errors/errors.go

// Package errors implements functions for manipulating errors.
//
// The tranditional 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. For example
//
//      _, err := ioutil.ReadAll(r)
//      if err != nil {
//              return errors.Wrap(err, "read failed")
//
// In addition, errors.Wrap records the file and line where it was called,
// allowing the programmer to retrieve the path to the original error.
//
// Retrieving the cause of an error
//
// Using errors.Wrap constructs a stack of errors, adding context to the
// preceeding error. Depending on the nature of the error it may be necessary
// to recerse 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 which will recursively retrieve the topmost
// error which does nor 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
//     }
package errors

import (
	"errors"
	"fmt"
	"io"
	"os"
	"runtime"
	"strings"
)

type loc uintptr

func (l loc) Location() (string, int) {
	pc := uintptr(l)
	fn := runtime.FuncForPC(pc)
	if fn == nil {
		return "unknown", 0
	}

	_, prefix, _, _ := runtime.Caller(0)
	file, line := fn.FileLine(pc)
	if i := strings.LastIndex(prefix, "github.com/pkg/errors"); i > 0 {
		file = file[i:]
	}
	return file, line
}

// New returns an error that formats as the given text.
func New(text string) error {
	pc, _, _, _ := runtime.Caller(1)
	return struct {
		error
		loc
	}{
		errors.New(text),
		loc(pc),
	}
}

type e struct {
	cause   error
	message string
	loc
}

func (e *e) Error() string {
	return e.message + ": " + e.cause.Error()
}

func (e *e) Cause() error {
	return e.cause
}

// Wrap returns an error annotating the cause with message.
// If cause is nil, Wrap returns nil.
func Wrap(cause error, message string) error {
	if cause == nil {
		return nil
	}
	pc, _, _, _ := runtime.Caller(1)
	return &e{
		cause:   cause,
		message: message,
		loc:     loc(pc),
	}
}

type causer interface {
	Cause() 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 {
	for err != nil {
		cause, ok := err.(causer)
		if !ok {
			break
		}
		err = cause.Cause()
	}
	return err
}

type locationer interface {
	Location() (string, int)
}

// Print prints the error to Stderr.
// If the error implements the Causer interface described in Cause
// Print will recurse into the error's cause.
// If the error implements the inteface:
//
//     type Location interface {
//            Location() (file string, line int)
//     }
//
// Print will also print the file and line of the error.
func Print(err error) {
	Fprint(os.Stderr, err)
}

// Fprint prints the error to the supplied writer.
// The format of the output is the same as Print.
// If err is nil, nothing is printed.
func Fprint(w io.Writer, err error) {
	for err != nil {
		location, ok := err.(locationer)
		if ok {
			file, line := location.Location()
			fmt.Fprintf(w, "%s:%d: ", file, line)
		}
		switch err := err.(type) {
		case *e:
			fmt.Fprintln(w, err.message)
		default:
			fmt.Fprintln(w, err.Error())
		}

		cause, ok := err.(causer)
		if !ok {
			break
		}
		err = cause.Cause()
	}
}