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.

2 files changed, 169 insertions, 136 deletions

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)