Errors x Stack x Info
===
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> :hash: errors
> [name=郭學聰 Hsueh-Tsung Kuo] [time=Wed, 23 Sep 2020] [color=red]
note: 2020-05-23
---
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## Who am I?
----
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* programmer from Rayark, a game company in Taiwan
* backend (and temporary frontend) engineer, focus on common service
* usually develop something related to my work in Python, Ruby, ECMAScript, Golang, C#
* ECMAScript hater since **Netscape** is dead
* built CDN-aware game asset update system
* built almost entire VOEZ game server by myself only
* supported Sdorica backend development
---
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## Outline
----
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4. TL;DR
5. Theory
6. Troll
7. Conclusion
8. Q&A
---
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## TL;DR
----
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* :no_entry_sign: Go 1.13 error system
* :no_entry_sign: golang/xerrors
* https://github.com/golang/xerrors
* https://godoc.org/golang.org/x/xerrors
----
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* :thumbsup: pkg/errors
* https://github.com/pkg/errors
* https://godoc.org/github.com/pkg/errors
----
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* for http server
* use the gin, luke
* <small>https://github.com/gin-gonic/gin#model-binding-and-validation</small>
----
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or follow the below
----
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```go=
package main
import (
"encoding/json"
"errors"
"fmt"
"io/ioutil"
"net/http"
"os"
"reflect"
"strings"
"github.com/julienschmidt/httprouter"
)
// utility function
func _wrapError(msg string, err error) error {
return fmt.Errorf("%s: %w", msg, err)
}
func GetErrorMessageTitle(err error) string {
return strings.SplitN(err.Error(), ":", 2)[0]
}
// ApiError
type ApiError struct {
statusCode int
msg string
err error
}
func NewApiError(statusCode int, msg string, err error) error {
if err != nil {
msg = fmt.Sprintf("%s: %s", msg, err.Error())
}
return &ApiError{statusCode: statusCode, msg: msg, err: err}
}
func (e *ApiError) Error() string {
return e.msg
}
func (e *ApiError) Unwrap() error {
return e.err
}
func (e *ApiError) StatusCode() int {
return e.statusCode
}
func ExtractErrorMessageChain(err error) string {
e := err
var b strings.Builder
for {
b.WriteString(e.Error() + "\n")
e = errors.Unwrap(e)
if e == nil {
break
}
}
return b.String()
}
func IsErrorEqual(e1 error, e2 error) bool {
if e1 == nil || e2 == nil {
return e1 == e2
} else {
return reflect.TypeOf(e1) == reflect.TypeOf(e2) && GetErrorMessageTitle(e1) == GetErrorMessageTitle(e2)
}
}
func WrapError(template error, err error) error {
apiErrorTemplate, ok := template.(*ApiError)
if ok {
err = NewApiError(apiErrorTemplate.StatusCode(), apiErrorTemplate.Error(), err)
} else {
err = _wrapError(template.Error(), err)
}
return err
}
// handler
type HandleE func(http.ResponseWriter, *http.Request, httprouter.Params) error
func ErrAwareHandle(h HandleE) httprouter.Handle {
return func(w http.ResponseWriter, r *http.Request, ps httprouter.Params) {
err := h(w, r, ps)
if err != nil {
switch e := err.(type) {
case *ApiError:
/*var respBody string
cause := errors.Unwrap(e)
if cause != nil {
respBody = e.Error() + ": " + cause.Error()
} else {
respBody = e.Error()
}
http.Error(w, respBody, e.StatusCode())*/
http.Error(w, e.Error(), e.StatusCode())
default:
http.Error(w, e.Error(), 500)
}
messageChain := ExtractErrorMessageChain(err)
fmt.Fprintf(os.Stderr, "error:\n%s\n", messageChain)
}
}
}
// usage
type Input struct {
Msg string `json:msg`
}
func example(w http.ResponseWriter, r *http.Request, ps httprouter.Params) error {
name := ps.ByName("name")
if name != "say" {
return NewApiError(404, "not found", errors.New("must be /error_test/say"))
}
body, err := ioutil.ReadAll(r.Body)
if err != nil {
return fmt.Errorf("body read failed: %v", err)
}
if len(body) < 2 {
return errors.New("body is not enough")
}
var input Input
if err = json.Unmarshal(body, &input); err != nil {
return NewApiError(400, "bad request", err)
}
if input.Msg == "" {
return NewApiError(400, "msg lost", nil)
}
if input.Msg == "FQ" {
return NewApiError(403, "you dirty", fmt.Errorf("変態 %w", errors.New("助兵衛")))
}
w.WriteHeader(200)
w.Write([]byte("OK\n"))
return nil
}
func main() {
router := httprouter.New()
router.POST("/error_test/:name", ErrAwareHandle(example))
err := http.ListenAndServe(":8000", router)
if err != nil {
panic(err)
}
}
// client example
// curl -v -X POST --data '{"msg": "FQ2"}' http://localhost:8000/error_test/say
```
---
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## Theory
* what
* error title
* which
* error type
* when
* line number
* where
* calling stack
----
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### :thumbsdown: Go
* [x] error title
* [ ] error type
* [ ] line number
* [ ] calling stack
----
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### Wrap
```go=
err = fmt.Errorf("... %w ...", ..., err, ...)
err.Unwrap()
```
----
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### Inspect Error Chain
```go=
errors.Is(err, ErrorInstance) // must be same address
var perr *ErrorType
errors.As(err, &perr)
```
----
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```go=
package main
import (
"errors"
"fmt"
)
type MyError struct {
msg string
err error
}
func NewMyError(msg string, err error) *MyError {
if err != nil {
msg = fmt.Sprintf("%s: %s", msg, err.Error())
}
return &MyError{
msg: msg,
err: err,
}
}
func (e *MyError) Error() string {
return e.msg
}
func (e *MyError) Unwrap() error {
return e.err
}
func main() {
var err error
err = errors.New("err1")
err = fmt.Errorf("err2: %w", err)
myErr := NewMyError("my err3", err)
myErr2 := NewMyError("my err3", err)
err = myErr
err = fmt.Errorf("err4: %w", err)
err = fmt.Errorf("err5: %w", err)
if err != nil {
fmt.Println("error is:", err)
if errors.Is(err, myErr) {
fmt.Println("It is myErr!")
}
if !errors.Is(err, myErr2) {
fmt.Println("It is NOT myErr2!")
}
fmt.Println("")
var myError *MyError
if errors.As(err, &myError) {
fmt.Println("failed:", err.Error())
fmt.Println("MyError:", myError.Error())
}
}
}
```
---
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## Troll
----
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# Go error system SUCKS
again, Go error system SUCKS
----
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### Rust
----
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```rust=
panic!()
Result<T, E>
Option<T>
myobj.myfn()? // ? operator
```
<small>https://doc.rust-lang.org/book/ch09-00-error-handling.html</small>
----
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```rust=
e.source()
e.backtrace()
```
<small>https://doc.rust-lang.org/std/error/trait.Error.html</small>
---
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## Conclusion
----
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* :thumbsup: pkg/errors
* https://github.com/pkg/errors
* https://godoc.org/github.com/pkg/errors
---
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## Q&A
---
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