Add --new-daemon, which runs in the foreground and does not fork

This is intended for modern init systems such as systemd,
which manage many of the traditional aspects of daemon behavior
themselves.  (Bug#2677)
* src/emacs.c (daemon_type): New integer.
(usage, standard_args): Add --old-daemon and --new-daemon.
(main): Handle --old-daemon and --new-daemon arguments.
Restrict all the forking and complicated daemon stuff to old-daemon.
(Fdaemon_initialized): Handle new-style daemon.
* src/lisp.h (IS_DAEMON, DAEMON_RUNNING) [!WINDOWNT]:
Replace daemon_pipe with daemon_type.
* doc/emacs/cmdargs.texi (Initial Options):
* doc/emacs/glossary.texi (Glossary):
* doc/emacs/misc.texi (Emacs Server):
* doc/lispref/display.texi (Window Systems):
* doc/lispref/os.texi (Startup Summary): Related doc updates.
* etc/NEWS: Mention this.
* etc/emacs.service: Use Type=simple and --new-daemon.

9 files changed, 202 insertions, 160 deletions

diff --git a/doc/emacs/cmdargs.texi b/doc/emacs/cmdargs.texi
index 444d30527f8..8d3d936b7b3 100644
--- a/doc/emacs/cmdargs.texi
+++ b/doc/emacs/cmdargs.texi
@@ -334,17 +334,18 @@ setting @code{inhibit-x-resources} to @code{t} (@pxref{Resources}).
 
 @item -daemon
 @opindex -daemon
-@itemx --daemon
+@itemx --daemon[=@var{name}]
 @opindex --daemon
+@itemx --old-daemon[=@var{name}]
+@itemx --new-daemon[=@var{name}]
 Start Emacs as a daemon---after Emacs starts up, it starts the Emacs
-server and disconnects from the terminal without opening any frames.
+server without opening any frames.
+(Optionally, you can specify an explicit @var{name} for the server.)
 You can then use the @command{emacsclient} command to connect to Emacs
 for editing.  @xref{Emacs Server}, for information about using Emacs
-as a daemon.
-
-@item -daemon=@var{SERVER-NAME}
-Start emacs in background as a daemon, and use @var{SERVER-NAME} as
-the server name.
+as a daemon.  An ``old-style'' daemon disconnects from the terminal
+and runs in the background (@samp{--daemon} is an alias for
+@samp{--old-daemon}).
 
 @item --no-desktop
 @opindex --no-desktop
diff --git a/doc/emacs/glossary.texi b/doc/emacs/glossary.texi
index bce97dacee2..d6489390ea5 100644
--- a/doc/emacs/glossary.texi
+++ b/doc/emacs/glossary.texi
@@ -338,8 +338,8 @@ or by rebinding key sequences (@pxref{Keymaps}).
 @item Daemon
 A daemon is a standard term for a system-level process that runs in the
 background.  Daemons are often started when the system first starts up.
-When Emacs runs in daemon-mode, it runs in the background and does not
-open a display.  You can then connect to it with the
+When Emacs runs in daemon-mode, it does not
+open a display.  You connect to it with the
 @command{emacsclient} program.  @xref{Emacs Server}.
 
 @item Default Argument
diff --git a/doc/emacs/misc.texi b/doc/emacs/misc.texi
index cb0a1160516..2290dec31e2 100644
--- a/doc/emacs/misc.texi
+++ b/doc/emacs/misc.texi
@@ -1583,11 +1583,10 @@ dies with the Emacs process.
 
 @cindex daemon, Emacs
 @item
-Run Emacs as a @dfn{daemon}, using the @samp{--daemon} command-line
-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.
+Run Emacs as a @dfn{daemon}, using one of the @samp{--daemon} command-line
+options.  @xref{Initial Options}.  When Emacs is started this way, it
+calls @code{server-start} after initialization and does not open an
+initial frame.  It then waits for edit requests from clients.
 
