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  1. Qt
  2. QTBUG-108822

QDBus: unable to connect the InterfacesAdded signal of org.freedesktop.DBus.ObjectManager on the system bus (unless using QDBUS_DEBUG)

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Details

    • Bug
    • Resolution: Fixed
    • P2: Important
    • 6.2.8, 6.4.3, 6.5.0, 6.6.0
    • 6.4.1
    • D-Bus
    • None
    • I first encountered that issue on a yocto-based, custom linux distro but it's reproducible on Ubuntu 22.10 both with X11 and Wayland.
    • Linux/Wayland, Linux/X11, Linux/Yocto
    • d619a952c (dev), 157c2d46a (6.5), d82f68f12 (tqtc/lts-6.2), 2230c1344 (6.4)

    Description

      Let's consider the following python code:

       provider.py:

      #!/usr/bin/env python3
# SPDX-License-Identifier: WTFPL

import pydbus
import pydbus_manager
import sys

from gi.repository import GLib
from functools import partial


class Provider:
    """
    <node>
        <interface name='org.foobar.Provider1'>
            <property name='FooBar' type='s' access='read'/>
        </interface>
    </node>
    """

    @property
    def FooBar(self):
        return "foobar foobar"


def add_interface(manager):
    print("Adding object")
    manager.register_object("/org/foobar/Provider1/FooBar", Provider(), None)
    return GLib.SOURCE_REMOVE


if __name__ == "__main__":
    loop = GLib.MainLoop()
    bus = (
        pydbus.SystemBus()
        if len(sys.argv) == 2 and sys.argv[1] == "system"
        else pydbus.SessionBus()
    )
    manager = pydbus_manager.Manager(bus, "org.foobar.Provider1")

    print("Registered the object manager")

    GLib.timeout_add_seconds(5, partial(add_interface, manager))

    print("Scheduled an interface event in 5 seconds")

    loop.run()

      receiver.py:

      #!/usr/bin/env python3
# SPDX-License-Identifier: WTFPL

import pydbus
import sys

from gi.repository import GLib


def on_interfaces_added(*args):
    print("InterfacesAdded: args: {}".format(args))


if __name__ == "__main__":
    loop = GLib.MainLoop()
    bus = (
        pydbus.SystemBus()
        if len(sys.argv) == 2 and sys.argv[1] == "system"
        else pydbus.SessionBus()
    )
    intf = bus.get("org.foobar.Provider1", "/org/foobar/Provider1")
    intf.InterfacesAdded.connect(on_interfaces_added)

    print("Connected to InterfacesAdded signal")

    loop.run()

      The /org/foobar/Provider1 object implements the org.freedesktop.DBus.ObjectManager interface and is visible in qdbus like this:

      $ qdbus org.foobar.Provider1 /org/foobar/Provider1
signal void org.freedesktop.DBus.Properties.PropertiesChanged(QString interface_name, QVariantMap changed_properties, QStringList invalidated_properties)
method QDBusVariant org.freedesktop.DBus.Properties.Get(QString interface_name, QString property_name)
method QVariantMap org.freedesktop.DBus.Properties.GetAll(QString interface_name)
method void org.freedesktop.DBus.Properties.Set(QString interface_name, QString property_name, QDBusVariant value)
method QString org.freedesktop.DBus.Introspectable.Introspect()
method QString org.freedesktop.DBus.Peer.GetMachineId()
method void org.freedesktop.DBus.Peer.Ping()
signal void org.freedesktop.DBus.ObjectManager.InterfacesAdded(QDBusObjectPath object_path, {D-Bus type "a{sa{sv}}"} interfaces_and_properties)
signal void org.freedesktop.DBus.ObjectManager.InterfacesRemoved(QDBusObjectPath object_path, QStringList interfaces)
method {D-Bus type "a{oa{sa{sv}}}"} org.freedesktop.DBus.ObjectManager.GetManagedObjects()

      The above python examples work just fine - if the receiver is run within 5 seconds of starting the provider, it receives the signal:

      $ ./python-receiver/receiver.py 
Connected to InterfacesAdded signal
InterfacesAdded: args: ('/org/foobar/Provider1/FooBar', {'org.freedesktop.DBus.Properties': {}, 'org.freedesktop.DBus.Introspectable': {}, 'org.freedesktop.DBus.Peer': {}, 'org.foobar.Provider1': {'FooBar': 'foobar foobar'}})

      Let's now implement a receiver in Qt6:

      receiver.hpp:

