fix: align in-memory authentication semantics

client.go

@@ -106,6 +106,9 @@ func NewClientWithAuthData(authData AuthData, options ...ClientOption) (*Client,
 	if err != nil {
 		return nil, err
 	}
+	if !authData.zero() {
+		authData = client.authDataWithIdentity(authData)
+	}
 	client.setAuthData(authData)
 	return client, nil
 }
@@ -131,8 +134,11 @@ func newClient(options ...ClientOption) (*Client, error) {
 	return client, nil
 }
 
-// WithAuthDataChanged configures serialized persistence for in-memory auth updates.
-// The callback runs under login serialization, but never while the auth data lock is held.
+// WithAuthDataChanged configures synchronous persistence for in-memory auth updates.
+// The callback is serialized by the client's login lock and may acquire unrelated
+// application locks, but it must not call Client methods other than AuthData.
+// In particular, calling Login or another method that may refresh authentication
+// will deadlock. Returning an error leaves the client's authentication unchanged.
 func WithAuthDataChanged(callback func(AuthData) error) ClientOption {
 	return func(client *Client) error {
 		if callback == nil {

memory_auth_test.go

@@ -10,6 +10,7 @@ import (
 	"path/filepath"
 	"reflect"
 	"strings"
+	"sync"
 	"testing"
 )
 
@@ -47,6 +48,31 @@ func TestNewClientWithAuthDataRejectsPartialData(t *testing.T) {
 	}
 }
 
+func TestNewClientWithAuthDataGeneratesMissingIdentityWithoutMutatingInput(t *testing.T) {
+	input := completeAuthData()
+	input.DeviceID = ""
+	input.PassO = ""
+
+	client, err := NewClientWithAuthData(input)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if input.DeviceID != "" || input.PassO != "" {
+		t.Fatalf("caller auth data mutated: %#v", input)
+	}
+	stored := client.AuthData()
+	if stored.DeviceID == "" || stored.PassO == "" {
+		t.Fatalf("stored identity not generated: %#v", stored)
+	}
+}
+
+func TestWithAuthDataChangedRejectsNil(t *testing.T) {
+	_, err := NewClientWithAuthData(AuthData{}, WithAuthDataChanged(nil))
+	if err == nil || !strings.Contains(err.Error(), "must not be nil") {
+		t.Fatalf("error = %v, want nil callback error", err)
+	}
+}
+
 func TestNewClientWithAuthDataDoesNotAccessFilesystem(t *testing.T) {
 	home := filepath.Join(t.TempDir(), "must-not-exist")
 	t.Setenv("HOME", home)
@@ -110,6 +136,43 @@ func TestMemoryRefreshCallbackCanReadCurrentAuth(t *testing.T) {
 	}
 }
 
+func TestMemoryRefreshCallbackCanAcquireApplicationLock(t *testing.T) {
+	var persistenceMu sync.Mutex
+	client, _, server := newMemoryRefreshClient(t, func(AuthData) error {
+		persistenceMu.Lock()
+		defer persistenceMu.Unlock()
+		return nil
+	})
+	defer server.Close()
+
+	if err := client.refreshToken(context.Background()); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestMemoryRefreshPersistsGeneratedIdentity(t *testing.T) {
+	initial := completeAuthData()
+	initial.DeviceID = ""
+	initial.PassO = ""
+	var changed AuthData
+	client, err := NewClientWithAuthData(initial, WithAuthDataChanged(func(authData AuthData) error {
+		changed = authData.clone()
+		return nil
+	}))
+	if err != nil {
+		t.Fatal(err)
+	}
+	server := configureRefreshServer(t, client)
+	defer server.Close()
+
+	if err := client.refreshToken(context.Background()); err != nil {
+		t.Fatal(err)
+	}
+	if changed.DeviceID == "" || changed.PassO == "" {
+		t.Fatalf("persisted identity not generated: %#v", changed)
+	}
+}
+
 func TestFreshMemoryClientQRLoginPersistsThroughCallback(t *testing.T) {
 	var changed AuthData
 	var server *httptest.Server