Libguestfs - Jul 2009 - [PATCH] Guestfish feature: remote control of guestfish over a pipe

-- 
Richard Jones, Emerging Technologies, Red Hat  http://et.redhat.com/~rjones
New in Fedora 11: Fedora Windows cross-compiler. Compile Windows
programs, test, and build Windows installers. Over 70 libraries supprt'd
http://fedoraproject.org/wiki/MinGW http://www.annexia.org/fedora_mingw
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>From 7ac4b60d979f841979d7503d9375ce2e5baf4306 Mon Sep 17 00:00:00 2001
From: Richard Jones <rjones at trick.home.annexia.org>
Date: Mon, 13 Jul 2009 13:06:26 +0100
Subject: [PATCH] Guestfish feature: remote control of guestfish over a pipe.

The use case is to have a long-running guestfish process in
a shell script, and thus to avoid the overhead of starting
guestfish each time.  Do:

 eval `guestfish --listen`

 guestfish --remote somecmd
 guestfish --remote someothercmd
 guestfish --remote exit

This patch also supports having multiple guestfish processes
at the same time.

The protocol is simple XDR messages over a Unix domain socket.
---
 TODO                       |    8 --
 fish/Makefile.am           |   19 +++
 fish/fish.c                |   68 ++++++++++-
 fish/fish.h                |    7 +-
 fish/rc.c                  |  267 ++++++++++++++++++++++++++++++++++++++++++++
 fish/rc_protocol.c         |   51 +++++++++
 fish/rc_protocol.h         |   59 ++++++++++
 fish/rc_protocol.x         |   36 ++++++
 guestfish.pod              |   78 +++++++++++++-
 regressions/Makefile.am    |    1 +
 regressions/test-remote.sh |   34 ++++++
 11 files changed, 611 insertions(+), 17 deletions(-)
 create mode 100644 fish/rc.c
 create mode 100644 fish/rc_protocol.c
 create mode 100644 fish/rc_protocol.h
 create mode 100644 fish/rc_protocol.x
 create mode 100755 regressions/test-remote.sh

diff --git a/TODO b/TODO
index 5b4c843..6a89409 100644
--- a/TODO
+++ b/TODO
@@ -178,11 +178,3 @@ Other initrd-* commands, such as:
 
 initrd-extract
 initrd-replace
-
-----------------------------------------------------------------------
-
-Control guestfish from a pipe.
-
-For shell scripts - they can start up a long-running guestfish process
-and intermittently send it commands.  Avoids the start-up overhead,
-but how do we reliably signal errors?
\ No newline at end of file
diff --git a/fish/Makefile.am b/fish/Makefile.am
index 03619f3..5255e85 100644
--- a/fish/Makefile.am
+++ b/fish/Makefile.am
@@ -29,10 +29,29 @@ guestfish_SOURCES = \
 	glob.c \
 	lcd.c \
 	more.c \
+	rc.c \
+	rc_protocol.c \
+	rc_protocol.h \
 	reopen.c \
 	time.c
 
+BUILT_SOURCES = \
+	rc_protocol.c \
+	rc_protocol.h
+
 guestfish_CFLAGS = \
 	-I$(top_srcdir)/src -I$(top_builddir)/src -Wall \
 	-DGUESTFS_DEFAULT_PATH='"$(libdir)/guestfs"'
 guestfish_LDADD = $(top_builddir)/src/libguestfs.la $(LIBREADLINE)
+
+if HAVE_RPCGEN
+rc_protocol.c: rc_protocol.x
+	rm -f $@-t
+	$(RPCGEN) -c -o $@-t $<
+	mv $@-t $@
+
+rc_protocol.h: rc_protocol.x
+	rm -f $@-t
+	$(RPCGEN) -h -o $@-t $<
+	mv $@-t $@
+endif
diff --git a/fish/fish.c b/fish/fish.c
index a093395..4042bbc 100644
--- a/fish/fish.c
+++ b/fish/fish.c
@@ -69,6 +69,9 @@ int read_only = 0;
 int quit = 0;
 int verbose = 0;
 int echo_commands = 0;
+int remote_control_listen = 0;
+int remote_control = 0;
+int exit_on_error = 1;
 
