Add problem library

README.md

@@ -1,4 +1,5 @@
 # PlayNet Libs
+
 [![Go Report Card](https://goreportcard.com/badge/github.com/playnet-public/libs)](https://goreportcard.com/report/github.com/playnet-public/libs)
 [![License: GPL v3](https://img.shields.io/badge/License-GPL%20v3-blue.svg)](https://www.gnu.org/licenses/gpl-3.0)
 [![Build Status](https://travis-ci.org/playnet-public/libs.svg?branch=master)](https://travis-ci.org/playnet-public/libs)
@@ -9,7 +10,8 @@ The repository containing various shared libs for the entire playnet project.
 ## Libs
 
 ### Logging
-Our logging setup using go.uber.org/zap.
+
+Our logging setup using `go.uber.org/zap`.
 Sentry and Jaeger are being added for production environments.
 
 ```go
@@ -27,6 +29,46 @@ If you provide a zap.Error tag, the related stacktrace will also be attached.
 
 Additionally there is a tracer(opentracing/jaeger) available in the logger which should be closed before exiting main.
 
+### Problems
+
+`problems` is a small library which implements the RFC7807 error response format standard for e.g. HTTP API's.
+
+The `problems` lib provides a struct called `Problem` and an interface called `ProblemInfo`.
+`Problem` implements the `error` interface, so you can simply return the problem as an error in your application.
+
+```go
+func returnError() error {
+    return problems.New()
+}
+```
+
+You can marshal the `Problem` struct to JSON:
+
+```go
+func makeJSON(problem *Problem) ([]byte, error) {
+    return json.Marshal(&problem)
+}
+```
+
+If you want to define a HTTP problem, you can do it like this:
+
+```go
+var problemNotFound = problems.New().SetTitle("NotFound").SetDetail("The requested url was not found.").SetStatus(404).SetType("https://example.com/problem/description")
+```
+
+For custom fields for more detailed problem information, you can implement `Problem` in your custom struct:
+
+```go
+type customInfo struct {
+    AdditionalField1 bool `json:"additional_field_1"`
+}
+
+type customProblem struct {
+    *problems.Problem
+    *customInfo
+}
+```
+
 ## Contributions
 
 Pull Requests and Issue Reports are welcome.

problems/problems.go

@@ -0,0 +1,147 @@
+package errors
+
+import (
+	"fmt"
+
+	"github.com/pkg/errors"
+)
+
+//DefaultType implements the default type content described in RFC 7807
+const DefaultType = "about:blank"
+
+//Problem implements the RFC 7807 "problem"/error standard
+//Additional error field can be defined in custom struct implementing the Problem struct
+type Problem struct {
+	Title    string `json:"title"`
+	Detail   string `json:"detail,omitempty"`
+	Type     string `json:"type,omitempty"`
+	Instance string `json:"instance,omitempty"`
+	Status   int    `json:"status,omitempty"`
+	cause    error
+}
+
+//Wrap is an alias to github.com/pkg/errors Wrap function. If Problem pointer passed as error, it sets the error and a new message of Problem.
+func Wrap(err error, msg string) error {
+	if err == nil {
+		return nil
+	}
+
+	problem, ok := err.(*Problem)
+	if !ok {
+		return errors.Wrap(err, msg)
+	}
+
+	problem.cause = errors.Wrap(problem.cause, msg)
+	return problem
+}
+
+//WithStack is an alias to github.com/pkg/errors WithStack function. If Problem pointer passed as error, it sets the error of Problem.
+func WithStack(err error) error {
+	if err == nil {
+		return nil
+	}
+
+	problem, ok := err.(*Problem)
+	if !ok {
+		return errors.WithStack(err)
+	}
+
+	problem.cause = errors.WithStack(problem.cause)
+	return problem
+}
+
+//WithMessage is an alias to github.com/pkg/errors WithMessage function. If Problem pointer passed as error, it sets the error, stacktrace and message of Problem.
+func WithMessage(err error, msg string) error {
+	if err == nil {
+		return nil
+	}
+
+	problem, ok := err.(*Problem)
+	if !ok {
+		return errors.WithMessage(err, msg)
+	}
+
+	problem.cause = errors.WithMessage(problem.cause, msg)
+	return problem
+}
+
+//New creates a new Problem with all object details described in RFC 7807
+func New(title, detail string, status int) error {
+	return newWithError(
+		errors.New(detail),
+		title,
+		detail,
+		status,
+	)
+}
+
+func newWithError(err error, title, detail string, status int) *Problem {
+	return &Problem{
+		Type:   DefaultType,
+		Title:  title,
+		Detail: err.Error(),
+		Status: status,
+		cause:  err,
+	}
+}
+
+//Format displays the contained error in a specific format
+func (p Problem) Format(s fmt.State, verb rune) {
+	if p.cause == nil {
+		fmt.Fprint(s, p.Detail)
+		return
+	}
+
+	formatter, ok := p.cause.(fmt.Formatter)
+	if !ok {
+		fmt.Fprint(s, p.Detail)
+		return
+	}
+
+	formatter.Format(s, verb)
+}
+
+//Cause returns the outermost error of Problem
+func (p Problem) Cause() error {
+	return p.cause
+}
+
+//Error returns the error of the problem
+func (p Problem) Error() string {
+	return p.cause.Error()
+}
+
+//SetTitle sets the title field of the problem, specified in RFC 7807
+func (p Problem) SetTitle(title string) *Problem {
+	p.Title = title
+	return &p
+}
+
+//SetDetail sets the detail field of the problem, specified in RFC 7807
+func (p Problem) SetDetail(detail string) *Problem {
+	p.Detail = detail
+	return &p
+}
+
+//SetType sets the detail field of the problem, specified in RFC 7807
+func (p Problem) SetType(t string) *Problem {
+	if t == "" {
+		p.Type = DefaultType
+	} else {
+		p.Type = t
+	}
+
+	return &p
+}
+
+//SetInstance sets the instance field of the problem, specified in RFC 7807
+func (p Problem) SetInstance(instance string) *Problem {
+	p.Instance = instance
+	return &p
+}
+
+//SetStatus sets the status field of the problem, specified in RFC 7807
+func (p Problem) SetStatus(status int) *Problem {
+	p.Status = status
+	return &p
+}