How to get the state of a service with sd-bus?

Question

I need to query, monitor and possibly change the state of a few systemd services from a C++ application. It looks like sd-bus is the right way to do this, but I'm having a terrible time finding an example.

So, how do I:

1) Query the current status of a service via sd-bus, similar to systemctl status foo.service?

2) Monitor the status of a service such that I get a callback whenever it changes?

3) Change the status of a service, similar to systemctl start/stop/restart?

Thanks!

Answer 1

Using the sd-bus API is absolutely correct (header #include <systemd/sd-bus.h>)

First you need get access to a bus object:

I do this:

Systemctl::Systemctl() :
    m_bus(nullptr)
{
    int r = sd_bus_default_system(&m_bus);

    if (r < 0)
        throw exception("Could not open systemd bus");
}

If you're having problems opening the bus:

1. Run as root/sudo
2. Make some polkit policies to grant your user/group access to this command
3. Run the _user bus instead of the _system bus

Don't forget to release the bus when you are done:

Systemctl::~Systemctl()
{
    sd_bus_unref(m_bus);
}

Now you had 3 questions:

1. Query the status

For each unit, I have a class which holds the escaped name (foo_2eservice) as m_name, and a reference to the bus in m_bus. Call this method with any property. You seem to be most interested in "ActiveState" or "SubState".

std::string Unit::GetPropertyString(const std::string& property) const
{
    sd_bus_error err = SD_BUS_ERROR_NULL;
    char* msg = nullptr;
    int r;

    r = sd_bus_get_property_string(m_bus,
        "org.freedesktop.systemd1",
        ("/org/freedesktop/systemd1/unit/" + m_unit).c_str(),
        "org.freedesktop.systemd1.Unit",
        property.c_str(),
        &err,
        &msg);

    if (r < 0)
    {
        std::string err_msg(err.message);
        sd_bus_error_free(&err);

        std::string err_str("Failed to get " + property + " for service "
                            + m_name + ". Error: " + err_msg);

        throw exception(err_str);
    }

    sd_bus_error_free(&err);

    // Free memory (avoid leaking)
    std::string ret(msg);
    free (msg);

    return ret;
}

2. Monitor the status of a service:

The first step is to set up a file-descriptor to subscribe to changes. In this case you are interested in subscribing to the "PropertiesChanged" signal. Note that you'll get a signal for any property changing, not just the state. In the sd_bus_add_match() call, there is room for a callback, though I haven't experimented with it.

void Systemctl::SubscribeToUnitChanges(const std::string& escaped_name)
{
    /* This function is an easier helper, but it as only introduced in systemd 237
     * Stretch is on 232 while buster is on 241 .  Need re replace this as long as
     * we still support stretch
    sd_bus_match_signal(
        m_bus,
        nullptr, // slot
        nullptr, // sender
        std::string("/org/freedesktop/systemd1/unit/" + escaped_name).c_str(), // path
        "org.freedesktop.DBus.Properties", // interface
        "PropertiesChanged", // member
        nullptr, // callback
        nullptr // userdata
    );
    */
    std::string match =  "type='signal'";
        match += ",path='/org/freedesktop/systemd1/unit/" + escaped_name + "'" ;
        match += ",interface='org.freedesktop.DBus.Properties'";
        match += ",member='PropertiesChanged'";

    sd_bus_add_match(
        m_bus,
        nullptr, // slot
        match.c_str(),
        nullptr, // callback
        nullptr // userdata
    );
}

Instead what I do is periodically poll the bus for the subscribed changes and update each unit:

bool Systemctl::ProcessBusChanges()
{
    bool changed = false;
    sd_bus_message* msg = nullptr;

    // for each new message
    std::list<std::string> escaped_names;
    while( sd_bus_process(m_bus, &msg) )
    {
        // Note:  Once sd_bus_process returns 0, We are supposed to call
        // sd_bus_wait, or check for changes on sd_bus_get_fd before calling
        // this function again.  We're breaking that rule.  I don't really know
        // the consequences.
        if (msg)
        {
            std::string path = strna( sd_bus_message_get_path(msg) );
            sd_bus_message_unref(msg);

            std::string escaped_name = path.erase(0, path.find_last_of('/')+1 );
            escaped_names.push_back(escaped_name);

            changed = true;
        }
    }

    escaped_names.sort();
    escaped_names.unique();
    for (auto unit : escaped_names)
    {
        auto it = m_units.find(unit);
        if (it != m_units.end())
            it->second.RefreshDynamicProperties();
    }

    return changed;
}

If it tells us that the bus has changed, then I go ahead and read all of my monitored units on that bus.

3. Change the status

This one is easy. I use the following, where method is one of "StartUnit", "StopUnit", or "RestartUnit".

static void CallMethodSS(sd_bus* bus,
                         const std::string& name,
                         const std::string& method)
{
    sd_bus_error err = SD_BUS_ERROR_NULL;
    sd_bus_message* msg = nullptr;
    int r;

    r = sd_bus_call_method(bus,
        "org.freedesktop.systemd1",         /* <service>   */
        "/org/freedesktop/systemd1",        /* <path>      */
        "org.freedesktop.systemd1.Manager", /* <interface> */
        method.c_str(),                     /* <method>    */
        &err,                               /* object to return error in */
        &msg,                               /* return message on success */
        "ss",                               /* <input_signature (string-string)> */
        name.c_str(),  "replace" );         /* <arguments...> */

    if (r < 0)
    {
        std::string err_str("Could not send " + method +
                            " command to systemd for service: " + name +
                            ". Error: " + err.message );

        sd_bus_error_free(&err);
        sd_bus_message_unref(msg);
        throw exception(err_str);
    }

    // Extra stuff that might be useful:  display the response...
    char* response;
    r = sd_bus_message_read(msg, "o", &response);
    if (r < 0)
    {
      LogError("Failed to parse response message: %s\n", strerror(-r) );
    }

    sd_bus_error_free(&err);
    sd_bus_message_unref(msg);
}