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Notifications, D-Bus Activation and NixOS

Yesterday, a bit of config hacking with Lassulus at c-base, resulted to a bunch of new ideas that I would like to implement. In short, I would like to mimick one specific feature that I remember from my Alfred usage days on OSX on my Linux setup. Think about typing mathmatical expressions and getting the answers presented through popups or notifications, for starters. Perhaps even copying it to the clipboard right away, or at least allowing the user to click the notification or a button on a notification to perform the copy operation.

This post documents my journey and learnings in my attempts to solve this issue figure out how D-Bus plays ball in NixOS.

At this stage I had already installed notify-osd on my NixOS :snowflake: mech before. Puzzled by notify-osd automatically starting after me logging in, without any reference to it in any of the services or dotfiles I wrote led me down a little investigation into the suspected culprits. :confused:.

A grep on the Nixpkgs codebase indicates that there is merely a package defined for notify-osd, that provides a “wrapped” binary and nothing more – no services or additions to startup scripts or dotfiles.

Furthermore, I found no services mentioning notify-osd and not a single reference in any of my existing dotfiles.

WTF is starting notify-osd? :confused:

The output of pstree after a fresh reboot displays notify-osd just one level under the init system (systemd), which would leave one to assume that either systemd sparked the process (and perhaps manages it too) or something else triggered a start of notify-osd. Since it isn’t forked under another application that could provide clues as to what started it, I still have to look elsewhere for answers.

vid@localhost> pstree -g 2                                                                                                                                                      ~
─┬◆ 00001 root systemd 
 ├──◆ 01615 rtkit /.../libexec/rtkit-daemon 
 ├──◆ 01575 vid /run/current-system/sw/bin/dbus-daemon --fork --print-pid 5 --print-address 7 --session 
 ├─── 01560 vid /.../bin/dbus-launch --exit-with-session /.../xsession none+xmonad
 ├─┬◆ 01443 vid /.../lib/systemd/systemd --user 
 │ ├─┬◆ 01631 vid /.../bin/pulseaudio --daemonize=no 
 │ │ └─── 01635 vid /.../libexec/pulse/gconf-helper 
 │ ├──◆ 01507 vid /.../bin/gpg-agent --supervised 
 │ ├─── 01448 vid (sd-pam)  
 │ └─┬◆ 01649 vid /.../bin/bash -e /.../bin/ibus-daemon-start 
 │   └─┬◆ 01650 vid /.../bin/.ibus-daemon-wrapped_ --cache=refresh 
 │     ├─── 01676 vid /.../libexec/ibus-engine-simple 
 │     ├─── 01661 vid /.../libexec/ibus-ui-gtk3 
 │     └─── 01659 vid /.../libexec/ibus-dconf 
 ├──◆ 01394 root /..._supplicant-2.6/sbin//wpa_supplicant -u 
 ├──◆ 01385 polkituser /.../lib/polkit-1/polkitd --no-debug 
 ├──◆ 01275 dnsmasq /.../bin/dnsmasq -k --enable-dbus --user=dnsmasq -C /...
 ├──◆ 01263 root agetty --login-program /.../bin/login --noclear --keep-baud tty1 115200,38400,9600 linux 
 ├──◆ 01259 privoxy /.../bin/privoxy --no-daemon --user privoxy /... 
 ├─┬◆ 01163 root /.../bin/slim 
 │ ├─┬─ 01490 vid /store/vidbina.home/.xmonad/xmonad-x86_64-linux 
 │ │ ├─┬◆ 01683 vid /run/current-system/sw/bin/termite 
 │ │ │ └─┬◆ 01704 vid /run/current-system/sw/bin/zsh 
 │ │ │   └──◆ 04224 vid pstree -g 2 
 │ │ ├──◆ 01667 vid xmobar -x0 
 │ │ ├─── 01657 vid /.../bin/python /.../bin/..blueman-applet-wrapped-w
 │ │ ├─── 01656 vid /run/current-system/sw/bin/nm-applet --sm-disable 
 │ │ └─── 01655 vid trayer --blahblah
 │ └──◆ 01244 root /.../bin/X -config /... -xkbdir /...
 ├──◆ 01128 root /.../lib/systemd/systemd-logind 
 ├──◆ 01030 messagebus /.../bin/dbus-daemon --system --address=systemd: --nofork --nopidfile --systemd-activation 
 ├──◆ 01024 dictd dictd 1.12.1: 0/0                                                                                                                                              
 ├─┬◆ 01017 root /.../sbin/NetworkManager --no-daemon 
 │ └──◆ 01747 root /.../bin/dhclient -d -q -sf /.../libexec/nm-dhc
 ├──◆ 01015 root /.../bin/illum-d 
 ├──◆ 01012 root /.../libexec/bluetooth/bluetoothd 
 ├──◆ 01011 root /.../sbin/rngd -f -v 
 ├──◆ 01008 root acpid --confdir /... 
 ├──◆ 00995 root /...+git20161120/bin/w3m /.../share/doc/nixos/index.html 
 ├──◆ 00955 systemd-timesync /.../lib/systemd/systemd-timesyncd 
 ├──◆ 00692 root /.../lib/systemd/systemd-udevd 
 ├──◆ 00686 root /.../lib/systemd/systemd-journald 
 ├─┬─ 02915 vid /run/current-system/sw/bin/zsh 
 │ └─── 02924 vid /.../bin/.firefox-wrapped 
 ├──◆ 01837 nscd nscd 
 ├─── 01829 vid /.../bin/notify-osd 
 └─── 01699 vid /.../libexec/bluetooth/obexd

