source: code/trunk/vendor/golang.org/x/xerrors/fmt.go@ 822

// 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.
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
}