Errors

Package is a fork of https://github.com/pkg/errors with additional functions for improving the relationship between structured logging and error handling in go.

Adding structured context to an error

Wraps the original error while providing structured context data

_, err := ioutil.ReadFile(fileName)
if err != nil {
        return errors.WithContext{"file": fileName}.Wrap(err, "read failed")
}

Retrieving the structured context

Using errors.WithContext{} stores the provided context for later retrieval by upstream code or structured logging systems

// Pass to logrus as structured logging
logrus.WithFields(errors.ToLogrus(err)).Error("open file error")

Stack information on the source of the error is also included

context := errors.ToMap(err)
context == map[string]interface{}{
      "file": "my-file.txt",
      "go-func": "loadFile()",
      "go-line": 146,
      "go-file": "with_context_example.go"
}

Conforms to the Causer interface

Errors wrapped with errors.WithContext{} are compatible with errors wrapped by github.com/pkg/errors

switch err := errors.Cause(err).(type) {
case *MyError:
        // handle specifically
default:
        // unknown error
}

Proper Usage

The context wrapped by errors.WithContext{} is not intended to be used to by code to decide how an error should be handled. It is a convenience where the failure is well known, but the context is dynamic. In other words, you know the database returned an unrecoverable query error, but creating a new error type with the details of each query error is overkill ErrorFetchPage{}, ErrorFetchAll{}, ErrorFetchAuthor{}, etc...

As an example

func (r *Repository) FetchAuthor(isbn string) (Author, error) {
    // Returns ErrorNotFound{} if not exist
    book, err := r.fetchBook(isbn)
    if err != nil {
        return nil, errors.WithContext{"isbn": isbn}.Wrap(err, "while fetching book")
    }
    // Returns ErrorNotFound{} if not exist
    author, err := r.fetchAuthorByBook(book)
    if err != nil {
        return nil, errors.WithContext{"book": book}.Wrap(err, "while fetching author")
    }
    return author, nil
}

You should continue to create and inspect error types

type ErrorAuthorNotFound struct {}

func isNotFound(err error) {
    _, ok := err.(*ErrorAuthorNotFound)
    return ok
}

func main() {
    r := Repository{}
    author, err := r.FetchAuthor("isbn-213f-23422f52356")
    if err != nil {
        // Fetch the original Cause() and determine if the error is recoverable
        if isNotFound(error.Cause(err)) {
                author, err := r.AddBook("isbn-213f-23422f52356", "charles", "darwin")
        }
        if err != nil {
                logrus.WithFields(errors.ToLogrus(err)).Errorf("while fetching author - %s", err)
                os.Exit(1)
        }
    }
    fmt.Printf("Author %+v\n", author)
}

Context for concrete error types

If the error implements the errors.HasContext interface the context can be retrieved

context, ok := err.(errors.HasContext)
if ok {
    fmt.Println(context.Context())
}

This makes it easy for error types to provide their context information.

type ErrorBookNotFound struct {
   ISBN string
}
// Implements the `HasContext` interface
func (e *ErrorBookNotFound) func Context() map[string]interface{} {
   return map[string]interface{}{
       "isbn": e.ISBN,
   }
}

Now we can create the error and logrus knows how to retrieve the context

func (* Repository) FetchBook(isbn string) (*Book, error) {
    var book Book
    err := r.db.Query("SELECT * FROM books WHERE isbn = ?").One(&book)
    if err != nil {
        return nil, ErrorBookNotFound{ISBN: isbn}
    }
}

func main() {
    r := Repository{}
    book, err := r.FetchBook("isbn-213f-23422f52356")
    if err != nil {
        logrus.WithFields(errors.ToLogrus(err)).Errorf("while fetching book - %s", err)
        os.Exit(1)
    }
    fmt.Printf("Book %+v\n", book)
}

A Complete example

The following is a complete example using http://github.com/mailgun/logrus-hooks/kafkahook to marshal the context into ES fields.

package main

import (
    "github.com/mailgun/holster/errors"
    "github.com/mailgun/logrus-hooks/kafkahook"
    "github.com/sirupsen/logrus"
    "log"
    "io/ioutil"
)

func OpenWithError(fileName string) error {
    _, err := ioutil.ReadFile(fileName)
    if err != nil {
            // pass the filename up via the error context
            return errors.WithContext{
                "file": fileName,
            }.Wrap(err, "read failed")
    }
    return nil
}

func main() {
    // Init the kafka hook logger
    hook, err := kafkahook.New(kafkahook.Config{
        Endpoints: []string{"kafka-n01", "kafka-n02"},
        Topic:     "udplog",
    })
    if err != nil {
        log.Fatal(err)
    }

    // Add the hook to logrus
    logrus.AddHook(hook)

    // Create an error and log it
    if err := OpenWithError("/tmp/non-existant.file"); err != nil {
        // This log line will show up in ES with the additional fields
        //
        // excText: "read failed"
        // excValue: "read failed: open /tmp/non-existant.file: no such file or directory"
        // excType: "*errors.WithContext"
        // filename: "/src/to/main.go"
        // funcName: "main()"
        // lineno: 25
        // context.file: "/tmp/non-existant.file"
        // context.domain.id: "some-id"
        // context.foo: "bar"
        logrus.WithFields(logrus.Fields{
            "domain.id": "some-id",
            "foo": "bar",
            "err": err,
        }).Error("log messge")
    }
}