有没有办法检查是否有人收听 dbus 信号？

Question

有没有办法检查 DBus 中的侦听客户端？

有可能吗？我正在使用 gdbus。

背景

我正在创建与串行端口接口的服务，我想在有人监听时隐式打开串行端口，并在最后一个客户端断开连接时自动关闭它。我可以用 open/close 方法来做到这一点，但是存在一个客户端在另一个客户端仍在监听时关闭连接的风险。

我的问题的另一个解决方案是连接计数，但也存在客户端忘记关闭端口或崩溃的风险。

您还有其他实现方法吗？

我的代码（缩短）

基于： https://github.com/bratsche/glib/blob/master/gio/tests/gdbus-example-server.c

#include <gio/gio.h>
#include <stdlib.h>

#ifdef G_OS_UNIX
#include <unistd.h>
#endif

/* ---------------------------------------------------------------------------------------------------- */

static GDBusNodeInfo *introspection_data = NULL;

/* Introspection data for the service we are exporting */
static const gchar introspection_xml[] =
  "<node>"
  "  <interface name='info.skorepa.serial.port'>"
  "    <signal name='DataRecieved'>"
  "      <arg type='ay' name='data'/>"
  "    </signal>"
  "  </interface>"
  "</node>";

/* ---------------------------------------------------------------------------------------------------- */

static void
handle_method_call (GDBusConnection       *connection,
                    const gchar           *sender,
                    const gchar           *object_path,
                    const gchar           *interface_name,
                    const gchar           *method_name,
                    GVariant              *parameters,
                    GDBusMethodInvocation *invocation,
                    gpointer               user_data)
{
  // nothing - signal only
}

static GVariant *
handle_get_property (GDBusConnection  *connection,
                     const gchar      *sender,
                     const gchar      *object_path,
                     const gchar      *interface_name,
                     const gchar      *property_name,
                     GError          **error,
                     gpointer          user_data)
{
  // nothing - signal only
}

static gboolean
handle_set_property (GDBusConnection  *connection,
                     const gchar      *sender,
                     const gchar      *object_path,
                     const gchar      *interface_name,
                     const gchar      *property_name,
                     GVariant         *value,
                     GError          **error,
                     gpointer          user_data)
{
  // nothing - no properties
}


/* for now */
static const GDBusInterfaceVTable interface_vtable =
{
  handle_method_call,
  handle_get_property,
  handle_set_property
};

/* ---------------------------------------------------------------------------------------------------- */
// Here I emit signal - for now I just emit every 2 seconds
static gboolean
on_timeout_cb (gpointer user_data)
{
  GDBusConnection *connection = G_DBUS_CONNECTION (user_data);
  GVariantBuilder *builder;
  GVariantBuilder *invalidated_builder;
  GError *error;

  error = NULL;
  printf("Constructing array\n");
  builder = g_variant_builder_new (G_VARIANT_TYPE ("ay"));
  printf("Adding 65\n");
  g_variant_builder_add (builder,
                         "y",
                         65);
  printf("Adding 66\n");
  g_variant_builder_add (builder,
                         "y",
                         66);
  printf("Emitting signal\n");
  g_dbus_connection_emit_signal (connection,
                                 NULL,
                                 "/info/skorepa/TestObject",
                                 "info.skorepa.serial.port",
                                 "DataRecieved",
                                 g_variant_new ("(ay)",
                                                builder),
                                 &error);
  printf("Checking for errors\n");
  g_assert_no_error (error);


  return TRUE;
}

/* ---------------------------------------------------------------------------------------------------- */

static void
on_bus_acquired (GDBusConnection *connection,
                 const gchar     *name,
                 gpointer         user_data)
{
  guint registration_id;

  registration_id = g_dbus_connection_register_object (connection,
                                                       "/info/skorepa/TestObject",
                                                       introspection_data->interfaces[0],
                                                       &interface_vtable,
                                                       NULL,  /* user_data */
                                                       NULL,  /* user_data_free_func */
                                                       NULL); /* GError** */
  g_assert (registration_id > 0);

  /* swap value of properties Foo and Bar every two seconds */
  g_timeout_add_seconds (2,
                         on_timeout_cb,
                         connection);
}

static void
on_name_acquired (GDBusConnection *connection,
                  const gchar     *name,
                  gpointer         user_data)
{
}

static void
on_name_lost (GDBusConnection *connection,
              const gchar     *name,
              gpointer         user_data)
{
  exit (1);
}

int
main (int argc, char *argv[])
{
  guint owner_id;
  GMainLoop *loop;

  g_type_init ();

  introspection_data = g_dbus_node_info_new_for_xml (introspection_xml, NULL);
  g_assert (introspection_data != NULL);

  owner_id = g_bus_own_name (G_BUS_TYPE_SESSION,
                             "info.skorepa.serial",
                             G_BUS_NAME_OWNER_FLAGS_NONE,
                             on_bus_acquired,
                             on_name_acquired,
                             on_name_lost,
                             NULL,
                             NULL);

  loop = g_main_loop_new (NULL, FALSE);
  g_main_loop_run (loop);

  g_bus_unown_name (owner_id);

  g_dbus_node_info_unref (introspection_data);

  return 0;
}

编译使用：

gcc signal-sample.c `pkg-config --cflags --libs glib-2.0 gio-2.0` -o test

谢谢

Answer 1

Is there a way to check for listening clients in DBus?

不，这是不可能的，因为 D-Bus 的设计方式。

当客户端想要订阅信号时，他们会向 D-Bus 守护进程发送一个 AddMatch method call，后者会在内部注册该状态。当您的服务发出信号时，它会将信号发送到 D-Bus 守护进程，然后将其转发给已订阅该信号的客户端（遵守有关广播和权限的各种策略规则）。您的服务无法了解 D-Bus 守护进程中的内部订阅状态。

处理这种事情的模式是让您的服务显式公开 subscribe 或 open 客户端必须调用的方法才能打开串行端口。您可以使用第二种方法在客户端完成后关闭串行端口；您还可以侦听客户端断开连接以自动关闭端口。（在 GDBus 中，这是通过使用 g_bus_watch_name() 并将客户端的唯一名称传递给它来完成的，它看起来像 :1.5。）

有没有办法检查是否有人收听 dbus 信号？

Is there a way to check if someone listens to dbus signal?

c

dbus

gdbus

背景

我的代码（缩短）