Fix server-name' and server-socket-dir' for (Bug#24218)

* lisp/server.el: (server-external-socket-initialised): New
(server-name): Compute server name from `get-external-sockname'.
(server-socket-dir): Compute socket dir from
`get-external-sockname'.
(server-start): Don't check for existing server when an
uninitialised external socket has been passed to Emacs.
* src/emacs.c: (main): Obtain socket name via getsockname and pass
to `init_process_emacs'.
* src/lisp.h: (init_process_emacs): Add second parameter.
* src/process.c: (external_sock_name): New.
(get-external-sockname): New.
(init_process_emacs): Set `external_sock_name' to `sockname'
parameter.

lisp/server.el

@@ -251,8 +251,16 @@ This means that the server should not kill the buffer when you say you
 are done with it in the server.")
 (make-variable-buffer-local 'server-existing-buffer)
 
-;;;###autoload
-(defcustom server-name "server"
+(defvar server-external-socket-initialised nil
+  "When an external socket is passed into Emacs, we need to call
+`server-start' in order to initialise the connection.  This flag
+prevents multiple initialisations when an external socket has
+been consumed.")
+
+(defcustom server-name
+  (if (get-external-sockname)
+      (file-name-nondirectory (get-external-sockname))
+    "server")
   "The name of the Emacs server, if this Emacs process creates one.
 The command `server-start' makes use of this.  It should not be
 changed while a server is running."
@@ -263,8 +271,10 @@ changed while a server is running."
 ;; We do not use `temporary-file-directory' here, because emacsclient
 ;; does not read the init file.
 (defvar server-socket-dir
-  (and (featurep 'make-network-process '(:family local))
-       (format "%s/emacs%d" (or (getenv "TMPDIR") "/tmp") (user-uid)))
+  (if (get-external-sockname)
+      (file-name-directory (get-external-sockname))
+    (and (featurep 'make-network-process '(:family local))
+         (format "%s/emacs%d" (or (getenv "TMPDIR") "/tmp") (user-uid))))
   "The directory in which to place the server socket.
 If local sockets are not supported, this is nil.")
 
@@ -618,23 +628,29 @@ To force-start a server, do \\[server-force-delete] and then
       (when server-process
 	;; kill it dead!
 	(ignore-errors (delete-process server-process)))
-      ;; Delete the socket files made by previous server invocations.
-      (if (not (eq t (server-running-p server-name)))
-	  ;; Remove any leftover socket or authentication file
-	  (ignore-errors
-	    (let (delete-by-moving-to-trash)
-	      (delete-file server-file)))
-	(setq server-mode nil) ;; already set by the minor mode code
-	(display-warning
-	 'server
-	 (concat "Unable to start the Emacs server.\n"
-		 (format "There is an existing Emacs server, named %S.\n"
-			 server-name)
-		 (substitute-command-keys
-                  "To start the server in this Emacs process, stop the existing
+      ;; Check to see if an uninitialised external socket has been
+      ;; passed in, if that is the case, skip checking
+      ;; `server-running-p' as this will return the wrong result.
+      (if (and (get-external-sockname)
+               (not server-external-socket-initialised))
+          (setq server-external-socket-initialised t)
+        ;; Delete the socket files made by previous server invocations.
+        (if (not (eq t (server-running-p server-name)))
+           ;; Remove any leftover socket or authentication file
+           (ignore-errors
+             (let (delete-by-moving-to-trash)
+               (delete-file server-file)))
+         (setq server-mode nil) ;; already set by the minor mode code
+         (display-warning
+          'server
+          (concat "Unable to start the Emacs server.\n"
+                  (format "There is an existing Emacs server, named %S.\n"
+                          server-name)
+                  (substitute-command-keys
+                    "To start the server in this Emacs process, stop the existing
 server or call `\\[server-force-delete]' to forcibly disconnect it."))
-	 :warning)
-	(setq leave-dead t))
+          :warning)
+         (setq leave-dead t)))
       ;; If this Emacs already had a server, clear out associated status.
       (while server-clients
 	(server-delete-client (car server-clients)))

src/emacs.c

@@ -60,6 +60,7 @@ along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.  */
 #ifdef HAVE_LIBSYSTEMD
 # include <systemd/sd-daemon.h>
 # include <sys/socket.h>
+# include <sys/un.h>
 #endif
 
