[PATCH v4] Add systemd socket launching support

[Matthew Leach]

Hi all,

Here is a new version of the systemd integration patches.  Main changes
include, documentation updates (document that the :use-systemd-socket
option may be silently ignored and include systemd units in the Emacs
manual), and rename config.h macro to HAVE_LIBSYSTEMD.

Feedback & comments welcome!

Thanks,
-- 
Matt

>From d5972913f8c6ed8b4de6054e14395a373f2ee96e Mon Sep 17 00:00:00 2001
From: Matthew Leach <address@hidden>
Date: Sat, 26 Mar 2016 16:41:17 +0000
Subject: [PATCH 1/4] Check for libsystemd when building Emacs.

* configure.ac: Add new default-on option systemd and check for
  libsystemd at configure time.
* src/Makefile.in: Add libsystemd library and C flags to the Emacs
  compilation options.
---
 configure.ac    | 13 +++++++++++++
 src/Makefile.in |  6 +++++-
 2 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/configure.ac b/configure.ac
index f3846f4..f53c670 100644
--- a/configure.ac
+++ b/configure.ac
@@ -330,6 +330,7 @@ OPTION_DEFAULT_ON([tiff],[don't compile with TIFF image 
support])
 OPTION_DEFAULT_ON([gif],[don't compile with GIF image support])
 OPTION_DEFAULT_ON([png],[don't compile with PNG image support])
 OPTION_DEFAULT_ON([rsvg],[don't compile with SVG image support])
+OPTION_DEFAULT_ON([libsystemd],[don't compile with libsystemd support])
 OPTION_DEFAULT_OFF([cairo],[compile with Cairo drawing (experimental)])
 OPTION_DEFAULT_ON([xml2],[don't compile with XML parsing support])
 OPTION_DEFAULT_ON([imagemagick],[don't compile with ImageMagick image support])
@@ -2716,6 +2717,18 @@ fi
 AC_SUBST(LIBGNUTLS_LIBS)
 AC_SUBST(LIBGNUTLS_CFLAGS)
 
+HAVE_LIBSYSTEMD=no
+if test "${with_libsystemd}" = "yes" ; then
+  EMACS_CHECK_MODULES([LIBSYSTEMD], [libsystemd >= 226],
+    [HAVE_LIBSYSTEMD=yes], [HAVE_LIBSYSTEMD=no])
+  if test "${HAVE_LIBSYSTEMD}" = "yes"; then
+    AC_DEFINE(HAVE_LIBSYSTEMD, 1, [Define if using libsystemd.])
+  fi
+fi
+
+AC_SUBST(LIBSYSTEMD_LIBS)
+AC_SUBST(LIBSYSTEMD_CFLAGS)
+
 NOTIFY_OBJ=
 NOTIFY_SUMMARY=no
 
diff --git a/src/Makefile.in b/src/Makefile.in
index c290a60..fc9360a 100644
--- a/src/Makefile.in
+++ b/src/Makefile.in
@@ -307,6 +307,9 @@ LIBSELINUX_LIBS = @LIBSELINUX_LIBS@
 LIBGNUTLS_LIBS = @LIBGNUTLS_LIBS@
 LIBGNUTLS_CFLAGS = @LIBGNUTLS_CFLAGS@
 
+LIBSYSTEMD_LIBS = @LIBSYSTEMD_LIBS@
+LIBSYSTEMD_CFLAGS = @LIBSYSTEMD_CFLAGS@
+
 INTERVALS_H = dispextern.h intervals.h composite.h
 
 GETLOADAVG_LIBS = @GETLOADAVG_LIBS@
@@ -372,6 +375,7 @@ ALL_CFLAGS=-Demacs $(MYCPPFLAGS) -I. -I$(srcdir) \
   $(WEBKIT_CFLAGS) \
   $(SETTINGS_CFLAGS) $(FREETYPE_CFLAGS) $(FONTCONFIG_CFLAGS) \
   $(LIBOTF_CFLAGS) $(M17N_FLT_CFLAGS) $(DEPFLAGS) \
+  $(LIBSYSTEMD_CFLAGS) \
   $(LIBGNUTLS_CFLAGS) $(NOTIFY_CFLAGS) $(CAIRO_CFLAGS) \
   $(WARN_CFLAGS) $(WERROR_CFLAGS) $(CFLAGS)
 ALL_OBJC_CFLAGS=$(ALL_CFLAGS) $(GNU_OBJC_CFLAGS)
@@ -489,7 +493,7 @@ LIBES = $(LIBS) $(W32_LIBS) $(LIBS_GNUSTEP) $(LIBX_BASE) 
$(LIBIMAGE) \
    $(LIBS_TERMCAP) $(GETLOADAVG_LIBS) $(SETTINGS_LIBS) $(LIBSELINUX_LIBS) \
    $(FREETYPE_LIBS) $(FONTCONFIG_LIBS) $(LIBOTF_LIBS) $(M17N_FLT_LIBS) \
    $(LIBGNUTLS_LIBS) $(LIB_PTHREAD) $(GETADDRINFO_A_LIBS) \
-   $(NOTIFY_LIBS) $(LIB_MATH) $(LIBZ) $(LIBMODULES)
+   $(NOTIFY_LIBS) $(LIB_MATH) $(LIBZ) $(LIBMODULES) $(LIBSYSTEMD_LIBS)
 