 int
 launch (guestfs_h *_g)
@@ -109,8 +112,10 @@ usage (void)
 	     "  -D|--no-dest-paths   Don't tab-complete paths from guest
fs\n"
 	     "  -f|--file file       Read commands from file\n"
 	     "  -i|--inspector       Run virt-inspector to get disk
mountpoints\n"
+	     "  --listen             Listen for remote commands\n"
 	     "  -m|--mount dev[:mnt] Mount dev on mnt (if omitted, /)\n"
 	     "  -n|--no-sync         Don't autosync\n"
+	     "  --remote[=pid]       Send commands to remote guestfish\n"
 	     "  -r|--ro              Mount read-only\n"
 	     "  -v|--verbose         Verbose messages\n"
 	     "  -x                   Echo each command before executing
it\n"
@@ -128,9 +133,11 @@ main (int argc, char *argv[])
     { "file", 1, 0, 'f' },
     { "help", 0, 0, '?' },
     { "inspector", 0, 0, 'i' },
+    { "listen", 0, 0, 0 },
     { "mount", 1, 0, 'm' },
     { "no-dest-paths", 0, 0, 'D' },
     { "no-sync", 0, 0, 'n' },
+    { "remote", 2, 0, 0 },
     { "ro", 0, 0, 'r' },
     { "verbose", 0, 0, 'v' },
     { "version", 0, 0, 'V' },
@@ -141,7 +148,9 @@ main (int argc, char *argv[])
   struct mp *mps = NULL;
   struct mp *mp;
   char *p, *file = NULL;
-  int c, inspector = 0;
+  int c;
+  int inspector = 0;
+  int option_index;
   struct sigaction sa;
 
   initialize_readline ();
@@ -176,10 +185,33 @@ main (int argc, char *argv[])
     guestfs_set_path (g, "appliance:" GUESTFS_DEFAULT_PATH);
 
   for (;;) {
-    c = getopt_long (argc, argv, options, long_options, NULL);
+    c = getopt_long (argc, argv, options, long_options, &option_index);
     if (c == -1) break;
 
     switch (c) {
+    case 0:			/* options which are long only */
+      if (strcmp (long_options[option_index].name, "listen") == 0)
+	remote_control_listen = 1;
+      else if (strcmp (long_options[option_index].name, "remote") ==
0) {
+	if (optarg) {
+	  if (sscanf (optarg, "%d", &remote_control) != 1) {
+	    fprintf (stderr, _("guestfish: --listen=PID: PID was not a number:
%s\n"), optarg);
+	    exit (1);
+	  }
+	} else {
+	  p = getenv ("GUESTFISH_PID");
+	  if (!p || sscanf (p, "%d", &remote_control) != 1) {
+	    fprintf (stderr, _("guestfish: remote: $GUESTFISH_PID must be set to
the PID of the remote process\n"));
+	    exit (1);
+	  }
+	}
+      } else {
+	fprintf (stderr, _("guestfish: unknown long option: %s (%d)\n"),
+		 long_options[option_index].name, option_index);
+	exit (1);
+      }
+      break;
+
     case 'a':
       if (access (optarg, R_OK) != 0) {
 	perror (optarg);
@@ -274,8 +306,8 @@ main (int argc, char *argv[])
     char cmd[1024];
     int r;
 
-    if (drvs || mps) {
-      fprintf (stderr, _("guestfish: cannot use -i option with -a or
-m\n"));
+    if (drvs || mps || remote_control_listen || remote_control) {
+      fprintf (stderr, _("guestfish: cannot use -i option with -a, -m,
--listen or --remote\n"));
       exit (1);
     }
     if (optind >= argc) {
@@ -327,6 +359,24 @@ main (int argc, char *argv[])
     mount_mps (mps);
   }
 
