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// Package errors provides simple error handling primitives.
//
// The traditional error handling idiom in Go is roughly akin to
//
// if err != nil {
// return err
// }
//
// which applied recursively up the call stack results in error reports
// without context or debugging information. The errors package allows
// programmers to add context to the failure path in their code in a way
// that does not destroy the original value of the error.
//
// Adding context to an error
//
// The errors.Wrap function returns a new error that adds context to the
// original error by recording a stack trace at the point Wrap is called,
// and the supplied message. For example
//
// _, err := ioutil.ReadAll(r)
// if err != nil {
// return errors.Wrap(err, "read failed")
// }
//
// If additional control is required the errors.WithStack and errors.WithMessage
// functions destructure errors.Wrap into its component operations of annotating
// an error with a stack trace and an a message, respectively.
//
// Retrieving the cause of an error
//
// Using errors.Wrap constructs a stack of errors, adding context to the
// preceding error. Depending on the nature of the error it may be necessary
// to reverse the operation of errors.Wrap to retrieve the original error
// for inspection. Any error value which implements this interface
//
// type causer interface {
// Cause() error
// }
//
// can be inspected by errors.Cause. errors.Cause will recursively retrieve
// the topmost error which does not implement causer, which is assumed to be
// the original cause. For example:
//
// switch err := errors.Cause(err).(type) {
// case *MyError:
// // handle specifically
// default:
// // unknown error
// }
//
// causer interface is not exported by this package, but is considered a part
// of stable public API.
//
// Formatted printing of errors
//
// All error values returned from this package implement fmt.Formatter and can
// be formatted by the fmt package. The following verbs are supported
//
// %s print the error. If the error has a Cause it will be
// printed recursively
// %v see %s
// %+v extended format. Each Frame of the error's StackTrace will
// be printed in detail.
//
// Retrieving the stack trace of an error or wrapper
//
// New, Errorf, Wrap, and Wrapf record a stack trace at the point they are
// invoked. This information can be retrieved with the following interface.
//
// type stackTracer interface {
// StackTrace() errors.StackTrace
// }
//
// Where errors.StackTrace is defined as
//
// type StackTrace []Frame
//
// The Frame type represents a call site in the stack trace. Frame supports
// the fmt.Formatter interface that can be used for printing information about
// the stack trace of this error. For example:
//
// if err, ok := err.(stackTracer); ok {
// for _, f := range err.StackTrace() {
// fmt.Printf("%+s:%d", f)
// }
// }
//
// stackTracer interface is not exported by this package, but is considered a part
// of stable public API.
//
// See the documentation for Frame.Format for more details.
package errors
import (
"fmt"
"io"
)
// New returns an error with the supplied message.
// New also records the stack trace at the point it was called.
func New(message string) error {
return &fundamental{
msg: message,
stack: callers(),
}
}
// Errorf formats according to a format specifier and returns the string
// as a value that satisfies error.
// Errorf also records the stack trace at the point it was called.
func Errorf(format string, args ...interface{}) error {
return &fundamental{
msg: fmt.Sprintf(format, args...),
stack: callers(),
}
}
// fundamental is an error that has a message and a stack, but no caller.
type fundamental struct {
msg string
*stack
}
func (f *fundamental) Error() string { return f.msg }
func (f *fundamental) Format(s fmt.State, verb rune) {
switch verb {
case 'v':
if s.Flag('+') {
io.WriteString(s, f.msg)
f.stack.Format(s, verb)
return
}
fallthrough
case 's':
io.WriteString(s, f.msg)
case 'q':
fmt.Fprintf(s, "%q", f.msg)
}
}
// WithStack annotates err with a stack trace at the point WithStack was called.
// If err is nil, WithStack returns nil.
func WithStack(err error) error {
if err == nil {
return nil
}
return &withStack{
err,
callers(),
}
}
type withStack struct {
error
*stack
}
func (w *withStack) Cause() error { return w.error }
func (w *withStack) Format(s fmt.State, verb rune) {
switch verb {
case 'v':
if s.Flag('+') {
fmt.Fprintf(s, "%+v", w.Cause())
w.stack.Format(s, verb)
return
}
fallthrough
case 's':
io.WriteString(s, w.Error())
case 'q':
fmt.Fprintf(s, "%q", w.Error())
}
}
// Wrap returns an error annotating err with a stack trace
// at the point Wrap is called, and the supplied message.
// If err is nil, Wrap returns nil.
func Wrap(err error, message string) error {
if err == nil {
return nil
}
err = &withMessage{
cause: err,
msg: message,
}
return &withStack{
err,
callers(),
}
}
// Wrapf returns an error annotating err with a stack trace
// at the point Wrapf is call, and the format specifier.
// If err is nil, Wrapf returns nil.
func Wrapf(err error, format string, args ...interface{}) error {
if err == nil {
return nil
}
err = &withMessage{
cause: err,
msg: fmt.Sprintf(format, args...),
}
return &withStack{
err,
callers(),
}
}
// WithMessage annotates err with a new message.
// If err is nil, WithMessage returns nil.
func WithMessage(err error, message string) error {
if err == nil {
return nil
}
return &withMessage{
cause: err,
msg: message,
}
}
type withMessage struct {
cause error
msg string
}
func (w *withMessage) Error() string { return w.msg + ": " + w.cause.Error() }
func (w *withMessage) Cause() error { return w.cause }
func (w *withMessage) Format(s fmt.State, verb rune) {
switch verb {
case 'v':
if s.Flag('+') {
fmt.Fprintf(s, "%+v\n", w.Cause())
io.WriteString(s, w.msg)
return
}
fallthrough
case 's', 'q':
io.WriteString(s, w.Error())
}
}
// Cause returns the underlying cause of the error, if possible.
// An error value has a cause if it implements the following
// interface:
//
// type causer interface {
// Cause() error
// }
//
// If the error does not implement Cause, the original error will
// be returned. If the error is nil, nil will be returned without further
// investigation.
func Cause(err error) error {
type causer interface {
Cause() error
}
for err != nil {
cause, ok := err.(causer)
if !ok {
break
}
err = cause.Cause()
}
return err
}