Timeout error when `GetManagedObjects` called on my service

[abroekhof]

Describe the bug
I'm trying to implement the BLE AdvertisementMonitor, and am getting a timeout when I call RegisterMonitor on org.bluez. It appears to be triggering a reverse GetManagedObjects on my service, which I thought would be handled by the Object Manager I created, but no luck.

To Reproduce
Here's the smallest reproducible version I could get. I can attach the glue files below if that would be helpful, but they were generated from the xml, so I don't expect the problem is there.

#include <signal.h>

#include <iostream>
#include <thread>

#include "adaptor/AdvertisementMonitor1_adaptor.h"
#include "proxy/AdvertisementMonitorManager1_proxy.h"

class ApplicationObjectManager
    : public sdbus::AdaptorInterfaces<sdbus::ObjectManager_adaptor> {
 public:
  ApplicationObjectManager(sdbus::IConnection& connection,
                           sdbus::ObjectPath objectPath)
      : AdaptorInterfaces(connection, std::move(objectPath)) {
    registerAdaptor();
  }

  ~ApplicationObjectManager() { unregisterAdaptor(); }
};

class AdvertisementMonitor
    : public sdbus::AdaptorInterfaces<org::bluez::AdvertisementMonitor1_adaptor,
                                      sdbus::ManagedObject_adaptor,
                                      sdbus::Properties_adaptor> {
 public:
  AdvertisementMonitor(sdbus::IConnection& connection,
                       sdbus::ObjectPath objectPath)
      : AdaptorInterfaces(connection, std::move(objectPath)) {
    registerAdaptor();
    emitInterfacesAddedSignal({sdbus::InterfaceName{
        org::bluez::AdvertisementMonitor1_adaptor::INTERFACE_NAME}});
  }

  ~AdvertisementMonitor() {
    emitInterfacesRemovedSignal({sdbus::InterfaceName{
        org::bluez::AdvertisementMonitor1_adaptor::INTERFACE_NAME}});
    unregisterAdaptor();
  }

 public:
  void Release() override {}
  void Activate() override {}
  void DeviceFound(const sdbus::ObjectPath& device) override {}
  void DeviceLost(const sdbus::ObjectPath& device) override {}

 protected:
  int16_t RSSILowThreshold() override { return 127; }
  int16_t RSSIHighThreshold() override { return 127; }
  uint16_t RSSILowTimeout() override { return 0; }
  uint16_t RSSIHighTimeout() override { return 0; }
  uint16_t RSSISamplingPeriod() override { return 0; }

  std::string Type() override { return "or_patterns"; }
  std::vector<sdbus::Struct<uint8_t, uint8_t, std::vector<uint8_t>>> Patterns()
      override {
    return {{0, 0xFF, {0x4c}}};
  }
};

class AdvertisementMonitorManager
    : public sdbus::ProxyInterfaces<
          org::bluez::AdvertisementMonitorManager1_proxy> {
 public:
  AdvertisementMonitorManager(sdbus::IConnection& connection,
                              sdbus::ServiceName destination,
                              sdbus::ObjectPath objectPath)
      : ProxyInterfaces{connection, std::move(destination),
                        std::move(objectPath)} {
    registerProxy();
  }

  ~AdvertisementMonitorManager() { unregisterProxy(); }
};

constexpr const char* BLUEZ_SERVICE = "org.bluez";
constexpr const char* DEVICE0 = "/org/bluez/hci0";

static std::atomic<bool> running{true};

static void sig_callback(int signum) {
  std::cout << "Signal received, shutting down..." << std::endl;
  running = false;
}

int main(int argc, char* argv[]) {
  if (signal(SIGINT, sig_callback) == SIG_ERR) {
    exit(-1);
  }

  auto connection = sdbus::createSystemBusConnection();
  connection->enterEventLoopAsync();

  auto application_object_manager = std::make_unique<ApplicationObjectManager>(
      *connection, sdbus::ObjectPath{"/org/example"});

  auto advertisement_monitor_manager =
      std::make_unique<AdvertisementMonitorManager>(
          *connection, sdbus::ServiceName{"org.bluez"},
          sdbus::ObjectPath{"/org/bluez/hci0"});

  advertisement_monitor_manager->RegisterMonitor(
      sdbus::ObjectPath{"/org/example/monitor"});

  std::cout << "Advertisement monitor registered. Press Ctrl+C to exit."
            << std::endl;

  while (running) {
    std::this_thread::sleep_for(std::chrono::milliseconds(100));
  }

  connection->leaveEventLoop();

  return 0;
}

Expected behavior
I expect RegisterMonitor to return normally.

