fix: propagate QR output failures

auth_test.go

@@ -12,10 +12,19 @@ import (
 	"os"
 	"path/filepath"
 	"strings"
+	"sync/atomic"
 	"testing"
 	"time"
 )
 
+var errQRWriterFailed = errors.New("QR writer failed")
+
+type failingQRWriter struct{}
+
+func (failingQRWriter) Write([]byte) (int, error) {
+	return 0, errQRWriterFailed
+}
+
 func TestParseServiceResponse(t *testing.T) {
 	var result struct {
 		Code int `json:"code"`
@@ -244,6 +253,40 @@ func TestLoginQRCoreFlow(t *testing.T) {
 	}
 }
 
+func TestLoginReturnsQROutputErrorBeforeLongPoll(t *testing.T) {
+	var longPollRequests atomic.Int32
+	var server *httptest.Server
+	server = httptest.NewServer(http.HandlerFunc(func(writer http.ResponseWriter, request *http.Request) {
+		switch request.URL.Path {
+		case "/serviceLogin":
+			_, _ = io.WriteString(writer, `&&&START&&&{"code":70016,"location":"`+server.URL+`/prepare"}`)
+		case "/loginUrl":
+			_, _ = io.WriteString(writer, `&&&START&&&{"code":0,"loginUrl":"https://qr.example/login","qr":"https://qr.example/image","lp":"`+server.URL+`/lp"}`)
+		case "/lp":
+			longPollRequests.Add(1)
+			_, _ = io.WriteString(writer, `&&&START&&&{"code":70016}`)
+		default:
+			http.NotFound(writer, request)
+		}
+	}))
+	defer server.Close()
+
+	client, err := NewClient(t.TempDir(), WithHTTPClient(server.Client()), WithQRWriter(failingQRWriter{}))
+	if err != nil {
+		t.Fatal(err)
+	}
+	client.serviceLoginURL = server.URL + "/serviceLogin"
+	client.loginURL = server.URL + "/loginUrl"
+
+	_, err = client.Login(context.Background())
+	if !errors.Is(err, errQRWriterFailed) {
+		t.Fatalf("Login() error = %v, want %v", err, errQRWriterFailed)
+	}
+	if requests := longPollRequests.Load(); requests != 0 {
+		t.Fatalf("long-poll requests = %d, want 0", requests)
+	}
+}
+
 func TestQRLoginRejectsIncompleteCallbackWithoutSaving(t *testing.T) {
 	var server *httptest.Server
 	server = httptest.NewServer(http.HandlerFunc(func(writer http.ResponseWriter, request *http.Request) {