+  /* Remote control? */
+  if (remote_control_listen && remote_control) {
+    fprintf (stderr, _("guestfish: cannot use --listen and --remote
options at the same time\n"));
+    exit (1);
+  }
+
+  if (remote_control_listen) {
+    if (optind < argc) {
+      fprintf (stderr, _("guestfish: extra parameters on the command line
with --listen flag\n"));
+      exit (1);
+    }
+    if (file) {
+      fprintf (stderr, _("guestfish: cannot use --listen and --file
options at the same time\n"));
+      exit (1);
+    }
+    rc_listen ();
+  }
+
   /* -f (file) parameter? */
   if (file) {
     close (0);
@@ -464,7 +514,6 @@ script (int prompt)
   char *argv[64];
   int i, len;
   int global_exit_on_error = !prompt;
-  int exit_on_error;
 
   if (prompt)
     printf (_("\n"
@@ -641,6 +690,8 @@ cmdline (char *argv[], int optind, int argc)
   const char *cmd;
   char **params;
 
+  exit_on_error = 1;
+
   if (optind >= argc) return;
 
   cmd = argv[optind++];
@@ -711,7 +762,12 @@ issue_command (const char *cmd, char *argv[], const char
*pipecmd)
   for (argc = 0; argv[argc] != NULL; ++argc)
     ;
 
-  if (strcasecmp (cmd, "help") == 0) {
+  /* If --remote was set, then send this command to a remote process. */
+  if (remote_control)
+    r = rc_remote (remote_control, cmd, argc, argv, exit_on_error);
+
+  /* Otherwise execute it locally. */
+  else if (strcasecmp (cmd, "help") == 0) {
     if (argc == 0)
       list_commands ();
     else
diff --git a/fish/fish.h b/fish/fish.h
index d0dc7a9..f911eed 100644
--- a/fish/fish.h
+++ b/fish/fish.h
@@ -1,4 +1,4 @@
-/* libguestfs - the guestfsd daemon
+/* libguestfs - guestfish shell
  * Copyright (C) 2009 Red Hat Inc.
  *
  * This program is free software; you can redistribute it and/or modify
@@ -77,6 +77,11 @@ extern int do_glob (const char *cmd, int argc, char *argv[]);
 /* in more.c */
 extern int do_more (const char *cmd, int argc, char *argv[]);
 
+/* in rc.c (remote control) */
+extern void rc_listen (void);
+extern int rc_remote (int pid, const char *cmd, int argc, char *argv[],
+		      int exit_on_error);
+
 /* in reopen.c */
 extern int do_reopen (const char *cmd, int argc, char *argv[]);
 