 #ifdef HAVE_WINDOW_SYSTEM
@@ -1002,6 +1003,7 @@ main (int argc, char **argv)
 
 
   int sockfd = -1;
+  char *sockname = NULL;
 
   if (argmatch (argv, argc, "-fg-daemon", "--fg-daemon", 10, NULL, &skip_args)
       || argmatch (argv, argc, "-fg-daemon", "--fg-daemon", 10, &dname_arg, &skip_args))
@@ -1061,8 +1063,16 @@ main (int argc, char **argv)
 		  "Try 'Accept=false' in the Emacs socket unit file.\n"));
       else if (systemd_socket == 1
 	       && (0 < sd_is_socket (SD_LISTEN_FDS_START,
-				     AF_UNSPEC, SOCK_STREAM, 1)))
-	sockfd = SD_LISTEN_FDS_START;
+				    AF_UNIX, SOCK_STREAM, 1)))
+	{
+	  struct sockaddr_un sockaddr;
+	  socklen_t sockaddr_sz = sizeof(sockaddr);
+
+	  sockfd = SD_LISTEN_FDS_START;
+
+	  if (!getsockname(sockfd, &sockaddr, &sockaddr_sz))
+	    sockname = strdup(sockaddr.sun_path);
+	}
 #endif /* HAVE_LIBSYSTEMD */
 
 #ifdef USE_GTK
@@ -1660,7 +1670,7 @@ Using an Emacs configured with --with-x-toolkit=lucid does not have this problem
   /* This can create a thread that may call getenv, so it must follow
      all calls to putenv and setenv.  Also, this sets up
      add_keyboard_wait_descriptor, which init_display uses.  */
-  init_process_emacs (sockfd);
+  init_process_emacs (sockfd, sockname);
 
   init_keyboard ();	/* This too must precede init_sys_modes.  */
   if (!noninteractive)

src/lisp.h

@@ -4316,7 +4316,7 @@ extern void delete_keyboard_wait_descriptor (int);
 extern void add_gpm_wait_descriptor (int);
 extern void delete_gpm_wait_descriptor (int);
 #endif
-extern void init_process_emacs (int);
+extern void init_process_emacs (int, char *);
 extern void syms_of_process (void);
 extern void setup_process_coding_systems (Lisp_Object);
 

src/process.c

@@ -276,6 +276,10 @@ static int max_desc;
    the file descriptor of a socket that is already bound.  */
 static int external_sock_fd;
 
+/* The name (path) of the socket that was passed to Emacs, when
+   `external_sock_fd' is not -1.  */
+static const char *external_sock_name = NULL;
+
 /* 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 *);
@@ -7972,10 +7976,21 @@ restore_nofile_limit (void)
 }
 
 
+DEFUN ("get-external-sockname", Fget_external_sockname, Sget_external_sockname, 0, 0, 0,
+       doc: /* Return the path of an external socket passed to Emacs.
+Otherwise return nil.  */)
+     (void)
+{
+    if (external_sock_name)
+        return make_string(external_sock_name, strlen(external_sock_name));
+    else
+        return Qnil;
+}
+
 /* This is not called "init_process" because that is the name of a
    Mach system call, so it would cause problems on Darwin systems.  */
 void
-init_process_emacs (int sockfd)
+init_process_emacs (int sockfd, char *sockname)
 {
 #ifdef subprocesses
   int i;
@@ -8010,6 +8025,7 @@ init_process_emacs (int sockfd)
 #endif
 
   external_sock_fd = sockfd;
+  external_sock_name = sockname;
   max_desc = -1;
   memset (fd_callback_info, 0, sizeof (fd_callback_info));
 
@@ -8304,4 +8320,5 @@ returns non-`nil'.  */);
   defsubr (&Sprocess_inherit_coding_system_flag);
   defsubr (&Slist_system_processes);
   defsubr (&Sprocess_attributes);
+  defsubr (&Sget_external_sockname);
 }