 $(leimdir)/leim-list.el: bootstrap-emacs$(EXEEXT)
        $(MAKE) -C ../leim leim-list.el EMACS="$(bootstrap_exe)"
-- 
2.7.4

>From 5f5ba1ce322a0c4b5ff7f2c7eb939928aec799c7 Mon Sep 17 00:00:00 2001
From: Matthew Leach <address@hidden>
Date: Sat, 26 Mar 2016 18:50:14 +0000
Subject: [PATCH 2/4] Read the number of sockets passed by systemd.

* src/emacs.c (systemd_socket): New variable for storing the socket
  descriptor passed in by systemd.
  (main): Call sd_listen_fds to read the number of sockets passed.
---
 src/emacs.c | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/src/emacs.c b/src/emacs.c
index 95d1905..67906e8 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 */
@@ -195,6 +200,12 @@ int daemon_pipe[2];
 HANDLE w32_daemon_event;
 #endif
 
+#ifdef HAVE_LIBSYSTEMD
+/* The socket descriptor passed by systemd.  If nothing has been
+   passed, this will be 0. */
+int systemd_socket = 0;
+#endif /* HAVE_LIBSYSTEMD */
+
 /* Save argv and argc.  */
 char **initial_argv;
 int initial_argc;
@@ -997,6 +1008,24 @@ 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");
+          systemd_socket = 0;
+        }
+      else if (systemd_socket == 1 &&
+               sd_is_socket (SD_LISTEN_FDS_START,
+                             AF_UNSPEC, SOCK_STREAM, 1) >= 0)
+        systemd_socket = SD_LISTEN_FDS_START;
+      else
+        systemd_socket = 0;
+#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\
-- 
2.7.4

>From 2ed8471b2c6157e5639570d865db98e9fe33fb36 Mon Sep 17 00:00:00 2001
From: Matthew Leach <address@hidden>
Date: Sat, 26 Mar 2016 19:37:10 +0000
Subject: [PATCH 3/4] Allow network processes to be made with a pre-allocated
 fd.

* 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 in :use-systemd-socket on
the parameter plist to use a socket descriptor that has been passed to
Emacs from systemd.
(wait_reading_process_output): Call socket() & bind() every time.
(syms_of_process): New symbol ":use-passed-socket".
* doc/lispref/processes.texi (Network Processes): Document new
  `make-network-process' option ':use-systemd-socket'.
---
 doc/lispref/processes.texi |  7 +++++++
 src/process.c              | 39 ++++++++++++++++++++++++++++++++-------
 2 files changed, 39 insertions(+), 7 deletions(-)

diff --git a/doc/lispref/processes.texi b/doc/lispref/processes.texi
index 8d3df55..83c1aa2 100644
--- a/doc/lispref/processes.texi
+++ b/doc/lispref/processes.texi
@@ -2367,6 +2367,13 @@ automatically for the given @var{host} and @var{service}.
 ignored.  @code{ipv4} and @code{ipv6} specify to use IPv4 and IPv6,
 respectively.
 