At this stage I recalled noticing a notify-osd popup every time I connected to a network leading me to assume that NetworkManager may have something to do with starting notify-osd. Since PID’s tend increase with time as new processes are started, the pstree listing suggests that the network manager (PID: 1017) along with my bluetooth daemons (PID: 1699) and some other junk were started before notify-osd.

In order to explore the hunch that NetworkManager may be involved, I kill notify-osd and reconnect to a network and voilà… notify-osd is ressurected.

Recalling reading something about D-Bus in conjunction NetworkManager far in the past, I decide to finally dive into a long overdue encounter with D-Bus to get to the bottom of this.

To get started, the tree indicates that two daemons are running for D-Bus, of which the second daemon seems to have been triggered by some launcher as evident in the following excerpt from my pstree output:

  1. dbus-daemon –system (PID: 1030)
  2. dbus-launch –exit-with-session /…/xsession blah (PID: 1560)
  3. dbus-daemon –session (PID: 1575)

Time to take a closer look…


It’s Saturday and the D-Bus homepage, under the section What is D-Bus, reads:

D-Bus is a message bus system, a simple way for applications to talk to one another. In addition to interprocess communication, D-Bus helps coordinate process lifecycle; it makes it simple and reliable to code a “single instance” application or daemon, and to launch applications and daemons on demand when their services are needed.

The D-Bus (Desktop bus) ecosystem introduces the concept of busses, connections, objects, interfaces and members.

D-Bus facilitates connections that host objects that expose methods. A user of D-Bus connects to the bus, and a specific connection in order to interact with an object through its methods.

A bus is the actual medium that serves as a carrier of all messages. Generally, a machine will have at least a system and session bus which, for example, would be accessible through a unix-domain socket expressed as a unix-domain path or a TCP connection expressed by hostname and port. :bus:

A connection is addressed by a bus name. The naming is slightly confusing but bear with me for a moment as I attempt to clarify. A bus name is the connection’s bus name (i.e.: name of the connection on the bus) and not the connection bus’s name. :name_badge:

Within a connection one may expect to find objects that contain members. Interfaces, as in OOP, specify members and can, as a whole, be implemented by an object. :package::wink:

To illustrate, the D-Bus Specification, documents an org.freedesktop.DBus.ListNames member for the org.freedesktop.DBus object, which lists the names currently registered on the bus. We could run

dbus-send \
  --session \
  --dest=org.freedesktop.DBus \
  --type=method_call \
  --print-reply \
  /org/freedesktop/DBus \

to get a listing of names on the session bus. The command is pretty self-explanatory but humor me for a moment. The --session flag indicates that we intent to communicate with the session bus, whereas the --system would indicate that we intent to communicate with the system bus. The --dest argument specifies the bus name, therefore specifying which connection on the bus we are targeting. With --type we specify we are issuing a method_call – another valid type is that of a signal. In our case we need a reply printed, hence the --print-reply. The object we are interacting with and the member of that object or the message name are the next two arguments respectively.

