fix crash when unknown endpoint is called (#179)

internal/server/api.go

@@ -102,7 +102,7 @@ func (a *API) Complete() {
 }
 
 // ServeHTTP handles requests to the server
-func (a *API) ServeHTTP(_ http.ResponseWriter, r *http.Request) {
+func (a *API) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	data, err := io.ReadAll(r.Body)
 	if err != nil {
 		err = fmt.Errorf("failed to read body: %w", err)
@@ -122,6 +122,12 @@ func (a *API) ServeHTTP(_ http.ResponseWriter, r *http.Request) {
 		a.pushError(err)
 	}
 
+	if actual == nil {
+		// indicates the kind (endpoint) isn't implemented in decodeWrapper, so return a 501
+		w.WriteHeader(http.StatusNotImplemented)
+		return
+	}
+
 	if kind == "increment_metric" {
 		// Let's just output the metrics data and stop
 		a.outputRequestData(kind, actual)

internal/server/api_test.go

@@ -1,6 +1,8 @@
 package server
 
 import (
+	"net/http"
+	"net/http/httptest"
 	"testing"
 )
 
@@ -12,3 +14,17 @@ func Test_decodeWrapper(t *testing.T) {
 		}
 	})
 }
+
+func TestAPI_ServeHTTP(t *testing.T) {
+	t.Run("doesn't crash when unknown endpoint is used", func(t *testing.T) {
+		request := httptest.NewRequest("POST", "/unexpected-endpoint", nil)
+		response := httptest.NewRecorder()
+
+		api := NewAPI(nil, nil)
+		api.ServeHTTP(response, request)
+
+		if response.Code != http.StatusNotImplemented {
+			t.Errorf("expected status code %d, got %d", http.StatusNotImplemented, response.Code)
+		}
+	})
+}