Catching runtime errors

Chapter 5.4.

Catching runtime errors

As soon as we begin adding dynamic behavior to our HTML templates there’s a risk of encountering runtime errors.

Let’s add a deliberate error to the view.tmpl template and see what happens:

File: ui/html/pages/view.tmpl

{{define "title"}}Snippet #{{.Snippet.ID}}{{end}}

{{define "main"}}
    {{with .Snippet}}
    <div class='snippet'>
        <div class='metadata'>
            <strong>{{.Title}}</strong>
            <span>#{{.ID}}</span>
        </div>
        {{len nil}} <!-- Deliberate error -->
        <pre><code>{{.Content}}</code></pre>
        <div class='metadata'>
            <time>Created: {{.Created}}</time>
            <time>Expires: {{.Expires}}</time>
        </div>
    </div>
    {{end}}
{{end}}

In this markup above we’ve added the line {{len nil}}, which should generate an error at runtime because in Go the value nil does not have a length.

Try running the application now. You’ll find that everything still compiles OK:

$ go run ./cmd/web
time=2024-03-18T11:29:23.000+00:00 level=INFO msg="starting server" addr=:4000

But if you use curl to make a request to http://localhost:4000/snippet/view/1 you’ll get a response which looks a bit like this.

$ curl -i http://localhost:4000/snippet/view/1
HTTP/1.1 200 OK
Date: Wed, 18 Mar 2024 11:29:23 GMT
Content-Length: 734
Content-Type: text/html; charset=utf-8


<!doctype html>
<html lang='en'>
    <head>
        <meta charset='utf-8'>
        <title>Snippet #1 - Snippetbox</title>
        <link rel='stylesheet' href='/static/css/main.css'>
        <link rel='shortcut icon' href='/static/img/favicon.ico' type='image/x-icon'>
        <link rel='stylesheet' href='https://fonts.googleapis.com/css?family=Ubuntu+Mono:400,700'>
    </head>
    <body>
        <header>
            <h1><a href='/'>Snippetbox</a></h1>
        </header>
        
 <nav>
    <a href='/'>Home</a>
</nav>

        <main>
            
    
    <div class='snippet'>
        <div class='metadata'>
            <strong>An old silent pond</strong>
            <span>#1</span>
        </div>
        Internal Server Error

This is pretty bad. Our application has thrown an error, but the user has wrongly been sent a 200 OK response. And even worse, they’ve received a half-complete HTML page.

To fix this we need to make the template render a two-stage process. First, we should make a ‘trial’ render by writing the template into a buffer. If this fails, we can respond to the user with an error message. But if it works, we can then write the contents of the buffer to our http.ResponseWriter.

Let’s update the render() helper to use this approach instead:

File: cmd/web/helpers.go

package main

import (
    "bytes" // New import
    "fmt"
    "net/http"
)

...

func (app *application) render(w http.ResponseWriter, r *http.Request, status int, page string, data templateData) {
    ts, ok := app.templateCache[page]
    if !ok {
        err := fmt.Errorf("the template %s does not exist", page)
        app.serverError(w, r, err)
        return
    }

    // Initialize a new buffer.
    buf := new(bytes.Buffer)

    // Write the template to the buffer, instead of straight to the
    // http.ResponseWriter. If there's an error, call our serverError() helper
    // and then return.
    err := ts.ExecuteTemplate(buf, "base", data)
    if err != nil {
        app.serverError(w, r, err)
        return
    }

    // If the template is written to the buffer without any errors, we are safe
    // to go ahead and write the HTTP status code to http.ResponseWriter.
    w.WriteHeader(status)

    // Write the contents of the buffer to the http.ResponseWriter. Note: this
    // is another time where we pass our http.ResponseWriter to a function that
    // takes an io.Writer.
    buf.WriteTo(w)
}

Restart the application and try making the same request again. You should now get a proper error message and 500 Internal Server Error response.

$ curl -i http://localhost:4000/snippet/view/1
HTTP/1.1 500 Internal Server Error
Content-Type: text/plain; charset=utf-8
X-Content-Type-Options: nosniff
Date: Wed, 18 Mar 2024 11:29:23 GMT
Content-Length: 22

Internal Server Error

Great stuff. That’s looking much better.

Before we move on to the next chapter, head back to the view.tmpl file and remove the deliberate error:

File: ui/html/pages/view.tmpl

{{define "title"}}Snippet #{{.Snippet.ID}}{{end}}

{{define "main"}}
    {{with .Snippet}}
    <div class='snippet'>
        <div class='metadata'>
            <strong>{{.Title}}</strong>
            <span>#{{.ID}}</span>
        </div>
        <pre><code>{{.Content}}</code></pre>
        <div class='metadata'>
            <time>Created: {{.Created}}</time>
            <time>Expires: {{.Expires}}</time>
        </div>
    </div>
    {{end}}
{{end}}