From 56fc0aef450e199bc93ea75b15dff778713de81f Mon Sep 17 00:00:00 2001
From: Matthew Leach <matthew@mattleach.net>
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-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/emacs/misc.texi | 38 ++++++++++++++++++++++++++++++++++++++
 etc/NEWS            |  7 +++++++
 lisp/server.el      |  1 +
 3 files changed, 46 insertions(+)

diff --git a/doc/emacs/misc.texi b/doc/emacs/misc.texi
index b5a2150..775cda9 100644
--- a/doc/emacs/misc.texi
+++ b/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
diff --git a/etc/NEWS b/etc/NEWS
index 66777e9..f395d45 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -26,6 +26,13 @@ otherwise leave it unmarked.
 * Installation Changes in Emacs 25.2
 
 +++
+** 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.
diff --git a/lisp/server.el b/lisp/server.el
index 5243820..2a9729e 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-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
-- 
2.8.0