systemd: add user unit

The user unit omits the "ProtectKernelModules" setting which fails
with modular kernels:

 Failed at step CAPABILITIES spawning /usr/bin/mpd: Operation not permitted

It is unfortunate that systemd (version 232) is unable to reduce its
own capabilities, because this requires us to split system and user
units.

 https://bugs.musicpd.org/view.php?id=4608

Makefile.am

@@ -1453,6 +1453,11 @@ systemdsystemunit_DATA = \
 	systemd/system/mpd.service
 endif
 
+if HAVE_SYSTEMD_USER
+systemduserunit_DATA = \
+	systemd/user/mpd.service
+endif
+
 
 #
 # Test programs

NEWS

@@ -2,6 +2,7 @@ ver 0.19.20 (not yet released)
 * decoder
   - ffmpeg: fix crash bug
 * fix unit test failure after recent "setprio" change
+* systemd: add user unit
 
 ver 0.19.20 (2016/12/09)
 * protocol

configure.ac

@@ -52,6 +52,22 @@ if test "x$with_systemdsystemunitdir" != xno; then
 fi
 AM_CONDITIONAL(HAVE_SYSTEMD, [test -n "$with_systemdsystemunitdir" -a "x$with_systemdsystemunitdir" != xno ])
 
+AC_ARG_WITH([systemduserunitdir],
+	    AS_HELP_STRING([--with-systemduserunitdir=DIR], [Directory for systemd service files]),
+	    [], [with_systemduserunitdir=no])
+if test "x$with_systemduserunitdir" = xyes; then
+	AC_MSG_CHECKING(for systemd)
+	with_systemduserunitdir=$($PKG_CONFIG --variable=systemduserunitdir systemd)
+	if test -z "$with_systemduserunitdir"; then
+		AC_MSG_ERROR([Failed to detect systemd])
+	fi
+	AC_MSG_RESULT([$with_systemduserunitdir])
+fi
+if test "x$with_systemduserunitdir" != xno; then
+	AC_SUBST([systemduserunitdir], [$with_systemduserunitdir])
+fi
+AM_CONDITIONAL(HAVE_SYSTEMD_USER, [test -n "$with_systemduserunitdir" -a "x$with_systemduserunitdir" != xno ])
+
 dnl ---------------------------------------------------------------------------
 dnl Declare Variables
 dnl ---------------------------------------------------------------------------
@@ -1906,6 +1922,7 @@ dnl ---------------------------------------------------------------------------
 AC_CONFIG_FILES(Makefile)
 AC_CONFIG_FILES(doc/doxygen.conf)
 AC_CONFIG_FILES(systemd/system/mpd.service)
+AC_CONFIG_FILES(systemd/user/mpd.service)
 AC_OUTPUT
 
 echo 'MPD is ready for compilation, type "make" to begin.'

doc/user.xml

@@ -167,6 +167,33 @@ systemctl start mpd.socket</programlisting>
         <varname>port</varname> settings.
       </para>
     </section>
+
+    <section id="systemd_user">
+      <title><filename>systemd</filename> user unit</title>
+
+      <para>
+        You can launch <application>MPD</application> as a
+        <filename>systemd</filename> user unit.  The service file will
+        only be installed when <application>MPD</application> was
+        configured with
+        <parameter>--with-systemduserunitdir=/usr/lib/systemd/user</parameter>
+        or
+        <parameter>--with-systemduserunitdir=$HOME/.local/share/systemd/user</parameter>.
+      </para>
+
+      <para>
+        Once the user unit is installed, you can start and stop
+        <application>MPD</application> like any other service:
+      </para>
+
+      <programlisting>systemctl --user start mpd</programlisting>
+
+      <para>
+        To auto-start <application>MPD</application> upon login, type:
+      </para>
+
+      <programlisting>systemctl --user enable mpd</programlisting>
+    </section>
   </chapter>
 
   <chapter id="config">

systemd/user/mpd.service.in

@@ -0,0 +1,29 @@
+[Unit]
+Description=Music Player Daemon
+After=network.target sound.target
+
+[Service]
+ExecStart=@prefix@/bin/mpd --no-daemon
+
+# allow MPD to use real-time priority 50
+LimitRTPRIO=50
+LimitRTTIME=infinity
+
+# disallow writing to /usr, /bin, /sbin, ...
+ProtectSystem=yes
+
+# more paranoid security settings
+NoNewPrivileges=yes
+ProtectKernelTunables=yes
+ProtectControlGroups=yes
+# AF_NETLINK is required by libsmbclient, or it will exit() .. *sigh*
+RestrictAddressFamilies=AF_INET AF_INET6 AF_UNIX AF_NETLINK
+RestrictNamespaces=yes
+
+# Note that "ProtectKernelModules=yes" is missing in the user unit
+# because systemd 232 is unable to reduce its own capabilities
+# ("Failed at step CAPABILITIES spawning /usr/bin/mpd: Operation not
+# permitted")
+
+[Install]
+WantedBy=default.target