fix: stop QR output after write failure

auth.go

@@ -258,8 +258,11 @@ type recordingWriter struct {
 }
 
 func (writer *recordingWriter) Write(payload []byte) (int, error) {
+	if writer.err != nil {
+		return 0, writer.err
+	}
 	written, err := writer.w.Write(payload)
-	if err != nil && writer.err == nil {
+	if err != nil {
 		writer.err = err
 	}
 	return written, err

auth_test.go

@@ -15,15 +15,52 @@ import (
 	"sync/atomic"
 	"testing"
 	"time"
+
+	"github.com/mdp/qrterminal/v3"
 )
 
 var errQRWriterFailed = errors.New("QR writer failed")
 
-type failingQRWriter struct{}
+type failThenBlockWriter struct {
+	calls       atomic.Int32
+	failOnCall  int32
+	blockWrites chan struct{}
+}
 
-func (failingQRWriter) Write([]byte) (int, error) {
+func (writer *failThenBlockWriter) Write(payload []byte) (int, error) {
+	call := writer.calls.Add(1)
+	if call == writer.failOnCall {
 		return 0, errQRWriterFailed
 	}
+	if call > writer.failOnCall {
+		<-writer.blockWrites
+	}
+	return len(payload), nil
+}
+
+func TestRecordingWriterStopsAfterFirstError(t *testing.T) {
+	underlying := &failThenBlockWriter{failOnCall: 1, blockWrites: make(chan struct{})}
+	writer := &recordingWriter{w: underlying}
+	done := make(chan struct{})
+	go func() {
+		qrterminal.GenerateHalfBlock("https://qr.example/login", qrterminal.L, writer)
+		close(done)
+	}()
+
+	select {
+	case <-done:
+	case <-time.After(time.Second):
+		close(underlying.blockWrites)
+		<-done
+		t.Fatal("GenerateHalfBlock blocked after the first write failure")
+	}
+	if calls := underlying.calls.Load(); calls != 1 {
+		t.Fatalf("underlying Write calls = %d, want 1", calls)
+	}
+	if !errors.Is(writer.err, errQRWriterFailed) {
+		t.Fatalf("recorded error = %v, want %v", writer.err, errQRWriterFailed)
+	}
+}
 
 func TestParseServiceResponse(t *testing.T) {
 	var result struct {
@@ -308,6 +345,7 @@ func TestLoginQRCoreFlow(t *testing.T) {
 
 func TestLoginReturnsQROutputErrorBeforeLongPoll(t *testing.T) {
 	var longPollRequests atomic.Int32
+	qrWriter := &failThenBlockWriter{failOnCall: 2, blockWrites: make(chan struct{})}
 	var server *httptest.Server
 	server = httptest.NewServer(http.HandlerFunc(func(writer http.ResponseWriter, request *http.Request) {
 		switch request.URL.Path {
@@ -324,14 +362,25 @@ func TestLoginReturnsQROutputErrorBeforeLongPoll(t *testing.T) {
 	}))
 	defer server.Close()
 
-	client, err := NewClient(t.TempDir(), WithHTTPClient(server.Client()), WithQRWriter(failingQRWriter{}))
+	client, err := NewClient(t.TempDir(), WithHTTPClient(server.Client()), WithQRWriter(qrWriter))
 	if err != nil {
 		t.Fatal(err)
 	}
 	client.serviceLoginURL = server.URL + "/serviceLogin"
 	client.loginURL = server.URL + "/loginUrl"
 
-	_, err = client.Login(context.Background())
+	done := make(chan error, 1)
+	go func() {
+		_, loginErr := client.Login(context.Background())
+		done <- loginErr
+	}()
+	select {
+	case err = <-done:
+	case <-time.After(time.Second):
+		close(qrWriter.blockWrites)
+		<-done
+		t.Fatal("Login() blocked after QR output failed")
+	}
 	if !errors.Is(err, errQRWriterFailed) {
 		t.Fatalf("Login() error = %v, want %v", err, errQRWriterFailed)
 	}