Error pages in go based web application

Question

We are building a web site with go using only the built-in stuff like http.HandleFuncto set up routing and the html.template package to render pages. Nothing fancy really and it works fine. Now I got around to thinking about error handling and would like to catch panics and whatnots and redirect to a static error page. What is the best way to do it with go?

The brute force approach I have been pondering first is to just redirect when an error is detected but it would be nice with a generic catch-all also. I didn't find anything in the docs about it.

(Also thinking about 404s along the same lines.)

Answer 1

There's a great write up on the golang blog about error handling and it specifically covers web related errors towards the end.

http://blog.golang.org/2011/07/error-handling-and-go.html

Basically, you wrap the regular http handlers with your own that can return an error. The wrapper checks if your new handlers return an error and if so react. I use something similar to this where i also call recover. Here's a snippet of the code I use (which is free and open).

While I dont yet provide a custom page for errors, it's been on my mind and should be trivial to add.

// Error is the expected return of a dae.Handler, or nil otherwise.
type Error struct {
    Error   error
    Code    int
    Message string
}

// NewError is a helper for creating an Error pointer.
func NewError(err error, code int, msg string) *Error {
    return &Error{err, code, msg}
}

// Handler is used to cast functions to its type to implement ServeHTTP.
// Code that panics is automatically recovered and delivers a server 500 error.
type Handler func(http.ResponseWriter, *http.Request) *Error

// NewHandler is a helper to chain multiple functions together.
func New(fn ...Handler) Handler {
    if len(fn) == 0 {
        panic("No Handlers passed in.")
    }

    h := Handler(fn[0])
    for i := 1; i < len(fn); i++ {
        h = h.Add(fn[i])
    }
    return h
}

// ServeHTTP implements the http.Handler interface. If an appHandler returns an
// error, the error is inspected and an appropriate response is written out.
func (fn Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
    defer func() {
        if r := recover(); r != nil {
            log.Printf("%v", r)
            http.Error(w, "A serious error has occured.", 500)
            if Debug {
                panic(r.(error))
            }
        }
    }()

    if e := fn(w, r); e != nil {
        log.Printf("Code: %v, Message: \"%s\", Error: %v", e.Code, e.Message, e.Error)
        switch e.Code {
        case 500:
            http.Error(w, e.Message, e.Code)
        case 404:
            http.NotFound(w, r)
            fmt.Fprint(w, e.Message)
        case 200:
            fmt.Fprint(w, e.Message)
        }
    }
}