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// 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 xerrors
import (
"fmt"
"strings"
"unicode"
"unicode/utf8"
"golang.org/x/xerrors/internal"
)
const percentBangString = "%!"
// Errorf formats according to a format specifier and returns the string as a
// value that satisfies error.
//
// The returned error includes the file and line number of the caller when
// formatted with additional detail enabled. If the last argument is an error
// the returned error's Format method will return it if the format string ends
// with ": %s", ": %v", or ": %w". If the last argument is an error and the
// format string ends with ": %w", the returned error implements an Unwrap
// method returning it.
//
// If the format specifier includes a %w verb with an error operand in a
// position other than at the end, the returned error will still implement an
// Unwrap method returning the operand, but the error's Format method will not
// return the wrapped error.
//
// It is invalid to include more than one %w verb or to supply it with an
// operand that does not implement the error interface. The %w verb is otherwise
// a synonym for %v.
//
// Note that as of Go 1.13, the fmt.Errorf function will do error formatting,
// but it will not capture a stack backtrace.
func Errorf(format string, a ...interface{}) error {
format = formatPlusW(format)
// Support a ": %[wsv]" suffix, which works well with xerrors.Formatter.
wrap := strings.HasSuffix(format, ": %w")
idx, format2, ok := parsePercentW(format)
percentWElsewhere := !wrap && idx >= 0
if !percentWElsewhere && (wrap || strings.HasSuffix(format, ": %s") || strings.HasSuffix(format, ": %v")) {
err := errorAt(a, len(a)-1)
if err == nil {
return &noWrapError{fmt.Sprintf(format, a...), nil, Caller(1)}
}
// TODO: this is not entirely correct. The error value could be
// printed elsewhere in format if it mixes numbered with unnumbered
// substitutions. With relatively small changes to doPrintf we can
// have it optionally ignore extra arguments and pass the argument
// list in its entirety.
msg := fmt.Sprintf(format[:len(format)-len(": %s")], a[:len(a)-1]...)
frame := Frame{}
if internal.EnableTrace {
frame = Caller(1)
}
if wrap {
return &wrapError{msg, err, frame}
}
return &noWrapError{msg, err, frame}
}
// Support %w anywhere.
// TODO: don't repeat the wrapped error's message when %w occurs in the middle.
msg := fmt.Sprintf(format2, a...)
if idx < 0 {
return &noWrapError{msg, nil, Caller(1)}
}
err := errorAt(a, idx)
if !ok || err == nil {
// Too many %ws or argument of %w is not an error. Approximate the Go
// 1.13 fmt.Errorf message.
return &noWrapError{fmt.Sprintf("%sw(%s)", percentBangString, msg), nil, Caller(1)}
}
frame := Frame{}
if internal.EnableTrace {
frame = Caller(1)
}
return &wrapError{msg, err, frame}
}
func errorAt(args []interface{}, i int) error {
if i < 0 || i >= len(args) {
return nil
}
err, ok := args[i].(error)
if !ok {
return nil
}
return err
}
// formatPlusW is used to avoid the vet check that will barf at %w.
func formatPlusW(s string) string {
return s
}
// Return the index of the only %w in format, or -1 if none.
// Also return a rewritten format string with %w replaced by %v, and
// false if there is more than one %w.
// TODO: handle "%[N]w".
func parsePercentW(format string) (idx int, newFormat string, ok bool) {
// Loosely copied from golang.org/x/tools/go/analysis/passes/printf/printf.go.
idx = -1
ok = true
n := 0
sz := 0
var isW bool
for i := 0; i < len(format); i += sz {
if format[i] != '%' {
sz = 1
continue
}
// "%%" is not a format directive.
if i+1 < len(format) && format[i+1] == '%' {
sz = 2
continue
}
sz, isW = parsePrintfVerb(format[i:])
if isW {
if idx >= 0 {
ok = false
} else {
idx = n
}
// "Replace" the last character, the 'w', with a 'v'.
p := i + sz - 1
format = format[:p] + "v" + format[p+1:]
}
n++
}
return idx, format, ok
}
// Parse the printf verb starting with a % at s[0].
// Return how many bytes it occupies and whether the verb is 'w'.
func parsePrintfVerb(s string) (int, bool) {
// Assume only that the directive is a sequence of non-letters followed by a single letter.
sz := 0
var r rune
for i := 1; i < len(s); i += sz {
r, sz = utf8.DecodeRuneInString(s[i:])
if unicode.IsLetter(r) {
return i + sz, r == 'w'
}
}
return len(s), false
}
type noWrapError struct {
msg string
err error
frame Frame
}
func (e *noWrapError) Error() string {
return fmt.Sprint(e)
}
func (e *noWrapError) Format(s fmt.State, v rune) { FormatError(e, s, v) }
func (e *noWrapError) FormatError(p Printer) (next error) {
p.Print(e.msg)
e.frame.Format(p)
return e.err
}
type wrapError struct {
msg string
err error
frame Frame
}
func (e *wrapError) Error() string {
return fmt.Sprint(e)
}
func (e *wrapError) Format(s fmt.State, v rune) { FormatError(e, s, v) }
func (e *wrapError) FormatError(p Printer) (next error) {
p.Print(e.msg)
e.frame.Format(p)
return e.err
}
func (e *wrapError) Unwrap() error {
return e.err
}
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