JSONLogger let's error interface take priority over json.Marshaler

[cam-stitt]

From what I can see, if you have an object that implements both json.Marshaler and error, it will always use the error interface. This is despite the fact that code states that json.Marshaler should always win here. Consider this (I know it's not perfect, but we have a stack trace in our error that we want logged):

type Error struct {
  Message string
  Status int
}

func (e Error) Error() string {
  return e.Message
}

func (e Error) MarshalJSON() ([]byte, error) {
  aux := struct{Message string, Status Int}{Message: e.Message, Status: e.Status}
  return json.Marshal(aux)
}

And you were to log it:

err := Error{msg: "Error impl", Status: 200}
logger.Log("msg", err)

The result will always be {"err": "Error impl"}

[cam-stitt]

From what I can see, it's because of this: https://github.com/go-kit/kit/blob/master/log/json_logger.go#L48

[ChrisHines]

Yes. I reproduced this bug and agree with your analysis. Would you like to submit a PR with a reproducing test case and a fix? I believe the fix is just removing the if statement you linked to above.

[ChrisHines]

Fixed by #581.