listen: implement systemd socket activation

INSTALL

@@ -139,6 +139,9 @@ For the sticker database.
 libcdio - http://www.gnu.org/software/libcdio/
 For playing audio CDs.
 
+libsystemd-daemon - http://freedesktop.org/wiki/Software/systemd/
+For systemd activation.
+
 
 pkg-config
 ----------

Makefile.am

@@ -34,6 +34,7 @@ src_mpd_LDADD = \
 	$(FILTER_LIBS) \
 	$(ENCODER_LIBS) \
 	$(MIXER_LIBS) \
+	$(SYSTEMD_DAEMON_LIBS) \
 	$(GLIB_LIBS)
 
 mpd_headers = \

NEWS

@@ -34,6 +34,7 @@ ver 0.17 (2011/??/??)
 * cue: show CUE track numbers
 * allow port specification in "bind_to_address" settings
 * support floating point samples
+* systemd socket activation
 
 
 ver 0.16.8 (2012/??/??)

configure.ac

@@ -353,6 +353,11 @@ AC_ARG_ENABLE(sqlite,
 		[enable support for the SQLite database]),,
 	[enable_sqlite=auto])
 
+AC_ARG_ENABLE(systemd-daemon,
+	AS_HELP_STRING([--enable-systemd-daemon],
+		[use the systemd daemon library (default=auto)]),,
+	[enable_systemd_daemon=auto])
+
 AC_ARG_ENABLE(tcp,
 	AS_HELP_STRING([--disable-tcp],
 		[disable support for clients connecting via TCP (default: enable)]),,
@@ -495,6 +500,13 @@ if
 	AC_MSG_ERROR([No client interfaces configured!])
 fi
 
+MPD_AUTO_PKG(systemd_daemon, SYSTEMD_DAEMON, libsystemd-daemon,
+	[systemd activation], [libsystemd-daemon not found])
+AM_CONDITIONAL(ENABLE_SYSTEMD_DAEMON, test x$enable_systemd_daemon = xyes)
+if test x$enable_systemd_daemon = xyes; then
+	AC_DEFINE([ENABLE_SYSTEMD_DAEMON], 1, [Define to use the systemd daemon library])
+fi
+
 dnl ---------------------------------------------------------------------------
 dnl LIBC Features
 dnl ---------------------------------------------------------------------------

doc/user.xml

@@ -99,6 +99,47 @@ cd mpd-version</programlisting>
 
       <programlisting>make install</programlisting>
     </section>
+
+    <section>
+      <title><filename>systemd</filename> socket activation</title>
+
+      <para>
+        Using <filename>systemd</filename>, you can launch
+        <filename>mpd</filename> on demand when the first client
+        attempts to connect.  Create two files in
+        <filename>/etc/systemd/system/</filename>; first
+        <filename>mpd.socket</filename>:
+      </para>
+
+      <programlisting>[Socket]
+ListenStream=/run/mpd.socket
+ListenStream=6600
+[Install]
+WantedBy=sockets.target</programlisting>
+
+      <para>
+        Now create <filename>mpd.service</filename>:
+      </para>
+
+      <programlisting>[Unit]
+Description=Music Player Daemon
+After=sound.target
+[Service]
+ExecStart=/usr/bin/mpd --stdout --no-daemon</programlisting>
+
+      <para>
+        Start the socket:
+      </para>
+
+      <programlisting>systemctl enable mpd.socket
+systemctl start mpd.socket</programlisting>
+
+      <para>
+        In this configuration, <filename>mpd</filename> will ignore
+        the <varname>bind_to_address</varname> and
+        <varname>port</varname> settings.
+      </para>
+    </section>
   </chapter>
 
   <chapter>

src/listen.c

@@ -28,6 +28,10 @@
 #include <string.h>
 #include <assert.h>
 
+#ifdef ENABLE_SYSTEMD_DAEMON
+#include <systemd/sd-daemon.h>
+#endif
+
 #undef G_LOG_DOMAIN
 #define G_LOG_DOMAIN "listen"
 
@@ -61,6 +65,30 @@ listen_add_config_param(unsigned int port,
 	}
 }
 
+static bool
+listen_systemd_activation(GError **error_r)
+{
+#ifdef ENABLE_SYSTEMD_DAEMON
+	int n = sd_listen_fds(true);
+	if (n <= 0) {
+		if (n < 0)
+			g_warning("sd_listen_fds() failed: %s",
+				  g_strerror(-n));
+		return false;
+	}
+
+	for (int i = SD_LISTEN_FDS_START, end = SD_LISTEN_FDS_START + n;
+	     i != end; ++i)
+		if (!server_socket_add_fd(listen_socket, i, error_r))
+			return false;
+
+	return true;
+#else
+	(void)error_r;
+	return false;
+#endif
+}
+
 bool
 listen_global_init(GError **error_r)
 {
@@ -72,6 +100,14 @@ listen_global_init(GError **error_r)
 
 	listen_socket = server_socket_new(listen_callback, NULL);
 
+	if (listen_systemd_activation(&error))
+		return true;
+
+	if (error != NULL) {
+		g_propagate_error(error_r, error);
+		return false;
+	}
+
 	if (param != NULL) {
 		/* "bind_to_address" is configured, create listeners
 		   for all values */