 @cindex systemd unit file
 @item
diff --git a/doc/lispref/display.texi b/doc/lispref/display.texi
index 381edbac48a..d050738d80f 100644
--- a/doc/lispref/display.texi
+++ b/doc/lispref/display.texi
@@ -7166,7 +7166,7 @@ Emacs is displaying the frame on a character-based terminal.
 @defvar initial-window-system
 This variable holds the value of @code{window-system} used for the
 first frame created by Emacs during startup.  (When Emacs is invoked
-with the @option{--daemon} option, it does not create any initial
+as a daemon, it does not create any initial
 frames, so @code{initial-window-system} is @code{nil}, except on
 MS-Windows, where it is still @code{w32}.  @xref{Initial Options,
 daemon,, emacs, The GNU Emacs Manual}.)
diff --git a/doc/lispref/os.texi b/doc/lispref/os.texi
index 97b086c0d37..3312705e167 100644
--- a/doc/lispref/os.texi
+++ b/doc/lispref/os.texi
@@ -112,8 +112,8 @@ compiled into the Emacs executable when it was built.
 It runs the normal hook @code{before-init-hook}.
 
 @item
-If appropriate, it creates a graphical frame.  This is not done if the
-options @samp{--batch} or @samp{--daemon} were specified.
+If appropriate, it creates a graphical frame.  This is not done in
+batch (noninteractive) or daemon mode.
 
 @item
 It initializes the initial frame's faces, and sets up the menu bar
@@ -256,10 +256,10 @@ options were specified.
 @c last few bits of command-line-1 are not done in batch mode.
 
 @item
-If the option @code{--daemon} was specified, it calls
-@code{server-start}, and on Posix systems also detaches from the
-controlling terminal.  @xref{Emacs Server,,, emacs, The GNU Emacs
-Manual}.
+If a daemon was requested, it calls @code{server-start}.
+(On Posix systems, if a background daemon was requested, it then
+detaches from the controlling terminal.)  @xref{Emacs
+Server,,, emacs, The GNU Emacs Manual}.
 
 @item
 If started by the X session manager, it calls
@@ -337,7 +337,10 @@ Do not display a splash screen.
 Run without an interactive terminal.  @xref{Batch Mode}.
 
 @item --daemon
-Do not initialize any display; just start a server in the background.
+@itemx --old-daemon
+@itemx --new-daemon
+Do not initialize any display; just start a server.
+(An ``old-style'' daemon automatically runs in the background.)
 
 @item --no-init-file
 @itemx -q
diff --git a/etc/NEWS b/etc/NEWS
index 34d64bfac33..15c264f582c 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -64,6 +64,12 @@ affected by this, as SGI stopped supporting IRIX in December 2013.
 
 * Startup Changes in Emacs 26.1
 
+** New option '--new-daemon'.  This is the same as '--daemon', except
+it runs in the foreground and does not fork.  This is intended for
+modern init systems such as systemd, which manage many of the traditional
+aspects of daemon behavior themselves.  '--old-daemon' is now an alias
+for '--daemon'.
+
 
 * Changes in Emacs 26.1
 
diff --git a/etc/emacs.service b/etc/emacs.service
index 92cdeb5cf49..d9f7fc569d2 100644
--- a/etc/emacs.service
+++ b/etc/emacs.service
@@ -7,8 +7,8 @@ Description=Emacs text editor
 Documentation=info:emacs man:emacs(1) https://gnu.org/software/emacs/
 
 [Service]
-Type=forking
-ExecStart=emacs --daemon
+Type=simple
+ExecStart=emacs --new-daemon
 ExecStop=emacsclient --eval "(kill-emacs)"
 Environment=SSH_AUTH_SOCK=%t/keyring/ssh
 Restart=on-failure
diff --git a/src/emacs.c b/src/emacs.c
index aeba9631696..92cbb086339 100644
--- a/src/emacs.c
+++ b/src/emacs.c
@@ -162,8 +162,8 @@ char *stack_bottom;
 static uprintmax_t heap_bss_diff;
 #endif
 
-/* To run as a daemon under Cocoa or Windows, we must do a fork+exec,
-   not a simple fork.
+/* To run as a background daemon under Cocoa or Windows,
+   we must do a fork+exec, not a simple fork.
 