address@hidden :use-systemd-socket @var{use-systemd-socket}
+If @var{use-systemd-socket} is address@hidden use any sockets passed
+by systemd instead of allocating one.  This is used by the Emacs
+server code to allow on-demand socket activation.  If Emacs hasn't
+been compiled with libsystemd support or systemd didn't pass a socket,
+this option is silently ignored.
+
 @item :local @var{local-address}
 For a server process, @var{local-address} is the address to listen on.
 It overrides @var{family}, @var{host} and @var{service}, so you
diff --git a/src/process.c b/src/process.c
index 198e7de..85ffd25 100644
--- a/src/process.c
+++ b/src/process.c
@@ -3075,7 +3075,8 @@ finish_after_tls_connection (Lisp_Object proc)
 #endif
 
 static void
-connect_network_socket (Lisp_Object proc, Lisp_Object ip_addresses)
+connect_network_socket (Lisp_Object proc, Lisp_Object ip_addresses,
+                        Lisp_Object use_systemd_socket_p)
 {
   ptrdiff_t count = SPECPDL_INDEX ();
   ptrdiff_t count1;
@@ -3089,6 +3090,16 @@ connect_network_socket (Lisp_Object proc, Lisp_Object 
ip_addresses)
   struct Lisp_Process *p = XPROCESS (proc);
   Lisp_Object contact = p->childp;
   int optbits = 0;
+  int systemd_socket_descriptor = 0;
+
+#ifdef HAVE_LIBSYSTEMD
+  if (!NILP (use_systemd_socket_p))
+    {
+      extern int systemd_socket;
+      systemd_socket_descriptor = systemd_socket;
+    }
+#endif /* HAVE_LIBSYSTEMD */
+
 
   /* Do this in case we never enter the while-loop below.  */
   count1 = SPECPDL_INDEX ();
@@ -3109,7 +3120,11 @@ connect_network_socket (Lisp_Object proc, Lisp_Object 
ip_addresses)
       sa = xmalloc (addrlen);
       conv_lisp_to_sockaddr (family, ip_address, sa, addrlen);
 
-      s = socket (family, p->socktype | SOCK_CLOEXEC, p->ai_protocol);
+      if (systemd_socket_descriptor)
+          s = systemd_socket_descriptor;
+      else
+          s = socket (family, p->socktype | SOCK_CLOEXEC, p->ai_protocol);
+
       if (s < 0)
        {
          xerrno = errno;
@@ -3168,8 +3183,11 @@ connect_network_socket (Lisp_Object proc, Lisp_Object 
ip_addresses)
                  report_file_error ("Cannot set reuse option on server 
socket", Qnil);
              }
 
-         if (bind (s, sa, addrlen))
-           report_file_error ("Cannot bind server socket", Qnil);
+          /* If we are passed a socket descriptor from systemd, it is
+             already bound. */
+         if (!systemd_socket_descriptor)
+           if (bind (s, sa, addrlen))
+             report_file_error ("Cannot bind server socket", Qnil);
 
 #ifdef HAVE_GETSOCKNAME
          if (p->port == 0)
@@ -3534,6 +3552,11 @@ The following network options can be specified for this 
connection:
                       (this is allowed by default for a server process).
 :bindtodevice NAME -- bind to interface NAME.  Using this may require
                       special privileges on some systems.
+:use-systemd-socket BOOL -- Use any pre-allocated sockets that have
+                            been passed to Emacs by systemd.  If Emacs
+                            hasn't been compiled with libsystemd support
+                            or systemd didn't pass a socket, this option
+                            is silently ignored.
 
 Consult the relevant system programmer's manual pages for more
 information on using these options.
@@ -3578,7 +3601,7 @@ usage: (make-network-process &rest ARGS)  */)
   EMACS_INT port = 0;
   Lisp_Object tem;
   Lisp_Object name, buffer, host, service, address;
-  Lisp_Object filter, sentinel;
+  Lisp_Object filter, sentinel, use_systemd_socket_p;
   Lisp_Object ip_addresses = Qnil;
   int socktype;
   int family = -1;
@@ -3618,6 +3641,7 @@ usage: (make-network-process &rest ARGS)  */)
   buffer = Fplist_get (contact, QCbuffer);
   filter = Fplist_get (contact, QCfilter);
   sentinel = Fplist_get (contact, QCsentinel);
+  use_systemd_socket_p = Fplist_get (contact, QCuse_systemd_socket);
 
   CHECK_STRING (name);
 
@@ -3914,7 +3938,7 @@ usage: (make-network-process &rest ARGS)  */)
     }
 #endif
 
-  connect_network_socket (proc, ip_addresses);
+  connect_network_socket (proc, ip_addresses, use_systemd_socket_p);
   return proc;
 }
 
