12 July 2011

Introduction

If you have written any Go code you have probably encountered the built-in error type. Go code uses error values to indicate an abnormal state. For example, the os.Open function returns a non-nil error value when it fails to open a file.

func Open(name string) (file *File, err error)

The following code uses os.Open to open a file. If an error occurs it calls log.Fatal to print the error message and stop.

f, err := os.Open("filename.ext")
if err != nil {
    log.Fatal(err)
}
// do something with the open *File f

You can get a lot done in Go knowing just this about the error type, but in this article we'll take a closer look at error and discuss some good practices for error handling in Go.

The error type

The error type is an interface type. An error variable represents any value that can describe itself as a string. Here is the interface's declaration:

type error interface {
    Error() string
}

The error type, as with all built in types, is predeclared in the universe block.

The most commonly-used error implementation is the errors package's unexported errorString type.

// errorString is a trivial implementation of error.
type errorString struct {
    s string
}

func (e *errorString) Error() string {
    return e.s
}

You can construct one of these values with the errors.New function. It takes a string that it converts to an errors.errorString and returns as an error value.

// New returns an error that formats as the given text.
func New(text string) error {
    return &errorString{text}
}

Here's how you might use errors.New:

func Sqrt(f float64) (float64, error) {
    if f < 0 {
        return 0, errors.New("math: square root of negative number")
    }
    // implementation
}

A caller passing a negative argument to Sqrt receives a non-nil error value (whose concrete representation is an errors.errorString value). The caller can access the error string ("math: square root of...") by calling the error's Error method, or by just printing it:

f, err := Sqrt(-1)
if err != nil {
    fmt.Println(err)
}

The fmt package formats an error value by calling its Error() string method.

It is the error implementation's responsibility to summarize the context. The error returned by os.Open formats as "open /etc/passwd: permission denied," not just "permission denied." The error returned by our Sqrt is missing information about the invalid argument.

To add that information, a useful function is the fmt package's Errorf. It formats a string according to Printf's rules and returns it as an error created by errors.New.

if f < 0 {
    return 0, fmt.Errorf("math: square root of negative number %g", f)
}

In many cases fmt.Errorf is good enough, but since error is an interface, you can use arbitrary data structures as error values, to allow callers to inspect the details of the error.

For instance, our hypothetical callers might want to recover the invalid argument passed to Sqrt. We can enable that by defining a new error implementation instead of using errors.errorString:

type NegativeSqrtError float64

func (f NegativeSqrtError) Error() string {
    return fmt.Sprintf("math: square root of negative number %g", float64(f))
}

A sophisticated caller can then use a type assertion to check for a NegativeSqrtError and handle it specially, while callers that just pass the error to fmt.Println or log.Fatal will see no change in behavior.

As another example, the json package specifies a SyntaxError type that the json.Decode function returns when it encounters a syntax error parsing a JSON blob.

type SyntaxError struct {
    msg    string // description of error
    Offset int64  // error occurred after reading Offset bytes
}

func (e *SyntaxError) Error() string { return e.msg }

The Offset field isn't even shown in the default formatting of the error, but callers can use it to add file and line information to their error messages:

if err := dec.Decode(&val); err != nil {
    if serr, ok := err.(*json.SyntaxError); ok {
        line, col := findLine(f, serr.Offset)
        return fmt.Errorf("%s:%d:%d: %v", f.Name(), line, col, err)
    }
    return err
}

(This is a slightly simplified version of some actual code from the Camlistore project.)

The error interface requires only a Error method; specific error implementations might have additional methods. For instance, the net package returns errors of type error, following the usual convention, but some of the error implementations have additional methods defined by the net.Error interface:

package net

type Error interface {
    error
    Timeout() bool   // Is the error a timeout?
    Temporary() bool // Is the error temporary?
}

Client code can test for a net.Error with a type assertion and then distinguish transient network errors from permanent ones. For instance, a web crawler might sleep and retry when it encounters a temporary error and give up otherwise.

if nerr, ok := err.(net.Error); ok && nerr.Temporary() {
    time.Sleep(1e9)
    continue
}
if err != nil {
    log.Fatal(err)
}

Simplifying repetitive error handling

In Go, error handling is important. The language's design and conventions encourage you to explicitly check for errors where they occur (as distinct from the convention in other languages of throwing exceptions and sometimes catching them). In some cases this makes Go code verbose, but fortunately there are some techniques you can use to minimize repetitive error handling.