    On Cocoa, CoreFoundation lib fails in forked process:
    http://developer.apple.com/ReleaseNotes/
@@ -190,9 +190,12 @@ bool build_details;
 /* Name for the server started by the daemon.*/
 static char *daemon_name;
 
+/* 0 not a daemon, 1 new-style (foreground), 2 old-style (background).  */
+int daemon_type;
+
 #ifndef WINDOWSNT
-/* Pipe used to send exit notification to the daemon parent at
-   startup.  */
+/* Pipe used to send exit notification to the background daemon parent at
+   startup.  On Windows, we use a kernel event instead.  */
 int daemon_pipe[2];
 #else
 HANDLE w32_daemon_event;
@@ -223,7 +226,8 @@ Initialization options:\n\
     "\
 --batch                     do not do interactive display; implies -q\n\
 --chdir DIR                 change to directory DIR\n\
---daemon[=NAME]             start a (named) server in the background\n\
+--daemon, --old-daemon[=NAME] start a (named) server in the background\n\
+--new-daemon[=NAME]         start a (named) server in the foreground\n\
 --debug-init                enable Emacs Lisp debugger for init file\n\
 --display, -d DISPLAY       use X server DISPLAY\n\
 ",
@@ -977,6 +981,8 @@ main (int argc, char **argv)
       exit (0);
     }
 
+  daemon_type = 0;
+
 #ifndef WINDOWSNT
   /* Make sure IS_DAEMON starts up as false.  */
   daemon_pipe[1] = 0;
@@ -987,38 +993,52 @@ main (int argc, char **argv)
 
   int sockfd = -1;
 
-  if (argmatch (argv, argc, "-daemon", "--daemon", 5, NULL, &skip_args)
-      || argmatch (argv, argc, "-daemon", "--daemon", 5, &dname_arg, &skip_args))
+  if (argmatch (argv, argc, "-new-daemon", "--new-daemon", 10, NULL, &skip_args)
+      || argmatch (argv, argc, "-new-daemon", "--new-daemon", 10, &dname_arg, &skip_args))
+    {
+      daemon_type = 1;           /* foreground */
+    }
+  else if (argmatch (argv, argc, "-daemon", "--daemon", 5, NULL, &skip_args)
+      || argmatch (argv, argc, "-daemon", "--daemon", 5, &dname_arg, &skip_args)
+      || argmatch (argv, argc, "-old-daemon", "--old-daemon", 10, NULL, &skip_args)
+      || argmatch (argv, argc, "-old-daemon", "--old-daemon", 10, &dname_arg, &skip_args))
+    {
+      daemon_type = 2;          /* background */
+    }
+
+
+  if (daemon_type > 0)
     {
 #ifndef DOS_NT
-      pid_t f;
-
-      /* Start as a daemon: fork a new child process which will run the
-	 rest of the initialization code, then exit.
-
-	 Detaching a daemon requires the following steps:
-	 - fork
-	 - setsid
-	 - exit the parent
-	 - close the tty file-descriptors
-
-	 We only want to do the last 2 steps once the daemon is ready to
-	 serve requests, i.e. after loading .emacs (initialization).
-	 OTOH initialization may start subprocesses (e.g. ispell) and these
-	 should be run from the proper process (the one that will end up
-	 running as daemon) and with the proper "session id" in order for
-	 them to keep working after detaching, so fork and setsid need to be
-	 performed before initialization.
-
-	 We want to avoid exiting before the server socket is ready, so
-	 use a pipe for synchronization.  The parent waits for the child
-	 to close its end of the pipe (using `daemon-initialized')
-	 before exiting.  */
-      if (emacs_pipe (daemon_pipe) != 0)
-	{
-	  fprintf (stderr, "Cannot pipe!\n");
-	  exit (1);
-	}
+      if (daemon_type == 2)
+        {
+          /* Start as a background daemon: fork a new child process which
+             will run the rest of the initialization code, then exit.
+
+             Detaching a daemon requires the following steps:
+             - fork
+             - setsid
+             - exit the parent
+             - close the tty file-descriptors
+
+             We only want to do the last 2 steps once the daemon is ready to
+             serve requests, i.e. after loading .emacs (initialization).
+             OTOH initialization may start subprocesses (e.g. ispell) and these
+             should be run from the proper process (the one that will end up
+             running as daemon) and with the proper "session id" in order for
+             them to keep working after detaching, so fork and setsid need to be
+             performed before initialization.
+
+             We want to avoid exiting before the server socket is ready, so
+             use a pipe for synchronization.  The parent waits for the child
+             to close its end of the pipe (using `daemon-initialized')
+             before exiting.  */
+          if (emacs_pipe (daemon_pipe) != 0)
+            {
+              fprintf (stderr, "Cannot pipe!\n");
+              exit (1);
+            }
+        } /* daemon_type == 2 */
 
