[mpd-devel] systemd and "zeroconf: No global port, disabling zeroconf"

[Punky]

Hi all,

I am now trying Debian jessie with 0.19.1. However, zeroconf is not
enabled, and /var/log/mpd/mpd.log shows "zeroconf: No global port,
disabling zeroconf".

Further looking into the code, I found the following in
zeroconf/ZeroconfGlue.cxx, line 52-57:

if (listen_port <= 0) {
LogWarning(zeroconf_domain,
"No global port, disabling zeroconf");
zeroconfEnabled = false;
return;
}

then in Listen.cxx, line 112-118

#ifdef ENABLE_SYSTEMD_DAEMON
if (listen_systemd_activation(error))
return true;

if (error.IsDefined())
return false;
#endif

//...line 149-151:
listen_port = port;
return true;
}

Since jessie now uses systemd instead of sysvinit,
listen_systemd_activation() is true. Then the function exit immediately
without setting listen_port = port.

Correct me if I am wrong as I just guess from reading the code.
--

[Max Kellermann]

Post by Punky
Since jessie now uses systemd instead of sysvinit,

But does it enable socket activation by default?

Post by Punky
listen_systemd_activation() is true. Then the function exit
immediately without setting listen_port = port.

Correct. Which value do you expect it to have, and where will it
obtain that value from?

[Punky]

Hi Max and all,

Post by Max Kellermann

Post by Punky
listen_systemd_activation() is true. Then the function exit
immediately without setting listen_port = port.

Correct. Which value do you expect it to have, and where will it
obtain that value from?

In zeroconf module, listen_port variable is used. listen_port variable
should be set to the value that MPD (successfully) listens to, and if TCP
port (default 6600) is being configured

I don't know much about systemd, and don't understand why systemd would
active the socket on behalf of the service (similar to inetd or xinetd!?).
But it seems to me listen_port is no longer set when systemd is enabled.

The problem here is when listen_port does not set, zeroconf is not enable.
I read through the codes on zeroconf module that it publishes the service
port using listen_port.

To make zeroconf works with systemd again, the argument is:

1. Just a added "listen_port=port;" after the test of
listen_systemd_activation()
is true? or
2. change zeroconf module to use port variable to publish the service? (but
port being configured does not mean mpd successfully listens to it)

It didn't read many mpd codes, so I don't understand the impact after any
of the above changes. I am happy to discuss and find a right solution.
--

[Max Kellermann]

Post by Punky
In zeroconf module, listen_port variable is used. listen_port variable
should be set to the value that MPD (successfully) listens to, and if TCP
port (default 6600) is being configured

But listen_port is ignored when systemd socket activation is used.
With socket activation, listen_port may or may not describe the port
that systemd listens to - there is no relation between the two.

Thus, MPD currently does not enable zeroconf when socket activation is
used, because MPD does not know what port to announce.

Post by Punky
I don't know much about systemd, and don't understand why systemd would
active the socket on behalf of the service (similar to inetd or xinetd!?).

systemd would activate the socket on behalf of the service because you
told it to. It is optional, and you enabled it ("systemctl enable
mpd.socket" instead of "systemctl enable mpd.service").

[Ben Boeckel]

Post by Max Kellermann
Thus, MPD currently does not enable zeroconf when socket activation is
used, because MPD does not know what port to announce.

Would getsockname(2) help?

--Ben

[Max Kellermann]

Post by Ben Boeckel

Post by Max Kellermann
Thus, MPD currently does not enable zeroconf when socket activation is
used, because MPD does not know what port to announce.

Would getsockname(2) help?

Yes, that could be used to find out, under certain circumstances.

But combining socket activation with zeroconf would be a rather
pointless exercise. You would only be able to find MPD after you
already connected to MPD.

[Ben Boeckel]

Post by Max Kellermann
But combining socket activation with zeroconf would be a rather
pointless exercise. You would only be able to find MPD after you
already connected to MPD.

Ah, indeed, that is a chicken-and-egg. I guess documentation would be
the best fix here then.

--Ben

[Punky]

Thanks Max,

Thus, MPD currently does not enable zeroconf when socket activation is

Post by Max Kellermann
used, because MPD does not know what port to announce.

I understand this now.

Post by Max Kellermann
systemd would activate the socket on behalf of the service because you
told it to. It is optional, and you enabled it ("systemctl enable
mpd.socket" instead of "systemctl enable mpd.service").

So to make zeroconf works under systemd, I need to "systemctl disable
mpd.socket" then "systemctl enable mpd.service" and "systemctl restart
mpd.service". Zeroconf works again.
--