diff --git a/fish/rc.c b/fish/rc.c
new file mode 100644
index 0000000..7277502
--- /dev/null
+++ b/fish/rc.c
@@ -0,0 +1,267 @@
+/* guestfish - the filesystem interactive shell
+ * Copyright (C) 2009 Red Hat Inc.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
+ */
+
+#include <config.h>
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+#include <fcntl.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <sys/un.h>
+#include <signal.h>
+
+#include <rpc/types.h>
+#include <rpc/xdr.h>
+
+#include "fish.h"
+#include "rc_protocol.h"
+
+#define UNIX_PATH_MAX 108
+
+static void
+create_sockpath (pid_t pid, char *sockpath, int len, struct sockaddr_un *addr)
+{
+  char dir[128];
+  uid_t euid = geteuid ();
+
+  snprintf (dir, sizeof dir, "/tmp/.guestfish-%d", euid);
+  mkdir (dir, 0700);
+
+  snprintf (sockpath, len, "/tmp/.guestfish-%d/socket-%d", euid,
pid);
+
+  addr->sun_family = AF_UNIX;
+  strcpy (addr->sun_path, sockpath);
+}
+
+/* Remote control server. */
+void
+rc_listen (void)
+{
+  char sockpath[128];
+  pid_t pid;
+  struct sockaddr_un addr;
+  int sock, s, i;
+  FILE *fp;
+  XDR xdr, xdr2;
+  guestfish_hello hello;
+  guestfish_call call;
+  guestfish_reply reply;
+  char **argv;
+  int argc;
+
+  memset (&hello, 0, sizeof hello);
+  memset (&call, 0, sizeof call);
+
+  pid = fork ();
+  if (pid == -1) {
+    perror ("fork");
+    exit (1);
+  }
+
+  if (pid > 0) {
+    /* Parent process. */
+    printf ("export GUESTFISH_PID=%d\n", pid);
+    fflush (stdout);
+    _exit (0);
+  }
+
+  /* Child process.
+   *
+   * Create the listening socket for accepting commands.
+   *
+   * Unfortunately there is a small but unavoidable race here.  We
+   * don't know the PID until after we've forked, so we cannot be
+   * sure the socket is created from the point of view of the parent
+   * (if the child is very slow).
+   */
+  pid = getpid ();
+  create_sockpath (pid, sockpath, sizeof sockpath, &addr);
+
+  sock = socket (AF_UNIX, SOCK_STREAM, 0);
+  if (sock == -1) {
+    perror ("socket");
+    exit (1);
+  }
+  unlink (sockpath);
+  if (bind (sock, (struct sockaddr *) &addr, sizeof addr) == -1) {
+    perror (sockpath);
+    exit (1);
+  }
+  if (listen (sock, 4) == -1) {
+    perror ("listen");
+    exit (1);
+  }
+
+  /* Now close stdout and substitute /dev/null.  This is necessary
+   * so that eval `guestfish --listen` doesn't block forever.
+   */
+  close (1);
+  open ("/dev/null", O_WRONLY);
+
+  /* Read commands and execute them. */
+  while (!quit) {
+    s = accept (sock, NULL, NULL);
+    if (s == -1)
+      perror ("accept");
+    else {
+      fp = fdopen (s, "r+");
+      xdrstdio_create (&xdr, fp, XDR_DECODE);
+
+      if (!xdr_guestfish_hello (&xdr, &hello)) {
+	fprintf (stderr, _("guestfish: protocol error: could not read
'hello' message\n"));
+	goto error;
+      }
+
+      if (strcmp (hello.vers, PACKAGE_VERSION) != 0) {
+	fprintf (stderr, _("guestfish: protocol error: version mismatch, server
version '%s' does not match client version '%s'.  The two
versions must match exactly.\n"),
+		 PACKAGE_VERSION,
+		 hello.vers);
+	xdr_free ((xdrproc_t) xdr_guestfish_hello, (char *) &hello);
+	goto error;
+      }
+      xdr_free ((xdrproc_t) xdr_guestfish_hello, (char *) &hello);
+
+      while (xdr_guestfish_call (&xdr, &call)) {
+	/* We have to extend and NULL-terminate the argv array. */
+	argc = call.args.args_len;
+	argv = realloc (call.args.args_val, (argc+1) * sizeof (char *));
+	if (argv == NULL) {
+	  perror ("realloc");
