Implement HTTP middlewares for EventsAPIServer

CHANGELOG.md

@@ -5,7 +5,9 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
-Nothing so far
+- Add new `Config.EventsAPIConfig.Middlewar` configuration and corresponding `WithMiddleware(…)` option.
+  This fixes #19 by allowing the user to inject a custom HTTP middleware for the
+  `EventsAPIServer`, e.g. in order to enable custom (health) endpoints.
 
 ## [v2.1.0] - 2020-07-25
 - Add new `EventsAPIAdapter` function to support integrating with Slack via the 

events_api.go

@@ -68,9 +68,14 @@ func NewEventsAPIServer(ctx context.Context, listenAddr string, conf Config) (*E
 		},
 	))
 
+	var handler http.Handler = http.HandlerFunc(a.httpHandler)
+	if conf.EventsAPI.Middleware != nil {
+		handler = conf.EventsAPI.Middleware(handler)
+	}
+
 	a.http = &http.Server{
 		Addr:         listenAddr,
-		Handler:      http.HandlerFunc(a.httpHandler),
+		Handler:      handler,
 		ErrorLog:     zap.NewStdLog(conf.Logger),
 		TLSConfig:    conf.EventsAPI.TLSConf,
 		ReadTimeout:  conf.EventsAPI.ReadTimeout,

events_api_test.go

@@ -4,9 +4,11 @@ import (
 	"bytes"
 	"context"
 	"encoding/json"
+	"fmt"
 	"io"
 	"net/http"
 	"net/http/httptest"
+	"strings"
 	"testing"
 
 	"github.com/go-joe/joe"
@@ -16,14 +18,21 @@ import (
 	"github.com/slack-go/slack/slackevents"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
+	"go.uber.org/zap"
 	"go.uber.org/zap/zaptest"
+	"go.uber.org/zap/zaptest/observer"
 )
 
-func newTestEventsAPIServer(t *testing.T) (_ *EventsAPIServer, finish func() (events []interface{})) {
+func newTestEventsAPIServer(t *testing.T, optionalConf ...Config) (_ *EventsAPIServer, finish func() (events []interface{})) {
+	var conf Config
+	if len(optionalConf) > 0 {
+		conf = optionalConf[0]
+	}
+
 	ctx := context.Background()
-	conf := Config{
-		Logger: zaptest.NewLogger(t),
-		Debug:  true,
+	conf.Debug = true
+	if conf.Logger == nil {
+		conf.Logger = zaptest.NewLogger(t)
 	}
 
 	slackAPI := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
@@ -153,6 +162,35 @@ func TestEventsAPIServer_HandleReactionAddedEvent(t *testing.T) {
 	assert.Equal(t, "+1", actual.Reaction.Shortcode)
 }
 
+func TestEventsAPIServer_HTTPHandlerMiddleware(t *testing.T) {
+	middleware := func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
+			if req.URL.Path == "/health" {
+				_, _ = fmt.Fprint(w, "Everything is fine!")
+				return
+			}
+
+			next.ServeHTTP(w, req)
+		})
+	}
+
+	obs, errorLogs := observer.New(zap.ErrorLevel)
+	s, _ := newTestEventsAPIServer(t, Config{
+		Logger: zap.New(obs),
+		EventsAPI: EventsAPIConfig{
+			Middleware: middleware,
+		},
+	})
+
+	req := httptest.NewRequest("GET", "/health", strings.NewReader("Are you okay?"))
+	resp := httptest.NewRecorder()
+
+	s.http.Handler.ServeHTTP(resp, req)
+
+	assert.Equal(t, "Everything is fine!", resp.Body.String())
+	assert.Empty(t, errorLogs.All())
+}
+
 func toJSON(req interface{}) io.Reader {
 	b := new(bytes.Buffer)
 	err := json.NewEncoder(b).Encode(req)

options.go

@@ -3,6 +3,7 @@ package slack
 import (
 	"crypto/tls"
 	"errors"
+	"net/http"
 	"time"
 
 	"github.com/slack-go/slack"
@@ -38,6 +39,7 @@ type Config struct {
 
 // EventsAPIConfig contains the configuration of an EventsAPIServer.
 type EventsAPIConfig struct {
+	Middleware        func(next http.Handler) http.Handler
 	ShutdownTimeout   time.Duration
 	ReadTimeout       time.Duration
 	WriteTimeout      time.Duration
@@ -167,3 +169,12 @@ func WithWriteTimeout(d time.Duration) Option {
 		return nil
 	}
 }
+
+// WithMiddleware is an option for the EventsAPIServer that allows the user to
+// inject an HTTP middleware to the HTTP server.
+func WithMiddleware(mw func(next http.Handler) http.Handler) Option {
+	return func(conf *Config) error {
+		conf.EventsAPI.Middleware = mw
+		return nil
+	}
+}

options_test.go

@@ -1,6 +1,7 @@
 package slack
 
 import (
+	"net/http"
 	"testing"
 
 	"github.com/go-joe/joe"
@@ -85,3 +86,21 @@ func TestWithListenPassive(t *testing.T) {
 	require.NoError(t, err)
 	assert.Equal(t, true, conf.ListenPassive)
 }
+
+func TestWithMiddleware(t *testing.T) {
+	var ok bool
+	middleware := func(next http.Handler) http.Handler {
+		ok = true // proof this function executed
+		return next
+	}
+
+	conf, err := newConf("my-secret-token", joeConf(t), []Option{
+		WithMiddleware(middleware),
+	})
+
+	require.NoError(t, err)
+	require.NotNil(t, conf.EventsAPI.Middleware)
+
+	conf.EventsAPI.Middleware(nil)
+	assert.True(t, ok)
+}