Middleware Plugins for KrakenD API Gateway

package main

import (
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"net/url"
)

// MiddlewareRegisterer is the symbol the plugin loader will be looking for. It must
// implement the MiddlewareRegisterer interface
//   type MiddlewareRegisterer interface {
//     RegisterMiddlewares(func(
//       string,
//       func(map[string]interface{}, func(context.Context, interface{}) (interface{}, error)) func(context.Context, interface{}) (interface{}, error),
//     ))
//   }
var MiddlewareRegisterer = registerer("mw-example")

var (
	logger                Logger          = nil
	ctx                   context.Context = context.Background()
	errUnkownRequestType                  = errors.New("unknown request type")
	errUnkownResponseType                 = errors.New("unknown response type")
)

type registerer string

// RegisterMiddlewares is the function the plugin loader will call to register the
// middleware(s) contained in the plugin using the function passed as argument.
// f will register the factoryFunc under the name.
func (r registerer) RegisterMiddlewares(f func(
	name string,
	middlewareFactory func(map[string]interface{}, func(context.Context, interface{}) (interface{}, error)) func(context.Context, interface{}) (interface{}, error),
),
) {
	f(string(r), r.middlewareFactory)
}

func (r registerer) middlewareFactory(cfg map[string]interface{}, next func(context.Context, interface{}) (interface{}, error)) func(context.Context, interface{}) (interface{}, error) {
	// If the middleware requires configuration, declare it under its "config" key.
	// ex: the following example adds a parameter "option" that the plugin can use.
	/*
	   "extra_config":{
	       "plugin/middleware":{
	           "plugins":[
	               {
	                   "name":"mw-example",
	                   "config":{
	                       "option": "value"
	                   }
	               }
	           ]
	       }
	   }

	// Uncomment if you have configuration:
	config, err := r.parseConfig(cfg)
	if err != nil {
		logger.Debug(fmt.Sprintf("[PLUGIN: %s] Cannot initialize configuration: %s", r, err))
		return nil
	}
	*/

	// This is an example of registered ctx usage, it can be used to, for example, connect to external services.
	select {
	case <-ctx.Done():
		logger.Debug(fmt.Sprintf("[PLUGIN: %s] Context is done, skipping middleware injection", r))
		return nil
	default:
	}

	logger.Debug(fmt.Sprintf("[PLUGIN: %s] Middleware injected", r))
	return func(ctx context.Context, req interface{}) (interface{}, error) {
		reqw, ok := req.(RequestWrapper)
		if !ok {
			return nil, errUnkownRequestType
		}

		resp, err := next(ctx, requestWrapper{
			params:  reqw.Params(),
			headers: reqw.Headers(),
			body:    reqw.Body(),
			method:  reqw.Method(),
			url:     reqw.URL(),
			query:   reqw.Query(),
			// Modifies the path:
			path:    reqw.Path() + "/fooo",
		})
		respw, ok := resp.(ResponseWrapper)
		if !ok {
			return nil, errUnkownResponseType
		}

		// Add an "acme" field in the response.
		data := respw.Data()
		data["acme"] = true

		return responseWrapper{
			ctx:        respw.Context(),
			request:    respw.Request(),
			data:       data,
			isComplete: respw.IsComplete(),
			metadata: metadataWrapper{
				headers:    respw.Headers(),
				statusCode: respw.StatusCode(),
			},
			io: respw.Io(),
		}, err
	}
}

type config struct {
	Opt string `json:"option"`
}

// parseConfig parses the configuration marshaling and unmarshaling into a struct.
// The received map is the content of "config", so you can also check its fields manually
func (r registerer) parseConfig(extra map[string]interface{}) (*config, error) {
	cfg := config{}
	b, err := json.Marshal(extra)
	if err != nil {
		return nil, err
	}
	if err = json.Unmarshal(b, &cfg); err != nil {
		return nil, err
	}
	return &cfg, nil
}