+	  exit (1);
+	}
+	call.args.args_val = argv;
+	argv[argc] = NULL;
+
+	if (verbose) {
+	  fprintf (stderr, "guestfish(%d): %s", pid, call.cmd);
+	  for (i = 0; i < argc; ++i)
+	    fprintf (stderr, " %s", argv[i]);
+	  fprintf (stderr, "\n");
+	}
+
+	/* Run the command. */
+	reply.r = issue_command (call.cmd, argv, NULL);
+
+	xdr_free ((xdrproc_t) xdr_guestfish_call, (char *) &call);
+
+	/* Send the reply. */
+	xdrstdio_create (&xdr2, fp, XDR_ENCODE);
+	(void) xdr_guestfish_reply (&xdr2, &reply);
+	xdr_destroy (&xdr2);
+
+	/* Exit on error? */
+	if (call.exit_on_error && reply.r == -1) {
+	  unlink (sockpath);
+	  exit (1);
+	}
+      }
+
+    error:
+      xdr_destroy (&xdr);	/* NB. This doesn't close 'fp'. */
+      fclose (fp);		/* Closes the underlying socket 's'. */
+    }
+  }
+
+  unlink (sockpath);
+  exit (0);
+}
+
+/* Remote control client. */
+int
+rc_remote (int pid, const char *cmd, int argc, char *argv[],
+	   int exit_on_error)
+{
+  guestfish_hello hello;
+  guestfish_call call;
+  guestfish_reply reply;
+  char sockpath[128];
+  struct sockaddr_un addr;
+  int sock;
+  FILE *fp;
+  XDR xdr;
+
+  memset (&reply, 0, sizeof reply);
+
+  /* This is fine as long as we never try to xdr_free this struct. */
+  hello.vers = (char *) PACKAGE_VERSION;
+
+  /* Check the other end is still running. */
+  if (kill (pid, 0) == -1) {
+    fprintf (stderr, _("guestfish: remote: looks like the server is not
running\n"));
+    return -1;
+  }
+
+  create_sockpath (pid, sockpath, sizeof sockpath, &addr);
+
+  sock = socket (AF_UNIX, SOCK_STREAM, 0);
+  if (sock == -1) {
+    perror ("socket");
+    return -1;
+  }
+
+  if (connect (sock, (struct sockaddr *) &addr, sizeof addr) == -1) {
+    perror (sockpath);
+    fprintf (stderr, _("guestfish: remote: looks like the server is not
running\n"));
+    close (sock);
+    return -1;
+  }
+
+  /* Send the greeting. */
+  fp = fdopen (sock, "r+");
+  xdrstdio_create (&xdr, fp, XDR_ENCODE);
+
+  if (!xdr_guestfish_hello (&xdr, &hello)) {
+    fprintf (stderr, _("guestfish: protocol error: could not send initial
greeting to server\n"));
+    fclose (fp);
+    xdr_destroy (&xdr);
+    return -1;
+  }
+
+  /* Send the command.  The server supports reading multiple commands
+   * per connection, but this code only ever sends one command.
+   */
+  call.cmd = (char *) cmd;
+  call.args.args_len = argc;
+  call.args.args_val = argv;
+  call.exit_on_error = exit_on_error;
+  if (!xdr_guestfish_call (&xdr, &call)) {
+    fprintf (stderr, _("guestfish: protocol error: could not send initial
greeting to server\n"));
+    fclose (fp);
+    xdr_destroy (&xdr);
+    return -1;
+  }
+  xdr_destroy (&xdr);
+
+  /* Wait for the reply. */
+  xdrstdio_create (&xdr, fp, XDR_DECODE);
+
+  if (!xdr_guestfish_reply (&xdr, &reply)) {
+    fprintf (stderr, _("guestfish: protocol error: could not decode reply
from server\n"));
+    fclose (fp);
+    xdr_destroy (&xdr);
+    return -1;
+  }
+
+  fclose (fp);
+  xdr_destroy (&xdr);
+
+  return reply.r;
+}
diff --git a/fish/rc_protocol.c b/fish/rc_protocol.c
new file mode 100644
index 0000000..451932f
--- /dev/null
+++ b/fish/rc_protocol.c
@@ -0,0 +1,51 @@
+/*
+ * Please do not edit this file.
+ * It was generated using rpcgen.
+ */
+
+#include "rc_protocol.h"
+
+bool_t
+xdr_str (XDR *xdrs, str *objp)
+{
+	register int32_t *buf;
+
+	 if (!xdr_string (xdrs, objp, ~0))
+		 return FALSE;
+	return TRUE;
+}
+
+bool_t
+xdr_guestfish_hello (XDR *xdrs, guestfish_hello *objp)
