// during Printf. Localization packages such as golang.org/x/text/message
+// typically provide their own implementations.
+type Printer interface {
+	// Print appends args to the message output.
+	Print(args ...interface{})
+
+	// Printf writes a formatted string.
+	Printf(format string, args ...interface{})
+
+	// Detail reports whether error detail is requested.
+	// After the first call to Detail, all text written to the Printer
+	// is formatted as additional detail, or ignored when
+	// detail has not been requested.
+	// If Detail returns false, the caller can avoid printing the detail at all.
+	Detail() bool
+}
diff --git a/src/errors/frame.go b/src/errors/frame.go
new file mode 100644
index 0000000000..a5369e5c36
--- /dev/null
+++ b/src/errors/frame.go
@@ -0,0 +1,56 @@
+// 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 (
+	"runtime"
+)
+
+// A Frame contains part of a call stack.
+type Frame struct {
+	// Make room for three PCs: the one we were asked for, what it called,
+	// and possibly a PC for skipPleaseUseCallersFrames. See:
+	// https://go.googlesource.com/go/+/032678e0fb/src/runtime/extern.go#169
+	frames [3]uintptr
+}
+
+// Caller returns a Frame that describes a frame on the caller's stack.
+// The argument skip is the number of frames to skip over.
+// Caller(0) returns the frame for the caller of Caller.
+func Caller(skip int) Frame {
+	var s Frame
+	runtime.Callers(skip+1, s.frames[:])
+	return s
+}
+
+// location reports the file, line, and function of a frame.
+//
+// The returned function may be "" even if file and line are not.
+func (f Frame) location() (function, file string, line int) {
+	frames := runtime.CallersFrames(f.frames[:])
+	if _, ok := frames.Next(); !ok {
+		return "", "", 0
+	}
+	fr, ok := frames.Next()
+	if !ok {
+		return "", "", 0
+	}
+	return fr.Function, fr.File, fr.Line
+}
+
+// Format prints the stack as error detail.
+// It should be called from an error's Format implementation,
+// before printing any other error detail.
+func (f Frame) Format(p Printer) {
+	if p.Detail() {
+		function, file, line := f.location()
+		if function != "" {
+			p.Printf("%s\n    ", function)
+		}
+		if file != "" {
+			p.Printf("%s:%d\n", file, line)
+		}
+	}
+}
diff --git a/src/errors/frame_test.go b/src/errors/frame_test.go
new file mode 100644
index 0000000000..864a6934d1
--- /dev/null
+++ b/src/errors/frame_test.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 errors_test
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"math/big"
+	"testing"
+)
+
+type myType struct{}
+
+func (myType) Format(s fmt.State, v rune) {
+	s.Write(bytes.Repeat([]byte("Hi! "), 10))
+}
+
+func BenchmarkErrorf(b *testing.B) {
+	err := errors.New("foo")
+	// pi := big.NewFloat(3.14) // Something expensive.
+	num := big.NewInt(5)
+	args := func(a ...interface{}) []interface{} { return a }
+	benchCases := []struct {
+		name   string
+		format string
+		args   []interface{}
+	}{
+		{"no_format", "msg: %v", args(err)},
+		{"with_format", "failed %d times: %v", args(5, err)},
+		{"method: mytype", "pi: %v", args("myfile.go", myType{}, err)},
+		{"method: number", "pi: %v", args("myfile.go", num, err)},
+	}
+	for _, bc := range benchCases {
+		b.Run(bc.name, func(b *testing.B) {
+			for i := 0; i < b.N; i++ {
+				_ = fmt.Errorf(bc.format, bc.args...)
+			}
+		})
+	}
+}