// This logger is replaced by the RegisterLogger method to load the one from KrakenD
func (r registerer) RegisterLogger(in interface{}) {
	l, ok := in.(Logger)
	if !ok {
		return
	}
	logger = l
	logger.Debug(fmt.Sprintf("[PLUGIN: %s] Logger loaded", r))
}

type Logger interface {
	Debug(v ...interface{})
	Info(v ...interface{})
	Warning(v ...interface{})
	Error(v ...interface{})
	Critical(v ...interface{})
	Fatal(v ...interface{})
}

// Empty logger implementation
type noopLogger struct{}

func (n noopLogger) Debug(_ ...interface{})    {}
func (n noopLogger) Info(_ ...interface{})     {}
func (n noopLogger) Warning(_ ...interface{})  {}
func (n noopLogger) Error(_ ...interface{})    {}
func (n noopLogger) Critical(_ ...interface{}) {}
func (n noopLogger) Fatal(_ ...interface{})    {}

// RegisterContext saves the KrakenD application context so KrakenD can inject
// the context like we do with the logger
func (r registerer) RegisterContext(c context.Context) {
	ctx = c
	logger.Debug(fmt.Sprintf("[PLUGIN: %s] Context loaded", r))
}

func main() {}

// RequestWrapper is an interface for passing proxy request between the krakend pipe
// and the loaded plugins
type RequestWrapper interface {
	Context() context.Context
	Params() map[string]string
	Headers() map[string][]string
	Body() io.ReadCloser
	Method() string
	URL() *url.URL
	Query() url.Values
	Path() string
}

type requestWrapper struct {
	ctx     context.Context
	method  string
	url     *url.URL
	query   url.Values
	path    string
	body    io.ReadCloser
	params  map[string]string
	headers map[string][]string
}

func (r requestWrapper) Context() context.Context     { return r.ctx }
func (r requestWrapper) Method() string               { return r.method }
func (r requestWrapper) URL() *url.URL                { return r.url }
func (r requestWrapper) Query() url.Values            { return r.query }
func (r requestWrapper) Path() string                 { return r.path }
func (r requestWrapper) Body() io.ReadCloser          { return r.body }
func (r requestWrapper) Params() map[string]string    { return r.params }
func (r requestWrapper) Headers() map[string][]string { return r.headers }

type metadataWrapper struct {
	headers    map[string][]string
	statusCode int
}

func (m metadataWrapper) Headers() map[string][]string { return m.headers }
func (m metadataWrapper) StatusCode() int              { return m.statusCode }

// ResponseWrapper is an interface for passing proxy response between the krakend pipe
// and the loaded plugins
type ResponseWrapper interface {
	Context() context.Context
	Request() interface{}
	Data() map[string]interface{}
	IsComplete() bool
	Headers() map[string][]string
	StatusCode() int
	Io() io.Reader
}

type responseWrapper struct {
	ctx        context.Context
	request    interface{}
	data       map[string]interface{}
	isComplete bool
	metadata   metadataWrapper
	io         io.Reader
}

func (r responseWrapper) Context() context.Context     { return r.ctx }
func (r responseWrapper) Request() interface{}         { return r.request }
func (r responseWrapper) Data() map[string]interface{} { return r.data }
func (r responseWrapper) IsComplete() bool             { return r.isComplete }
func (r responseWrapper) Io() io.Reader                { return r.io }
func (r responseWrapper) Headers() map[string][]string { return r.metadata.headers }
func (r responseWrapper) StatusCode() int              { return r.metadata.statusCode }

{
  "version": 4,
  "extra_config": {
    "plugin": {
      "pattern": ".so",
      "folder": "/opt/krakend/plugins/"
    }
  },
  "endpoints": [
    {
      "endpoint": "/foo",
      "extra_config": {
        "plugin/middleware": {
          "plugins": [
            {
              "name": "mw-example",
              "config": {
                "option": "value"
              }
            }
          ]
        }
      },
      "backend": [
        {
          "host": ["http://localhost:8080"],
          "url_pattern": "/__debug/"
        }
      ]
    }
  ]
}