Real (buggy) behavior
Here are logs from dbus-monitor:

// Register Monitor is called.
method call time=1750287421.989479 sender=:1.230 -> destination=org.bluez serial=2 path=/org/bluez/hci0; interface=org.bluez.AdvertisementMonitorManager1; member=RegisterMonitor
   object path "/org/example/monitor"
method call time=1750287421.989673 sender=:1.166 -> destination=org.freedesktop.DBus serial=296 path=/org/freedesktop/DBus; interface=org.freedesktop.DBus; member=AddMatch
   string "type='signal',sender='org.freedesktop.DBus',path='/org/freedesktop/DBus',interface='org.freedesktop.DBus',member='NameOwnerChanged',arg0=':1.230'"
method return time=1750287421.989686 sender=org.freedesktop.DBus -> destination=:1.166 serial=176 reply_serial=296
method call time=1750287421.989857 sender=:1.166 -> destination=org.freedesktop.DBus serial=297 path=/org/freedesktop/DBus; interface=org.freedesktop.DBus; member=GetNameOwner
   string ":1.230"
method return time=1750287421.989867 sender=org.freedesktop.DBus -> destination=:1.166 serial=177 reply_serial=297
   string ":1.230"
method call time=1750287421.989921 sender=:1.166 -> destination=org.freedesktop.DBus serial=298 path=/org/freedesktop/DBus; interface=org.freedesktop.DBus; member=AddMatch
   string "type='signal',sender=':1.230',path='/org/example/monitor',interface='org.freedesktop.DBus.ObjectManager',member='InterfacesAdded'"
method return time=1750287421.989931 sender=org.freedesktop.DBus -> destination=:1.166 serial=178 reply_serial=298
method call time=1750287421.990135 sender=:1.166 -> destination=org.freedesktop.DBus serial=299 path=/org/freedesktop/DBus; interface=org.freedesktop.DBus; member=AddMatch
   string "type='signal',sender=':1.230',path='/org/example/monitor',interface='org.freedesktop.DBus.ObjectManager',member='InterfacesRemoved'"
method return time=1750287421.990162 sender=org.freedesktop.DBus -> destination=:1.166 serial=179 reply_serial=299
method call time=1750287421.990350 sender=:1.166 -> destination=org.freedesktop.DBus serial=300 path=/org/freedesktop/DBus; interface=org.freedesktop.DBus; member=AddMatch
   string "type='signal',sender=':1.230',path_namespace='/org/example/monitor'"
method return time=1750287421.990378 sender=org.freedesktop.DBus -> destination=:1.166 serial=180 reply_serial=300

// Tries to GetManagedObjects
method call time=1750287421.990468 sender=:1.166 -> destination=:1.230 serial=301 path=/org/example/monitor; interface=org.freedesktop.DBus.ObjectManager; member=GetManagedObjects

// A while later, it errors out
error time=1750287447.016276 sender=:1.230 -> destination=:1.166 error_name=org.freedesktop.DBus.Error.UnknownObject reply_serial=301
   string "Unknown object '/org/example/monitor'."

Additional context
Running on Ubuntu 24.04.2. I enabled experimental mode on bluetoothd, and have been able to add a monitor from bluetoothctl, so this should be working.

I'm building with Bazel, but I don't think that should matter?

cc_library(
    name = "sdbus-cpp",
    srcs = glob([
        "src/*.h",
        "src/*.c",
        "src/*.cpp",
    ]),
    hdrs = glob([
        "include/sdbus-c++/**/*.h",
        "include/sdbus-c++/**/*.inl",
    ]),
    includes = ["include"],
    copts = [
        '-DSDBUS_HEADER=\"<systemd/sd-bus.h>\"',
    ],
    linkopts = select({
        "@//tools/toolchains:linux_aarch64_config": [],
        "//conditions:default": [
            "-lsystemd",
        ],
    }),
)

[sangelovic]

@abroekhof The reason seems to be quite obvious. In your example, you construct application_object_manager under object path "/org/example", but you pass path "/org/example/monitor" to RegisterMonitor(). However, no object exists under that path in your example. Hence UnknownObject error. Shouldn't you pass "/org/example" to RegisterMonitor()?

After you fix this, you may encounter another issue -- deadlock. RegisterMonitor() blocks on the connection, waiting for the reply. But if the service does a recursive blocking call back to objects on our connection, the request is enqueued but not processed, and you have a deadlock. The solution is to either use an async variant of RegisterMonitor() call, or use a separate connection for the proxy object (if the proxy object is a so-called short-lived proxy, it doesn't even need an event loop thread on that connection -- you could just do std::make_unique<AdvertisementMonitorManager>(sdbus::ServiceName{"org.bluez"}, sdbus::ObjectPath{"/org/bluez/hci0"});.) See sdbus-c++ tutorial for more info.