Consider an App Engine application with an HTTP handler that retrieves a record from the datastore and formats it with a template.

func init() {
    http.HandleFunc("/view", viewRecord)
}

func viewRecord(w http.ResponseWriter, r *http.Request) {
    c := appengine.NewContext(r)
    key := datastore.NewKey(c, "Record", r.FormValue("id"), 0, nil)
    record := new(Record)
    if err := datastore.Get(c, key, record); err != nil {
        http.Error(w, err.Error(), 500)
        return
    }
    if err := viewTemplate.Execute(w, record); err != nil {
        http.Error(w, err.Error(), 500)
    }
}

This function handles errors returned by the datastore.Get function and viewTemplate's Execute method. In both cases, it presents a simple error message to the user with the HTTP status code 500 ("Internal Server Error"). This looks like a manageable amount of code, but add some more HTTP handlers and you quickly end up with many copies of identical error handling code.

To reduce the repetition we can define our own HTTP appHandler type that includes an error return value:

type appHandler func(http.ResponseWriter, *http.Request) error

Then we can change our viewRecord function to return errors:

func viewRecord(w http.ResponseWriter, r *http.Request) error {
    c := appengine.NewContext(r)
    key := datastore.NewKey(c, "Record", r.FormValue("id"), 0, nil)
    record := new(Record)
    if err := datastore.Get(c, key, record); err != nil {
        return err
    }
    return viewTemplate.Execute(w, record)
}

This is simpler than the original version, but the http package doesn't understand functions that return error. To fix this we can implement the http.Handler interface's ServeHTTP method on appHandler:

func (fn appHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
    if err := fn(w, r); err != nil {
        http.Error(w, err.Error(), 500)
    }
}

The ServeHTTP method calls the appHandler function and displays the returned error (if any) to the user. Notice that the method's receiver, fn, is a function. (Go can do that!) The method invokes the function by calling the receiver in the expression fn(w, r).

Now when registering viewRecord with the http package we use the Handle function (instead of HandleFunc) as appHandler is an http.Handler (not an http.HandlerFunc).

func init() {
    http.Handle("/view", appHandler(viewRecord))
}

With this basic error handling infrastructure in place, we can make it more user friendly. Rather than just displaying the error string, it would be better to give the user a simple error message with an appropriate HTTP status code, while logging the full error to the App Engine developer console for debugging purposes.

To do this we create an appError struct containing an error and some other fields:

type appError struct {
    Error   error
    Message string
    Code    int
}

Next we modify the appHandler type to return *appError values:

type appHandler func(http.ResponseWriter, *http.Request) *appError

(It's usually a mistake to pass back the concrete type of an error rather than error, for reasons discussed in the Go FAQ, but it's the right thing to do here because ServeHTTP is the only place that sees the value and uses its contents.)

And make appHandler's ServeHTTP method display the appError's Message to the user with the correct HTTP status Code and log the full Error to the developer console:

func (fn appHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
    if e := fn(w, r); e != nil { // e is *appError, not os.Error.
        c := appengine.NewContext(r)
        c.Errorf("%v", e.Error)
        http.Error(w, e.Message, e.Code)
    }
}

Finally, we update viewRecord to the new function signature and have it return more context when it encounters an error:

func viewRecord(w http.ResponseWriter, r *http.Request) *appError {
    c := appengine.NewContext(r)
    key := datastore.NewKey(c, "Record", r.FormValue("id"), 0, nil)
    record := new(Record)
    if err := datastore.Get(c, key, record); err != nil {
        return &appError{err, "Record not found", 404}
    }
    if err := viewTemplate.Execute(w, record); err != nil {
        return &appError{err, "Can't display record", 500}
    }
    return nil
}

This version of viewRecord is the same length as the original, but now each of those lines has specific meaning and we are providing a friendlier user experience.

It doesn't end there; we can further improve the error handling in our application. Some ideas:

• give the error handler a pretty HTML template,

• make debugging easier by writing the stack trace to the HTTP response when the user is an administrator,

• write a constructor function for appError that stores the stack trace for easier debugging,

• recover from panics inside the appHandler, logging the error to the console as "Critical," while telling the user "a serious error has occurred." This is a nice touch to avoid exposing the user to inscrutable error messages caused by programming errors. See the Defer, Panic, and Recover article for more details.

Conclusion

Proper error handling is an essential requirement of good software. By employing the techniques described in this post you should be able to write more reliable and succinct Go code.