Ensure Interceptor request uses HTTP POST.

Fixes #437

docs/eventlisteners.md

@@ -122,7 +122,7 @@ contains the default values defined in
 To access logs for the EventListener sink, you can query for pods with the
 `eventlistener` label set to the name of your EventListener resource:
 
-```shell script
+```shell
 kubectl get pods --selector eventlistener=my-eventlistener
 ```
 
@@ -186,6 +186,7 @@ To be an Event Interceptor, a Kubernetes object should:
 
 - Be fronted by a regular Kubernetes v1 Service over port 80
 - Accept JSON payloads over HTTP
+- Accept HTTP POST requests with JSON payloads.
 - Return a HTTP 200 OK Status if the EventListener should continue processing
   the event
 - Return a JSON body back. This will be used by the EventListener as the event

pkg/sink/sink.go

@@ -163,6 +163,7 @@ func (r Sink) executeInterceptors(t *triggersv1.EventListenerTrigger, in *http.R
 
 	// The request body to the first interceptor in the chain should be the received event body.
 	request := &http.Request{
+		Method: http.MethodPost,
 		Header: in.Header,
 		Body:   ioutil.NopCloser(bytes.NewBuffer(event)),
 	}
@@ -190,6 +191,7 @@ func (r Sink) executeInterceptors(t *triggersv1.EventListenerTrigger, in *http.R
 		// Set the next request to be the output of the last response to enable
 		// request chaining.
 		request = &http.Request{
+			Method: http.MethodPost,
 			Header: resp.Header,
 			Body:   ioutil.NopCloser(resp.Body),
 		}

pkg/sink/sink_test.go

@@ -643,6 +643,10 @@ type sequentialInterceptor struct {
 
 func (f *sequentialInterceptor) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	f.called = true
+	if r.Method != http.MethodPost {
+		w.WriteHeader(http.StatusMethodNotAllowed)
+		return
+	}
 	var data map[string]int
 	if err := json.NewDecoder(r.Body).Decode(&data); err != nil {
 		w.WriteHeader(http.StatusInternalServerError)
@@ -697,25 +701,39 @@ func TestExecuteInterceptor(t *testing.T) {
 		Interceptors: []*triggersv1.EventInterceptor{a, a},
 	}
 
-	req, err := http.NewRequest(http.MethodPost, "/", nil)
-	if err != nil {
-		t.Fatalf("http.NewRequest: %v", err)
-	}
-	resp, header, err := r.executeInterceptors(trigger, req, []byte(`{}`), "", logger)
-	if err != nil {
-		t.Fatalf("executeInterceptors: %v", err)
-	}
-
-	var got map[string]int
-	if err := json.Unmarshal(resp, &got); err != nil {
-		t.Fatalf("json.Unmarshal: %v", err)
-	}
-	want := map[string]int{"i": 2}
-	if diff := cmp.Diff(want, got); diff != "" {
-		t.Errorf("Body: -want +got: %s", diff)
-	}
-	if diff := cmp.Diff([]string{"1", "2"}, header["Foo"]); diff != "" {
-		t.Errorf("Header: -want +got: %s", diff)
+	for _, method := range []string{
+		http.MethodGet,
+		http.MethodHead,
+		http.MethodPost,
+		http.MethodPut,
+		http.MethodPatch,
+		http.MethodDelete,
+		http.MethodConnect,
+		http.MethodOptions,
+		http.MethodTrace,
+	} {
+		t.Run(method, func(t *testing.T) {
+			req, err := http.NewRequest(method, "/", nil)
+			if err != nil {
+				t.Fatalf("http.NewRequest: %v", err)
+			}
+			resp, header, err := r.executeInterceptors(trigger, req, []byte(`{}`), "", logger)
+			if err != nil {
+				t.Fatalf("executeInterceptors: %v", err)
+			}
+
+			var got map[string]int
+			if err := json.Unmarshal(resp, &got); err != nil {
+				t.Fatalf("json.Unmarshal: %v", err)
+			}
+			want := map[string]int{"i": 2}
+			if diff := cmp.Diff(want, got); diff != "" {
+				t.Errorf("Body: -want +got: %s", diff)
+			}
+			if diff := cmp.Diff([]string{"1", "2"}, header["Foo"]); diff != "" {
+				t.Errorf("Header: -want +got: %s", diff)
+			}
+		})
 	}
 }