:bulb: Installing dbus-map and executing the command dbus-map --session --dump-methods gives an overview of all methods for the different connections on the session bus.

A quick read of the Introduction to D-Bus article, mentions activations as a mechanism for triggering an executable. Basically activations allow for a service to subscribe to a given type of message in order to trigger an executable (if it isn’t already running) upon the delivery of a message.

In order to explore whether activations were indeed the cause of the notify-osd resurrection, I decided to try my hand at invoking a method call to the member named org.freedesktop.DBus.StartServiceByName.

Killing notify-osd and sending the StartServiceByName message with the name of the service, being “org.freedesktop.Notifications”, and an empty 32-bit integer as arguments, by running:

kill `pidof notify-osd`
dbus-send \
  --session \
  --dest=org.freedesktop.DBus \
  --type=method_call \
  --print-reply \
  /org/freedesktop/DBus \
  org.freedesktop.DBus.StartServiceByName string:org.freedesktop.Notifications uint32:0

seems to do the trick and produces a reply indicating that the service was succesfully started. :trophy:

method return time=1516484477.248379 sender=org.freedesktop.DBus -> destination=:1.163 serial=3 reply_serial=2 uint32 1

The remaining question is now, where the .service is defined. As far as I can tell from the documentation this should be recorded in a file somewhere on my filesystem.

:book: On Unix systems, the system bus should default to searching for .service files in /usr/local/share/dbus-1/system-services, /usr/share/dbus-1/system-services and /lib/dbus-1/system-services, with that order of precedence. It may also search other implementation-specific locations, but should not vary these locations based on environment variables.

:book: On Unix systems, the session bus should search for .service files in $XDG_DATA_DIRS/dbus-1/services as defined by the XDG Base Directory Specification. Implementations may also search additional locations, with a higher or lower priority than the XDG directories.

Since I haven’t yet found it, I’m afraid that perhaps D-Bus allows an application to register for service activation on first boot, but haven’t found anything in the documentation to support this theory. :confused:

After a while of sleuthing on the web and poking around my file system, I remember1 that NixOS :snowflake: allows a package to provide complementary files by placing them in the $out/doc, $out/lib and $out/share directories, for example.

The notify-osd package does exactly that: producing a service file in share/dbus-1/services. Problem solved! Now I know that uninstalling notify-osd will also remove the service for activation.

Just not sure if NixOS will restart the D-Bus daemon once notify-osd is uninstalled or perhaps D-Bus keeps an eye on the dbus-1/services directory to appropriately respond to file changes.

I’ll check it out later… I had a b-day bash to attend at nine and it’s quarter to midnight. :stuck_out_tongue_closed_eyes::confetti_ball:


It’s Sunday and this post is already too long, so I’ll work on the features I wanted on another occassion.

I removed the notify-osd package and installed dunst instead, but D-Bus didn’t take notice of the change. For the entire duration of my session, notify-osd would pop up to any relevant message sent to the org.freedekstop.Notifications connection.

Since the D-Bus session bus lasts for the duration of the user session, I basically had to log out and log back in to get dunst to pick up my notifications.

Actually, I even tried killing the D-Bus session daemon in order to observe if that would trigger a restart. It didn’t :laughing: but it did take down all of my windows rather spectacularly. :boom: I had a good laugh.

Someday I will have to figure out what the point of dbus-launch is.

Someday… :rainbow:

  1. If you manage to figure out where this is documented, please let me know. As with many things NixOS :snowflake: related, the documentation is continually improving but as of yet still hard to search sometimes.