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'.

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])
@@ -2732,6 +2733,18 @@ 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
 

doc/emacs/misc.texi

@@ -1580,6 +1580,44 @@ 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.
+
+@cindex socket activation, systemd, Emacs
+@item
+An external process can invoke the Emacs server when a connection
+event occurs upon a specified socket and pass the socket to the new
+Emacs server process.  An instance of this is @command{systemd}'s
+socket functionaly: the @command{systemd} service creates a socket and
+listens for connections on it; when @command{emacsclient} connects to
+it for the first time, @command{systemd} can launch the Emacs server
+and hand over the socket to it for servicing @command{emacsclient}
+connections.  A setup to use this functionality could be:
+
+@file{~/.config/systemd/user/emacs.service}:
+@example
+[Unit]
+Description=Emacs
+
+[Service]
+Type=forking
+ExecStart=/path/to/emacs --daemon
+ExecStop=/path/to/emacsclient --eval "(kill-emacs)"
+Restart=always
+
+[Install]
+WantedBy=default.target
+@end example
+
+@file{~/.config/systemd/user/emacs.socket}:
+@example
+[Socket]
+ListenStream=/path/to/.emacs.socket
+
+[Install]
+WantedBy=sockets.target
+@end 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

doc/lispref/processes.texi

@@ -2367,6 +2367,12 @@ automatically for the given @var{host} and @var{service}.
 ignored.  @code{ipv4} and @code{ipv6} specify to use IPv4 and IPv6,
 respectively.
 
+@item :use-external-socket @var{use-external-socket}
+If @var{use-external-socket} is non-@code{nil} use any sockets passed
+to Emacs on invocation instead of allocating one.  This is used by the
+Emacs server code to allow on-demand socket activation.  If Emacs
+wasn't passed 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

etc/NEWS

@@ -32,6 +32,13 @@ the default in developer builds.  As before, use
 '--enable-gcc-warnings' to stop the build if GCC issues warnings.
 
 +++
+** Emacs server now has socket-launching support.  This allows socket
+based activation, where an external process can invoke the Emacs
+server process upon a socket connection event and hand over the socket
+to Emacs.  Emacs will use this socket for servicing emacsclient
+commands.  systemd can make use of this new functionally but can be
+disabled with the configure option '--disable-libsystemd'.
+
 ** 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.

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-external-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

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)"

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\

src/lisp.h

@@ -4184,6 +4184,7 @@ extern void delete_gpm_wait_descriptor (int);
 extern void init_process_emacs (void);
 extern void syms_of_process (void);
 extern void setup_process_coding_systems (Lisp_Object);
+extern void set_external_socket_descriptor (int);
 
 /* Defined in callproc.c.  */
 #ifndef DOS_NT

src/process.c

@@ -267,6 +267,9 @@ static int max_process_desc;
 /* The largest descriptor currently in use for input; -1 if none.  */
 static int max_input_desc;
 
+/* The descriptor  of any sockets passed to Emacs; -1 if none. */
+static int external_sock_fd = -1;
+
 /* Indexed by descriptor, gives the process (if any) for that descriptor.  */
 static Lisp_Object chan_process[FD_SETSIZE];
 static void wait_for_socket_fds (Lisp_Object, char const *);
@@ -3075,7 +3078,8 @@ 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_external_socket_p)
 {
   ptrdiff_t count = SPECPDL_INDEX ();
   ptrdiff_t count1;
@@ -3089,6 +3093,16 @@ 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 socket_to_use = -1;
+
+  if (!NILP (use_external_socket_p))
+    {
+      socket_to_use = external_sock_fd;
+
+      /* Ensure we don't consume the external socket twice. */
+      external_sock_fd = -1;
+    }
+
 
   /* Do this in case we never enter the while-loop below.  */
   count1 = SPECPDL_INDEX ();
@@ -3109,7 +3123,11 @@ 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 (socket_to_use != -1)
+          s = socket_to_use;
+      else
+          s = socket (family, p->socktype | SOCK_CLOEXEC, p->ai_protocol);
+
       if (s < 0)
 	{
 	  xerrno = errno;
@@ -3168,8 +3186,11 @@ 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, it should be
+             already bound. */
+	  if (socket_to_use == -1)
+	    if (bind (s, sa, addrlen))
+	      report_file_error ("Cannot bind server socket", Qnil);
 
 #ifdef HAVE_GETSOCKNAME
 	  if (p->port == 0)
@@ -3534,6 +3555,11 @@ 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-external-socket BOOL -- Use any pre-allocated sockets that have
+                             been passed to Emacs.  If Emacs wasn't
+                             passed a socket, this option is silently
+                             ignored.
+
 
 Consult the relevant system programmer's manual pages for more
 information on using these options.
@@ -3578,7 +3604,7 @@ 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_external_socket_p;
   Lisp_Object ip_addresses = Qnil;
   int socktype;
   int family = -1;
@@ -3618,6 +3644,7 @@ usage: (make-network-process &rest ARGS)  */)
   buffer = Fplist_get (contact, QCbuffer);
   filter = Fplist_get (contact, QCfilter);
   sentinel = Fplist_get (contact, QCsentinel);
+  use_external_socket_p = Fplist_get (contact, QCuse_external_socket);
 
   CHECK_STRING (name);
 
@@ -3914,7 +3941,7 @@ usage: (make-network-process &rest ARGS)  */)
     }
 #endif
 
-  connect_network_socket (proc, ip_addresses);
+  connect_network_socket (proc, ip_addresses, use_external_socket_p);
   return proc;
 }
 
@@ -4848,7 +4875,7 @@ 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;
 		  }
@@ -7709,6 +7736,16 @@ catch_child_signal (void)
 }
 #endif	/* subprocesses */
 
+/* Set the external socket descriptor for Emacs to use when
+   `make-network-process' is called with a non-nil
+   `:use-external-socket' option.  The fd should have been checked to
+   ensure it is a valid socket and is already bound.  */
+void
+set_external_socket_descriptor(int fd)
+{
+    external_sock_fd = fd;
+}
+
 
 /* This is not called "init_process" because that is the name of a
    Mach system call, so it would cause problems on Darwin systems.  */
@@ -7837,6 +7874,7 @@ syms_of_process (void)
   DEFSYM (QCserver, ":server");
   DEFSYM (QCnowait, ":nowait");
   DEFSYM (QCsentinel, ":sentinel");
+  DEFSYM (QCuse_external_socket, ":use-external-socket");
   DEFSYM (QCtls_parameters, ":tls-parameters");
   DEFSYM (Qnsm_verify_connection, "nsm-verify-connection");
   DEFSYM (QClog, ":log");