Add external socket launching support

* src/process.c (connect_network_socket): Allow a pre-allocated socket
descriptor to be used if passed to Emacs, avoiding the call to
'socket' and 'bind'.
(Fmake_network_process): Allow users to pass ':use-external-socket' in
the parameter plist to use any sockets that have been passed to Emacs.
(wait_reading_process_output): Call 'socket' and 'bind' every time.
(syms_of_process): New symbol ':use-external-socket'.
(set_external_socket_descriptor): New function.
(external_sock_fd): New variable.
* src/lisp.h: (set_external_socket_descriptor): New declaration.
* src/emacs.c (main): Call 'sd_listen_fds' to read the number of sockets
passed and call 'set_external_socket_descriptor' to set the external
socket.
* src/Makefile.in: Add libsystemd library and C flags to the Emacs
compilation options.

* configure.ac: Add new default-on option "systemd" and check for
libsystemd at configure time.

* lisp/server.el (server-start): Set ':use-external-socket' to 't' when
calling 'make-network-process'.

* etc/NEWS: Document new socket-passing functionality and the configure
option to disable systemd interaction.

* doc/emacs/misc.texi (Emacs Server): Document systemd socket passing
functionality and provide systemd unit examples.
* doc/lispref/processes.texi (Network Processes): Document new
'make-network-process' option ':use-external-socket'.

1 files changed, 22 insertions, 0 deletions

diff --git a/src/emacs.c b/src/emacs.c
index c21c9e3971c..a51df0903b1 100644
--- a/src/emacs.c
+++ b/src/emacs.c
@@ -56,6 +56,11 @@ along with GNU Emacs.  If not, see <http://www.gnu.org/licenses/>.  */
 #include <binary-io.h>
 #endif
 
+#ifdef HAVE_LIBSYSTEMD
+#include <systemd/sd-daemon.h>
+#include <sys/socket.h>
+#endif /* HAVE_LIBSYSTEMD */
+
 #ifdef HAVE_WINDOW_SYSTEM
 #include TERM_HEADER
 #endif /* HAVE_WINDOW_SYSTEM */
@@ -676,6 +681,9 @@ main (int argc, char **argv)
   char dname_arg2[80];
 #endif
   char *ch_to_dir = 0;
+#ifdef HAVE_LIBSYSTEMD
+  int systemd_socket;
+#endif
 
   /* If we use --chdir, this records the original directory.  */
   char *original_pwd = 0;
@@ -999,6 +1007,20 @@ main (int argc, char **argv)
 	  exit (1);
 	}
 
+#ifdef HAVE_LIBSYSTEMD
+      /* Read the number of sockets passed through by systemd. */
+      systemd_socket = sd_listen_fds(1);
+
+      if (systemd_socket > 1)
+        fprintf (stderr, "\nWarning: systemd has passed more than one socket to the Emacs process.\n\
+Try adding 'Accept=false' in the Emacs socket unit file.\n");
+
+      else if (systemd_socket == 1 &&
+               sd_is_socket (SD_LISTEN_FDS_START,
+                             AF_UNSPEC, SOCK_STREAM, 1) >= 0)
+        set_external_socket_descriptor (SD_LISTEN_FDS_START);
+#endif /* HAVE_LIBSYSTEMD */
+
 #ifndef DAEMON_MUST_EXEC
 #ifdef USE_GTK
       fprintf (stderr, "\nWarning: due to a long standing Gtk+ bug\nhttp://bugzilla.gnome.org/show_bug.cgi?id=85715\n\