 #ifdef HAVE_LIBSYSTEMD
       /* Read the number of sockets passed through by systemd.  */
@@ -1035,99 +1055,105 @@ main (int argc, char **argv)
 	sockfd = 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\
 Emacs might crash when run in daemon mode and the X11 connection is unexpectedly lost.\n\
 Using an Emacs configured with --with-x-toolkit=lucid does not have this problem.\n");
 #endif /* USE_GTK */
-      f = fork ();
-#else /* DAEMON_MUST_EXEC */
-      if (!dname_arg || !strchr (dname_arg, '\n'))
-	  f = fork ();  /* in orig */
-      else
-	  f = 0;  /* in exec'd */
-#endif /* !DAEMON_MUST_EXEC */
-      if (f > 0)
-	{
-	  int retval;
-	  char buf[1];
-
-	  /* Close unused writing end of the pipe.  */
-	  emacs_close (daemon_pipe[1]);
 
-	  /* Just wait for the child to close its end of the pipe.  */
-	  do
-	    {
-	      retval = read (daemon_pipe[0], &buf, 1);
-	    }
-	  while (retval == -1 && errno == EINTR);
-
-	  if (retval < 0)
-	    {
-	      fprintf (stderr, "Error reading status from child\n");
-	      exit (1);
-	    }
-	  else if (retval == 0)
-	    {
-	      fprintf (stderr, "Error: server did not start correctly\n");
-	      exit (1);
-	    }
+      if (daemon_type == 2)
+        {
+          pid_t f;
+#ifndef DAEMON_MUST_EXEC
 
-	  emacs_close (daemon_pipe[0]);
-	  exit (0);
-	}
-      if (f < 0)
-	{
-	  emacs_perror ("fork");
-	  exit (EXIT_CANCELED);
-	}
+          f = fork ();
+#else /* DAEMON_MUST_EXEC */
+          if (!dname_arg || !strchr (dname_arg, '\n'))
+            f = fork ();  /* in orig */
+          else
+            f = 0;  /* in exec'd */
+#endif /* !DAEMON_MUST_EXEC */
+          if (f > 0)
+            {
+              int retval;
+              char buf[1];
+
+              /* Close unused writing end of the pipe.  */
+              emacs_close (daemon_pipe[1]);
+
+              /* Just wait for the child to close its end of the pipe.  */
+              do
+                {
+                  retval = read (daemon_pipe[0], &buf, 1);
+                }
+              while (retval == -1 && errno == EINTR);
+
+              if (retval < 0)
+                {
+                  fprintf (stderr, "Error reading status from child\n");
+                  exit (1);
+                }
+              else if (retval == 0)
+                {
+                  fprintf (stderr, "Error: server did not start correctly\n");
+                  exit (1);
+                }
+
+              emacs_close (daemon_pipe[0]);
+              exit (0);
+            }
+          if (f < 0)
+            {
+              emacs_perror ("fork");
+              exit (EXIT_CANCELED);
+            }
 