      /* SPDX-License-Identifier: WTFPL */

#include <QObject>
#include <QtDBus/QDBusObjectPath>
#include <QMap>
#include <QVariant>
#include <QString>

typedef QMap<QString, QDBusVariant> QDBusVariantMap;
Q_DECLARE_METATYPE(QDBusVariantMap);
typedef QMap<QString, QDBusVariantMap> QDBusVariantMapMap;
Q_DECLARE_METATYPE(QDBusVariantMapMap);

class Receiver : public QObject
{
	Q_OBJECT

public:
	Receiver(QObject *parent = nullptr);
	virtual ~Receiver();

private slots:
	void InterfacesAdded(QDBusObjectPath object_path,
			     QDBusVariantMapMap interfaces_and_properties);
};

      main.cpp:

      // SPDX-License-Identifier: WTFPL

#include <QtGlobal>
#include <QDebug>
#include <QtWidgets/QApplication>
#include <QtDBus/QDBusInterface>
#include <QtDBus/QDBusMetaType>

#include "receiver.hpp"

Receiver::Receiver(QObject *parent) : QObject(parent) { }

Receiver::~Receiver() { }

void Receiver::InterfacesAdded(QDBusObjectPath object_path,
			       QDBusVariantMapMap interfaces_and_properties)
{
	Q_UNUSED(interfaces_and_properties);

	qInfo() << "InterfacesAdded: " << object_path.path();
}

int main(int argc, char **argv)
{
	QApplication app(argc, argv);

	auto bus = (argc == 2 && QString(argv[1]) == "system") ?
		QDBusConnection::systemBus() : QDBusConnection::sessionBus();
	Receiver receiver;

	qDBusRegisterMetaType<QDBusVariantMap>();
	qDBusRegisterMetaType<QDBusVariantMapMap>();

	qInfo() << "Before connection";
	qInfo() << "Is connected " << bus.isConnected();

	qInfo() << "Trying to connect";
	auto ret = bus.connect(
		"org.foobar.Provider1",
		"/org/foobar/Provider1",
		"org.freedesktop.DBus.ObjectManager",
		"InterfacesAdded",
		&receiver,
		SLOT(InterfacesAdded(QDBusObjectPath, QDBusVariantMapMap))
	);

	qInfo() << "connect() result: " << ret;
	if (!ret)
		qInfo() << "lastError: " << bus.lastError();

	return app.exec();
}

      Runs fine as well:

      $ ./receiver 
Before connection
Is connected  true
Trying to connect
connect() result:  true
InterfacesAdded:  "/org/foobar/Provider1/FooBar"

      The above examples are using the session bus. Now let's use the following config under /etc/dbus-1/system.d/org.foobar.Provider1.conf:

      <!-- SPDX-License-Identifier: WTFPL -->

<!DOCTYPE busconfig PUBLIC "-//freedesktop//DTD D-BUS Bus Configuration 1.0//EN"
 "http://www.freedesktop.org/standards/dbus/1.0/busconfig.dtd">

<busconfig>
  <policy user="root">
    <allow own="org.foobar.Provider1"/>
    <allow send_destination="org.foobar.Provider1"/>
    <allow receive_sender="org.foobar.Provider1"/>
  </policy>
</busconfig>

      And run the examples with the "system" argument. Python still works just fine. But running the same C++ code results in the following:

      $ sudo ./receiver system
QStandardPaths: XDG_RUNTIME_DIR not set, defaulting to '/tmp/runtime-root'
Before connection
Is connected  true
Trying to connect
connect() result:  false
lastError:  QDBusError("", "")

      As you can see, the connection fails, bus.connect() returns false, the signal is never received but bus.lastError() doesn't indicate any error code.

      Ok, let's try to debug this with QDBUS_DEBUG=1:

