feat(clint): add OnTransfer callback

Support external locking for auth transfer.

telegram/client.go

@@ -128,6 +128,9 @@ type Client struct {
 
 	// Tracing.
 	tracer trace.Tracer
+
+	// onTransfer is called on authorization transfer.
+	onTransfer func(ctx context.Context, tx func(ctx context.Context) error) error
 }
 
 // NewClient creates new unstarted client.
@@ -161,6 +164,7 @@ func NewClient(appID int, appHash string, opt Options) *Client {
 		migrationTimeout: opt.MigrationTimeout,
 		noUpdatesMode:    opt.NoUpdates,
 		mw:               opt.Middlewares,
+		onTransfer:       opt.OnTransfer,
 	}
 	if opt.TracerProvider != nil {
 		client.tracer = opt.TracerProvider.Tracer(oteltg.Name)

telegram/options.go

@@ -94,6 +94,14 @@ type Options struct {
 
 	// OpenTelemetry.
 	TracerProvider trace.TracerProvider
+
+	// OnTransfer is called on authorization transfer to acquire external lock.
+	//
+	// The function should call tx function to perform transfer, serializing it
+	// with external lock.
+	//
+	// The function must return error if tx function returned error.
+	OnTransfer func(ctx context.Context, tx func(context.Context) error) error
 }
 
 func (opt *Options) setDefaults() {
@@ -140,6 +148,9 @@ func (opt *Options) setDefaults() {
 			return nil
 		})
 	}
+	if opt.OnTransfer == nil {
+		opt.OnTransfer = noopOnTransfer
+	}
 }
 
 func defaultBackoff(c clock.Clock) func() backoff.BackOff {

telegram/transfer.go

@@ -17,20 +17,29 @@ func (c *Client) exportAuth(ctx context.Context, dcID int) (*tg.AuthExportedAuth
 	return export, nil
 }
 
+func noopOnTransfer(ctx context.Context, fn func(context.Context) error) error { return fn(ctx) }
+
 // transfer exports current authorization and imports it to another DC.
 // See https://core.telegram.org/api/datacenter#authorization-transfer.
 func (c *Client) transfer(ctx context.Context, to *tg.Client, dc int) (tg.AuthAuthorizationClass, error) {
-	auth, err := c.exportAuth(ctx, dc)
-	if err != nil {
-		return nil, errors.Wrapf(err, "export to %d", dc)
-	}
-
-	req := &tg.AuthImportAuthorizationRequest{}
-	req.FillFrom(auth)
-	r, err := to.AuthImportAuthorization(ctx, req)
-	if err != nil {
-		return nil, errors.Wrapf(err, "import from %d", dc)
+	var out tg.AuthAuthorizationClass
+	if err := c.onTransfer(ctx, func(ctx context.Context) error {
+		auth, err := c.exportAuth(ctx, dc)
+		if err != nil {
+			return errors.Wrapf(err, "export to %d", dc)
+		}
+
+		req := &tg.AuthImportAuthorizationRequest{}
+		req.FillFrom(auth)
+		r, err := to.AuthImportAuthorization(ctx, req)
+		if err != nil {
+			return errors.Wrapf(err, "import from %d", dc)
+		}
+
+		out = r
+		return nil
+	}); err != nil {
+		return nil, errors.Wrap(err, "onTransfer")
 	}
-
-	return r, nil
+	return out, nil
 }