 #ifdef DAEMON_MUST_EXEC
-      {
-        /* In orig process, forked as child, OR in exec'd. */
-        if (!dname_arg || !strchr (dname_arg, '\n'))
-          {  /* In orig, child: now exec w/special daemon name. */
-            char fdStr[80];
-	    int fdStrlen =
-	      snprintf (fdStr, sizeof fdStr,
-			"--daemon=\n%d,%d\n%s", daemon_pipe[0],
-			daemon_pipe[1], dname_arg ? dname_arg : "");
-
-	    if (! (0 <= fdStrlen && fdStrlen < sizeof fdStr))
-              {
-                fprintf (stderr, "daemon: child name too long\n");
-                exit (EXIT_CANNOT_INVOKE);
+          {
+            /* In orig process, forked as child, OR in exec'd. */
+            if (!dname_arg || !strchr (dname_arg, '\n'))
+              {  /* In orig, child: now exec w/special daemon name. */
+                char fdStr[80];
+                int fdStrlen =
+                  snprintf (fdStr, sizeof fdStr,
+                            "--old-daemon=\n%d,%d\n%s", daemon_pipe[0],
+                            daemon_pipe[1], dname_arg ? dname_arg : "");
+
+                if (! (0 <= fdStrlen && fdStrlen < sizeof fdStr))
+                  {
+                    fprintf (stderr, "daemon: child name too long\n");
+                    exit (EXIT_CANNOT_INVOKE);
+                  }
+
+                argv[skip_args] = fdStr;
+
+                fcntl (daemon_pipe[0], F_SETFD, 0);
+                fcntl (daemon_pipe[1], F_SETFD, 0);
+                execvp (argv[0], argv);
+                emacs_perror (argv[0]);
+                exit (errno == ENOENT ? EXIT_ENOENT : EXIT_CANNOT_INVOKE);
               }
 
-            argv[skip_args] = fdStr;
-
-	    fcntl (daemon_pipe[0], F_SETFD, 0);
-	    fcntl (daemon_pipe[1], F_SETFD, 0);
-            execvp (argv[0], argv);
-	    emacs_perror (argv[0]);
-	    exit (errno == ENOENT ? EXIT_ENOENT : EXIT_CANNOT_INVOKE);
-          }
-
-        /* In exec'd: parse special dname into pipe and name info. */
-        if (!dname_arg || !strchr (dname_arg, '\n')
-            || strlen (dname_arg) < 1 || strlen (dname_arg) > 70)
+            /* In exec'd: parse special dname into pipe and name info. */
+            if (!dname_arg || !strchr (dname_arg, '\n')
+                || strlen (dname_arg) < 1 || strlen (dname_arg) > 70)
           {
             fprintf (stderr, "emacs daemon: daemon name absent or too long\n");
             exit (EXIT_CANNOT_INVOKE);
           }
-        dname_arg2[0] = '\0';
-        sscanf (dname_arg, "\n%d,%d\n%s", &(daemon_pipe[0]), &(daemon_pipe[1]),
-                dname_arg2);
-        dname_arg = *dname_arg2 ? dname_arg2 : NULL;
-	fcntl (daemon_pipe[1], F_SETFD, FD_CLOEXEC);
-      }
+            dname_arg2[0] = '\0';
+            sscanf (dname_arg, "\n%d,%d\n%s", &(daemon_pipe[0]), &(daemon_pipe[1]),
+                    dname_arg2);
+            dname_arg = *dname_arg2 ? dname_arg2 : NULL;
+            fcntl (daemon_pipe[1], F_SETFD, FD_CLOEXEC);
+          }
 #endif /* DAEMON_MUST_EXEC */
 
-      /* Close unused reading end of the pipe.  */
-      emacs_close (daemon_pipe[0]);
+          /* Close unused reading end of the pipe.  */
+          emacs_close (daemon_pipe[0]);
 
