Bluetooth scan failure on service discovery start

[ocgltd]

I'm learning about Bluetooth (on Linux), and I tried the BtChat app. It fails to work, reporting "qt.bluetooth.bluez: SDP scan failure QProcess::NormalExit 3" when I scan for peers. Looks like Service discovery agent isn't working.

I tried the BtScanner app and while it can find devices (Device discovery agent), it does not find any services for any selected device (Service discovery agent isn't working).

So I tried to create a simple Bt scanner based on the above examples. I paired BTChat down to the basics to just print out what it finds (no UUID filtering). I'm running Qt 5.15.2 on Kubuntu 23 64-bit.

Although it runs, the discovery seems stalled for a minute or more when it first starts, as the first 3 errors print out (below). It might repeat this error and give up, or after a minute start printing out discovered services.

What is causing the "SDP scan failure" error? and long delay. I have tried this on 2 differnt Linux PC's with the same result. I found reference to this bug (https://bugreports.qt.io/browse/QTBUG-53041) but I'm not sure if that's the cause since it shows resolved as of Qt 5.7.1. And there is this almost identical question (https://forum.qt.io/topic/80849/qt-bluetooth-bluez-sdp-scan-failure-qprocess-exitstatus-normalexit-3 ) but there is no answer...

Starting discovery
qt.bluetooth.bluez: SDP scan failure QProcess::NormalExit 3
qt.bluetooth.bluez: SDP scan failure QProcess::NormalExit 3
qt.bluetooth.bluez: SDP scan failure QProcess::NormalExit 3
Discovered service on "58-B0-2D-4F-71-2E" "58:B0:2D:4F:71:2E"
	Service name: ""
	Description: ""
	Provider: ""
	L2CAP protocol service multiplexer: 31
	RFCOMM server channel: -1
Discovered service on "58-B0-2D-4F-71-2E" "58:B0:2D:4F:71:2E"
	Service name: ""
	Description: ""
	Provider: ""
	L2CAP protocol service multiplexer: 31

In case it matters, here's the class:

#include "bt.h"

#include <QtBluetooth/qbluetoothdeviceinfo.h>
#include <QtBluetooth/qbluetoothlocaldevice.h>
#include <QtBluetooth/qbluetoothuuid.h>

static const QUuid serviceUuid = QUuid("e8e10f95-1a70-4b27-9ccf-02010264e9c8");

BlueToothController::BlueToothController()
{
}


BlueToothController::~BlueToothController()
{
    // Ensure no discovery is running
    stopDiscovery();
}


void BlueToothController::setupLocalDevice() {

    // Get a list of all bluetooth devices (adapter) on this host
    localAdapters = QBluetoothLocalDevice::allDevices();
    qDebug() << "Num adap found" << localAdapters.count();

    // If no adapters found, clear name
    if (localAdapters.count() < 1) {
        qDebug() << "Few Num adap found" << localAdapters.count();
        localName.clear();
        localAddress = QBluetoothAddress();
    }
    // Else power on and make the first adapter discoverable
    else {
        //we ignore more than two adapters
        QBluetoothLocalDevice adapter(localAdapters.at(0).address());
        adapter.setHostMode(QBluetoothLocalDevice::HostDiscoverable);
        QBluetoothLocalDevice().setHostMode(QBluetoothLocalDevice::HostDiscoverable); //add this
        localName = QBluetoothLocalDevice().name();
        localAddress = localAdapters.at(0).address();

    }

}



void BlueToothController::startDiscovery(const QBluetoothUuid &uuid)
{
    qDebug() << "Starting discovery";

    m_discoveryAgent = new QBluetoothServiceDiscoveryAgent(localAddress);

    connect(m_discoveryAgent, &QBluetoothServiceDiscoveryAgent::serviceDiscovered,
            this, &BlueToothController::serviceDiscovered);
    connect(m_discoveryAgent, &QBluetoothServiceDiscoveryAgent::finished,
            this, &BlueToothController::discoveryFinished);
    connect(m_discoveryAgent, &QBluetoothServiceDiscoveryAgent::canceled,
            this, &BlueToothController::discoveryFinished);

    // If already running then abort current discovery
    if (m_discoveryAgent->isActive()) m_discoveryAgent->stop();

//    m_discoveryAgent->setUuidFilter(uuid);

    // Force full discovery (don't use SDP cache only)
    m_discoveryAgent->start(QBluetoothServiceDiscoveryAgent::FullDiscovery);
//    m_discoveryAgent->start(QBluetoothServiceDiscoveryAgent::MinimalDiscovery);
}



void BlueToothController::stopDiscovery() {
    if (m_discoveryAgent == nullptr) return;
    m_discoveryAgent->stop();
    delete m_discoveryAgent;
    m_discoveryAgent = nullptr;
}


void BlueToothController::serviceDiscovered(const QBluetoothServiceInfo &serviceInfo)
{

    qDebug() << "Discovered service on device.name "
             << serviceInfo.device().name() << " address " << serviceInfo.device().address().toString();
    qDebug() << "\tService name:" << serviceInfo.serviceName();
    qDebug() << "\tService UUID:" << serviceInfo.serviceUuid();
    qDebug() << "\tService Class UUIDs:" << serviceInfo.serviceClassUuids();
    qDebug() << "\tDescription:"
             << serviceInfo.attribute(QBluetoothServiceInfo::ServiceDescription).toString();
    qDebug() << "\tProvider:"
             << serviceInfo.attribute(QBluetoothServiceInfo::ServiceProvider).toString();
    qDebug() << "\tL2CAP protocol service multiplexer:"
             << serviceInfo.protocolServiceMultiplexer();
    qDebug() << "\tRFCOMM server channel:" << serviceInfo.serverChannel();

    const QBluetoothAddress address = serviceInfo.device().address();
    for (const QBluetoothServiceInfo &info : std::as_const(m_discoveredServices)) {
        if (info.device().address() == address)
            return;
    }

    QString remoteName;
    if (serviceInfo.device().name().isEmpty())
        remoteName = address.toString();
    else
        remoteName = serviceInfo.device().name();

    m_discoveredServices.append(serviceInfo);
}



void BlueToothController::discoveryFinished()
{
    qDebug() << ("Discovery finished");
}

[ocgltd]

To help out others facing similar problems, here's what i have (tentatively) concluded:

The device/service discovery process uses an external program called sdpcanner, which is launched using QProcess. If the sdpscanner encounters a device/service which does not respond as expected it times out/crashes. That results in the error shown.

Qt discovery launces many sdpscanner procresses. If one succeeds it will respond with what it found. If none succeed, you get now discovered devices/services. (Not very reliable)

If someone knowledgeable can add to / correct the above please jump in.