PMM-13658 fix updates error response

pkg/api/update/update.go

@@ -93,7 +93,8 @@ func (handle *Handler) Handle(w http.ResponseWriter, r *http.Request) {
 
 func handleError(w http.ResponseWriter, err error) {
 	if err != nil {
-		if errors.Is(err, &types.ValidationError{}) {
+		var validationErr *types.ValidationError
+		if errors.As(err, &validationErr) {
 			log.Warning(err)
 			w.WriteHeader(http.StatusPreconditionFailed)
 			w.Write([]byte(err.Error()))

pkg/api/update/update_test.go

@@ -0,0 +1,46 @@
+package update
+
+import (
+	"errors"
+	"github.com/containrrr/watchtower/pkg/types"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+)
+
+func TestHandleError(t *testing.T) {
+	tests := []struct {
+		name           string
+		error          error
+		expectedStatus int
+	}{
+		{
+			name:           "no error returns OK",
+			error:          nil,
+			expectedStatus: http.StatusOK,
+		},
+		{
+			name:           "validation error returns message",
+			error:          types.NewValidationError("no new image available"),
+			expectedStatus: http.StatusPreconditionFailed,
+		},
+		{
+			name:           "generic server error",
+			error:          errors.New("server error"),
+			expectedStatus: http.StatusInternalServerError,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			_ = httptest.NewRequest(http.MethodGet, "/v1/update", nil)
+			rr := httptest.NewRecorder()
+
+			handleError(rr, tt.error)
+
+			if status := rr.Result().StatusCode; status != tt.expectedStatus {
+				t.Errorf("the handler wrote status code %d, expected: %d", status, tt.expectedStatus)
+			}
+		})
+	}
+}