-      setsid ();
+          setsid ();
+        } /* daemon_type == 2 */
 #elif defined(WINDOWSNT)
       /* Indicate that we want daemon mode.  */
       w32_daemon_event = CreateEvent (NULL, TRUE, FALSE, W32_DAEMON_EVENT);
@@ -1138,7 +1164,7 @@ Using an Emacs configured with --with-x-toolkit=lucid does not have this problem
           exit (1);
         }
 #else /* MSDOS */
-      fprintf (stderr, "This platform does not support the -daemon flag.\n");
+      fprintf (stderr, "This platform does not support daemon mode.\n");
       exit (1);
 #endif /* MSDOS */
       if (dname_arg)
@@ -1684,6 +1710,8 @@ static const struct standard_args standard_args[] =
   { "-batch", "--batch", 100, 0 },
   { "-script", "--script", 100, 1 },
   { "-daemon", "--daemon", 99, 0 },
+  { "-old-daemon", "--old-daemon", 99, 0 },
+  { "-new-daemon", "--new-daemon", 99, 0 },
   { "-help", "--help", 90, 0 },
   { "-nl", "--no-loadup", 70, 0 },
   { "-nsl", "--no-site-lisp", 65, 0 },
@@ -2407,27 +2435,33 @@ from the parent process and its tty file descriptors.  */)
   if (NILP (Vafter_init_time))
     error ("This function can only be called after loading the init files");
 #ifndef WINDOWSNT
-  int nfd;
-
-  /* Get rid of stdin, stdout and stderr.  */
-  nfd = emacs_open ("/dev/null", O_RDWR, 0);
-  err |= nfd < 0;
-  err |= dup2 (nfd, STDIN_FILENO) < 0;
-  err |= dup2 (nfd, STDOUT_FILENO) < 0;
-  err |= dup2 (nfd, STDERR_FILENO) < 0;
-  err |= emacs_close (nfd) != 0;
-
-  /* Closing the pipe will notify the parent that it can exit.
-     FIXME: In case some other process inherited the pipe, closing it here
-     won't notify the parent because it's still open elsewhere, so we
-     additionally send a byte, just to make sure the parent really exits.
-     Instead, we should probably close the pipe in start-process and
-     call-process to make sure the pipe is never inherited by
-     subprocesses.  */
-  err |= write (daemon_pipe[1], "\n", 1) < 0;
-  err |= emacs_close (daemon_pipe[1]) != 0;
+
+  if (daemon_type == 2)
+    {
+      int nfd;
+
+      /* Get rid of stdin, stdout and stderr.  */
+      nfd = emacs_open ("/dev/null", O_RDWR, 0);
+      err |= nfd < 0;
+      err |= dup2 (nfd, STDIN_FILENO) < 0;
+      err |= dup2 (nfd, STDOUT_FILENO) < 0;
+      err |= dup2 (nfd, STDERR_FILENO) < 0;
+      err |= emacs_close (nfd) != 0;
+
+      /* Closing the pipe will notify the parent that it can exit.
+         FIXME: In case some other process inherited the pipe, closing it here
+         won't notify the parent because it's still open elsewhere, so we
+         additionally send a byte, just to make sure the parent really exits.
+         Instead, we should probably close the pipe in start-process and
+         call-process to make sure the pipe is never inherited by
+         subprocesses.  */
+      err |= write (daemon_pipe[1], "\n", 1) < 0;
+      err |= emacs_close (daemon_pipe[1]) != 0;
+    }
+
   /* Set it to an invalid value so we know we've already run this function.  */
-  daemon_pipe[1] = -1;
+  daemon_type = -1;
+
 #else  /* WINDOWSNT */
   /* Signal the waiting emacsclient process.  */
   err |= SetEvent (w32_daemon_event) == 0;
diff --git a/src/lisp.h b/src/lisp.h
index aaa44232720..e087828d94f 100644
--- a/src/lisp.h
+++ b/src/lisp.h
@@ -4139,12 +4139,11 @@ extern bool no_site_lisp;
 /* True means put details like time stamps into builds.  */
 extern bool build_details;
 
-/* Pipe used to send exit notification to the daemon parent at
-   startup.  On Windows, we use a kernel event instead.  */
 #ifndef WINDOWSNT
-extern int daemon_pipe[2];
-#define IS_DAEMON (daemon_pipe[1] != 0)
-#define DAEMON_RUNNING (daemon_pipe[1] >= 0)
+/* 0 not a daemon, 1 new-style (foreground), 2 old-style (background).  */
+extern int daemon_type;
+#define IS_DAEMON (daemon_type != 0)
+#define DAEMON_RUNNING (daemon_type >= 0)
 #else  /* WINDOWSNT */
 extern void *w32_daemon_event;
 #define IS_DAEMON (w32_daemon_event != NULL)