      $ sudo QDBUS_DEBUG=1 ./qt6-receiver/receiver system
QStandardPaths: XDG_RUNTIME_DIR not set, defaulting to '/tmp/runtime-root'
QDBusConnectionPrivate(0x7fab3c004f70) : connected successfully
QDBusConnectionPrivate(0x7fab3c004f70) got message (signal): QDBusMessage(type=Signal, service="org.freedesktop.DBus", path="/org/freedesktop/DBus", interface="org.freedesktop.DBus", member="NameAcquired", signature="s", contents=(":1.165") )
QDBusConnectionPrivate(0x7fab3c004f70) delivery is suspended
Before connection
Is connected  true
Trying to connect
QDBusConnectionPrivate(0x7fab3c004f70) Adding rule: "type='signal',sender='org.foobar.Provider1',path='/org/foobar/Provider1',interface='org.freedesktop.DBus.ObjectManager',member='InterfacesAdded'"
QDBusConnectionPrivate(0x7fab3c004f70) sending message: QDBusMessage(type=MethodCall, service="org.freedesktop.DBus", path="/org/freedesktop/DBus", interface="org.freedesktop.DBus", member="GetNameOwner", signature="", contents=("org.foobar.Provider1") )
QDBusConnectionPrivate(0x7fab3c004f70) got message reply: QDBusMessage(type=MethodReturn, service="org.freedesktop.DBus", signature="s", contents=(":1.164") )
QDBusConnectionPrivate(0x7fab3c004f70) Watching service "org.foobar.Provider1" for owner changes (current owner: ":1.164" )
connect() result:  true
QDBusConnectionPrivate(0x7fab3c00fa40) : connected successfully
QDBusConnectionPrivate(0x7fab3c00fa40) got message (signal): QDBusMessage(type=Signal, service="org.freedesktop.DBus", path="/org/freedesktop/DBus", interface="org.freedesktop.DBus", member="NameAcquired", signature="s", contents=(":1.4") )
QDBusConnectionPrivate(0x7fab3c00fa40) delivery is suspended
QDBusConnectionPrivate(0x7fab3c00fa40) Adding rule: "type='signal',sender='org.freedesktop.DBus',interface='org.freedesktop.DBus',member='NameOwnerChanged',arg0='org.a11y.Bus',arg1=''"
QDBusConnectionPrivate(0x7fab3c00fa40) sending message: QDBusMessage(type=MethodCall, service="org.freedesktop.DBus", path="/org/freedesktop/DBus", interface="org.freedesktop.DBus", member="NameHasOwner", signature="", contents=("org.a11y.Bus") )
QDBusConnectionPrivate(0x7fab3c00fa40) got message reply: QDBusMessage(type=MethodReturn, service="org.freedesktop.DBus", signature="b", contents=(false) )
QDBusConnectionPrivate(0x7fab3c015220) : connected successfully
QDBusConnectionPrivate(0x7fab3c015220) got message (signal): QDBusMessage(type=Signal, service="org.freedesktop.DBus", path="/org/freedesktop/DBus", interface="org.freedesktop.DBus", member="NameAcquired", signature="s", contents=(":1.34") )
QDBusConnectionPrivate(0x7fab3c004f70) dequeueing message QDBusMessage(type=Signal, service="org.freedesktop.DBus", path="/org/freedesktop/DBus", interface="org.freedesktop.DBus", member="NameAcquired", signature="s", contents=(":1.165") )
QDBusConnectionPrivate(0x7fab3c00fa40) dequeueing message QDBusMessage(type=Signal, service="org.freedesktop.DBus", path="/org/freedesktop/DBus", interface="org.freedesktop.DBus", member="NameAcquired", signature="s", contents=(":1.4") )
QDBusConnectionPrivate(0x7fab3c004f70) got message (signal): QDBusMessage(type=Signal, service=":1.164", path="/org/foobar/Provider1", interface="org.freedesktop.DBus.ObjectManager", member="InterfacesAdded", signature="oa{sa{sv}}", contents=([ObjectPath: /org/foobar/Provider1/FooBar], [Argument: a{sa{sv}} {"org.freedesktop.DBus.Properties" = [Argument: a{sv} {}], "org.freedesktop.DBus.Introspectable" = [Argument: a{sv} {}], "org.freedesktop.DBus.Peer" = [Argument: a{sv} {}], "org.foobar.Provider1" = [Argument: a{sv} {"FooBar" = [Variant(QString): "foobar foobar"]}]}]) )
InterfacesAdded:  "/org/foobar/Provider1/FooBar"
QDBusConnectionPrivate(0x7fab3c004f70) got message (signal): QDBusMessage(type=Signal, service="org.freedesktop.DBus", path="/org/freedesktop/DBus", interface="org.freedesktop.DBus", member="NameOwnerChanged", signature="sss", contents=("org.foobar.Provider1", ":1.164", "") )
QDBusConnectionPrivate(0x7fab3c004f70) Updating name "org.foobar.Provider1" from ":1.164" to ""

      Ha! Now it works.

      The examples I posted are available here. I suspect it's some timing issue that's hidden when enabling debug?

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            thiago Thiago Macieira
            brgl Bartosz Golaszewski
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              Resolved:

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