@@ -4848,7 +4872,7 @@ wait_reading_process_output (intmax_t time_limit, int 
nsecs, int read_kbd,
                  {
                    Lisp_Object ip_addresses = check_for_dns (aproc);
                    if (!NILP (ip_addresses) && !EQ (ip_addresses, Qt))
-                     connect_network_socket (aproc, ip_addresses);
+                     connect_network_socket (aproc, ip_addresses, Qnil);
                    else
                      retry_for_async = true;
                  }
@@ -7837,6 +7861,7 @@ syms_of_process (void)
   DEFSYM (QCserver, ":server");
   DEFSYM (QCnowait, ":nowait");
   DEFSYM (QCsentinel, ":sentinel");
+  DEFSYM (QCuse_systemd_socket, ":use-systemd-socket");
   DEFSYM (QCtls_parameters, ":tls-parameters");
   DEFSYM (Qnsm_verify_connection, "nsm-verify-connection");
   DEFSYM (QClog, ":log");
-- 
2.7.4

>From a09250b5c53c8da1157135f1ace2ecd4748d6510 Mon Sep 17 00:00:00 2001
From: Matthew Leach <address@hidden>
Date: Sat, 26 Mar 2016 20:43:26 +0000
Subject: [PATCH 4/4] When Emacs is passed a socket descriptor, make the server
 listen on it.

* lisp/server.el (server-start): Set :use-systemd-socket to `t' when
  calling `make-network-process'.
* etc/NEWS: Document new systemd functionality and build option to
  disable it.
* doc/emacs/misc.texi (Emacs Server): Document systemd socket passing
  functionality and provide systemd unit examples.
---
 doc/emacs/misc.texi | 37 +++++++++++++++++++++++++++++++++++++
 etc/NEWS            |  7 +++++++
 lisp/server.el      |  1 +
 3 files changed, 45 insertions(+)

diff --git a/doc/emacs/misc.texi b/doc/emacs/misc.texi
index b5a2150..2622592 100644
--- a/doc/emacs/misc.texi
+++ b/doc/emacs/misc.texi
@@ -1580,6 +1580,43 @@ option.  @xref{Initial Options}.  When Emacs is started 
this way, it
 calls @code{server-start} after initialization, and returns control to
 the calling terminal instead of opening an initial frame; it then
 waits in the background, listening for edit requests.
+
address@hidden socket activation, systemd, Emacs
address@hidden
+If Emacs has been built with @command{systemd} support, the Emacs
+server can be started by socket activation.  The @command{systemd}
+service creates a socket and listens for connections on it; when
address@hidden connects to it for the first time,
address@hidden can launch the Emacs server and hands over the
+socket to it for servicing @command{emacsclient} connections.  A setup
+to use this functionality could be:
+
address@hidden/.config/systemd/user/emacs.service}:
address@hidden
+[Unit]
+Description=Emacs
+
+[Service]
+Type=forking
+ExecStart=/path/to/emacs --daemon
+ExecStop=/path/to/emacsclient --eval "(kill-emacs)"
+Restart=always
+
+[Install]
+WantedBy=default.target
address@hidden example
+
address@hidden/.config/systemd/user/emacs.socket}:
address@hidden
+[Socket]
+ListenStream=/path/to/.emacs.socket
+
+[Install]
+WantedBy=sockets.target
address@hidden example
+
+The @code{ListenStream} path will be the path that Emacs listens for
+connections from @command{emacsclient}; this is a file of your choice.
 @end itemize
 
 @cindex @env{TEXEDIT} environment variable
diff --git a/etc/NEWS b/etc/NEWS
index 66777e9..9afed5b 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -26,6 +26,13 @@ otherwise leave it unmarked.
 * Installation Changes in Emacs 25.2
 
 +++
+** Emacs now has systemd support.  This allows socket based
+activation, where systemd can invoke the Emacs process upon a
+socket connection and hand over the socket to Emacs.  Emacs will
+use this socket for communicating with the emacsclient program.
+This can be disabled with the new configure option
+'--disable-systemd'.
+
 ** New configure option '--disable-build-details' attempts to build an
 Emacs that is more likely to be reproducible; that is, if you build
 and install Emacs twice, the second Emacs is a copy of the first.
diff --git a/lisp/server.el b/lisp/server.el
index 5243820..2c9113c 100644
--- a/lisp/server.el
+++ b/lisp/server.el
@@ -655,6 +655,7 @@ server or call `\\[server-force-delete]' to forcibly 
disconnect it."))
                       :noquery t
                       :sentinel #'server-sentinel
                       :filter #'server-process-filter
+                      :use-systemd-socket t
                       ;; We must receive file names without being decoded.
                       ;; Those are decoded by server-process-filter according
                       ;; to file-name-coding-system.  Also don't get
-- 
2.7.4