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D-BUS API查找与调用

上篇文章<<D-Bus实例介绍>>中简要介绍了D-Bus的基本概念,其中提到systemdNetworkManager等系统服务导出了D-Bus API供其他程序来调用。本文通过示例来说明这些API的查找与调用。

上篇文章我们提到D-Busobject可以实现多个InterfaceD-Bus规范中标准化了一些接口,这些接口对于我们调用其他服务提供的D-Bus API非常有帮助。

我们主要来看其中的两个:

它有一个方法:

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org.freedesktop.DBus.Introspectable.Introspect (out STRING xml_data)

它会返回一个包含有对象(object), 接口(interface), 方法(methods), 信号(signals),属性(properties)等信息的XML字符串。对象如果实现这个接口, 我们就可以通过调用该方法了解这个对象对外提供的所有信息。

XML字符串的解析方法可以参考官方文档:
https://dbus.freedesktop.org/doc/dbus-specification.html#introspection-format

它有三个方法:

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org.freedesktop.DBus.Properties.Get (in STRING interface_name,
in STRING property_name,
out VARIANT value);
org.freedesktop.DBus.Properties.Set (in STRING interface_name,
in STRING property_name,
in VARIANT value);
org.freedesktop.DBus.Properties.GetAll (in STRING interface_name,
out DICT<STRING,VARIANT> props);

如果对象实现了该接口, 那么可以通过调用这些方法来对该对象的属性(properties)进行操作。

NetworkManager服务都实现了这些标准接口。我们通过busctl工具来看一下具体的信息。

busctlsystemd提供的一个D-Bus操作工具,功能非常强大,在之前的文章中我们也用它来查看当前的D-Bus服务。具体用法可以参考Man Page

首先,使用busctl来查看org.freedesktop.NetworkManager服务提供了哪些对象(objects):

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[root@centos1 dbus]# busctl tree org.freedesktop.NetworkManager
├─/com
│ └─/com/redhat
│ └─/com/redhat/ifcfgrh1
└─/org
└─/org/freedesktop
└─/org/freedesktop/NetworkManager
├─/org/freedesktop/NetworkManager/ActiveConnection
│ └─/org/freedesktop/NetworkManager/ActiveConnection/0
├─/org/freedesktop/NetworkManager/AgentManager
├─/org/freedesktop/NetworkManager/Devices
│ ├─/org/freedesktop/NetworkManager/Devices/0
│ ├─/org/freedesktop/NetworkManager/Devices/1
│ └─/org/freedesktop/NetworkManager/Devices/2
├─/org/freedesktop/NetworkManager/IP4Config
│ ├─/org/freedesktop/NetworkManager/IP4Config/0
│ ├─/org/freedesktop/NetworkManager/IP4Config/1
│ └─/org/freedesktop/NetworkManager/IP4Config/2
├─/org/freedesktop/NetworkManager/IP6Config
│ ├─/org/freedesktop/NetworkManager/IP6Config/0
│ ├─/org/freedesktop/NetworkManager/IP6Config/1
│ └─/org/freedesktop/NetworkManager/IP6Config/2
└─/org/freedesktop/NetworkManager/Settings
└─/org/freedesktop/NetworkManager/Settings/0

接着,来查看某个对象实现的接口及其成员:

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[root@centos1 dbus]# busctl introspect org.freedesktop.NetworkManager /org/freedesktop/NetworkManager/Devices/0
NAME TYPE SIGNATURE RESULT/VALUE FLAGS
org.freedesktop.DBus.Introspectable interface - - -
.Introspect method - s -
org.freedesktop.DBus.Properties interface - - -
.Get method ss v -
.GetAll method s a{sv} -
.Set method ssv - -
org.freedesktop.NetworkManager.Device interface - - -
.Delete method - - -
.Disconnect method - - -
.ActiveConnection property o "/" emits-change
.Autoconnect property b false emits-change writable
.AvailableConnections property ao 0 emits-change
.Capabilities property u 7 emits-change
.DeviceType property u 14 emits-change
.Dhcp4Config property o "/" emits-change
.Dhcp6Config property o "/" emits-change
.Driver property s "unknown" emits-change
.DriverVersion property s "" emits-change
.FirmwareMissing property b false emits-change
.FirmwareVersion property s "" emits-change
.Interface property s "lo" emits-change
.Ip4Address property u 16777343 emits-change
.Ip4Config property o "/org/freedesktop/NetworkManager/IP4C... emits-change
.Ip6Config property o "/org/freedesktop/NetworkManager/IP6C... emits-change
.IpInterface property s "lo" emits-change
.Managed property b false emits-change
.Mtu property u 65536 emits-change
.PhysicalPortId property s "" emits-change
.State property u 10 emits-change
.StateReason property (uu) 10 0 emits-change
.Udi property s "/sys/devices/virtual/net/lo" emits-change
.StateChanged signal uuu - -
org.freedesktop.NetworkManager.Device.Generic interface - - -
.HwAddress property s "00:00:00:00:00:00" emits-change
.TypeDescription property s "loopback" emits-change
.PropertiesChanged signal a{sv} - -

返回结果中显示了该对象实现的接口及其方法(methods)、信息(signals)、属性(properties)等信息,属性值也被列出。

如果我们需要获取特定的属性值,还可以使用busctlget-property子命令单独获取。

比如,我们获取某个网卡的名称:

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[root@centos1 python]# busctl get-property org.freedesktop.NetworkManager /org/freedesktop/NetworkManager/Devices/1 org.freedesktop.NetworkManager.Device Interface
s "enp0s3"

上面我们使用busctl工具来示范了D-Bus服务API的调用方式。下面,我们再通一个Python程序来说明如何在程序中实现属性获取。

源码如下:

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import dbus
from pprint import pprint
from dbus import SystemBus
from dbus import Interface
from xml.etree import ElementTree

def print_network_ifaces(bus, service, object_path):
try:
if object_path == "/org/freedesktop/NetworkManager/Devices":
return

obj = bus.get_object(service, object_path)
interface = Interface(obj, 'org.freedesktop.DBus.Properties')

m = interface.get_dbus_method('Get', dbus_interface=None)

ifacename = m('org.freedesktop.NetworkManager.Device', 'Interface')
print(ifacename)

except Exception, err:
print err

def rec_intro(bus, service, object_path, callback):
callback(bus, service, object_path)
obj = bus.get_object(service, object_path)
interface = Interface(obj, 'org.freedesktop.DBus.Introspectable')
xmlstring = interface.Introspect()
for child in ElementTree.fromstring(xmlstring):
if child.tag == 'node':
if object_path == '/':
object_path = ''
new_path = '/' . join((object_path, child.attrib['name']))
rec_intro(bus, service, new_path, callback)

bus = SystemBus()
rec_intro(bus, 'org.freedesktop.NetworkManager', '/org/freedesktop/NetworkManager/Devices', print_network_ifaces)

我们的程序中首先调用标准接口org.freedesktop.DBus.Introspectable的方法Introspect拿到所有网络接口对象,然后调用标准接口org.freedesktop.DBus.Properties的方法Get获取属性Interface的值,从而获取网络接口名称。

系统本身的许多服务都提供了D-Bus API, 在许多场景下通过调用这些API可以非常便捷地实现我们的功能需求。