+{
+	register int32_t *buf;
+
+	 if (!xdr_string (xdrs, &objp->vers, ~0))
+		 return FALSE;
+	return TRUE;
+}
+
+bool_t
+xdr_guestfish_call (XDR *xdrs, guestfish_call *objp)
+{
+	register int32_t *buf;
+
+	 if (!xdr_string (xdrs, &objp->cmd, ~0))
+		 return FALSE;
+	 if (!xdr_array (xdrs, (char **)&objp->args.args_val, (u_int *)
&objp->args.args_len, ~0,
+		sizeof (str), (xdrproc_t) xdr_str))
+		 return FALSE;
+	 if (!xdr_bool (xdrs, &objp->exit_on_error))
+		 return FALSE;
+	return TRUE;
+}
+
+bool_t
+xdr_guestfish_reply (XDR *xdrs, guestfish_reply *objp)
+{
+	register int32_t *buf;
+
+	 if (!xdr_int (xdrs, &objp->r))
+		 return FALSE;
+	return TRUE;
+}
diff --git a/fish/rc_protocol.h b/fish/rc_protocol.h
new file mode 100644
index 0000000..0233b91
--- /dev/null
+++ b/fish/rc_protocol.h
@@ -0,0 +1,59 @@
+/*
+ * Please do not edit this file.
+ * It was generated using rpcgen.
+ */
+
+#ifndef _RC_PROTOCOL_H_RPCGEN
+#define _RC_PROTOCOL_H_RPCGEN
+
+#include <rpc/rpc.h>
+
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+
+typedef char *str;
+
+struct guestfish_hello {
+	char *vers;
+};
+typedef struct guestfish_hello guestfish_hello;
+
+struct guestfish_call {
+	char *cmd;
+	struct {
+		u_int args_len;
+		str *args_val;
+	} args;
+	bool_t exit_on_error;
+};
+typedef struct guestfish_call guestfish_call;
+
+struct guestfish_reply {
+	int r;
+};
+typedef struct guestfish_reply guestfish_reply;
+
+/* the xdr functions */
+
+#if defined(__STDC__) || defined(__cplusplus)
+extern  bool_t xdr_str (XDR *, str*);
+extern  bool_t xdr_guestfish_hello (XDR *, guestfish_hello*);
+extern  bool_t xdr_guestfish_call (XDR *, guestfish_call*);
+extern  bool_t xdr_guestfish_reply (XDR *, guestfish_reply*);
+
+#else /* K&R C */
+extern bool_t xdr_str ();
+extern bool_t xdr_guestfish_hello ();
+extern bool_t xdr_guestfish_call ();
+extern bool_t xdr_guestfish_reply ();
+
+#endif /* K&R C */
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* !_RC_PROTOCOL_H_RPCGEN */
diff --git a/fish/rc_protocol.x b/fish/rc_protocol.x
new file mode 100644
index 0000000..9d8f0e9
--- /dev/null
+++ b/fish/rc_protocol.x
@@ -0,0 +1,36 @@
+/* libguestfs - guestfish remote control protocol -*- c -*-
+ * Copyright (C) 2009 Red Hat Inc.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+typedef string str<>;
+
+struct guestfish_hello {
+  /* Client and server version strings must match exactly.  We change
+   * this protocol whenever we want to.
+   */
+  string vers<>;
+};
+
+struct guestfish_call {
+  string cmd<>;
+  str args<>;
+  bool exit_on_error;
+};
+
+struct guestfish_reply {
+  int r;			/* 0 or -1 only. */
+};
diff --git a/guestfish.pod b/guestfish.pod
index 0100311..4f8cf95 100644
--- a/guestfish.pod
+++ b/guestfish.pod
@@ -66,6 +66,11 @@ Remove C</boot/grub/menu.lst> (in reality not such a
great idea):
  sfdisk /dev/sda 0 0 0 ,
  mkfs ext2 /dev/sda1
 
+=head2 Remote control
+
+ eval `guestfish --listen`
+ guestfish --remote cmd
+
 =head1 DESCRIPTION
 
 Guestfish is a shell and command-line tool for examining and modifying
@@ -125,11 +130,17 @@ Typical usage is either:
 
  guestfish -i /dev/Guests/MyGuest
 
-You cannot use I<-a> or I<-m> in conjunction with this option, and
-options other than I<--ro> might not behave correctly.
+You cannot use I<-a>, I<-m>, I<--listen> or I<--remote>
in conjunction
+with this option, and options other than I<--ro> might not behave
+correctly.
 
 See also: L<virt-inspector(1)>.
 
+=item B<--listen>
+
+Fork into the background and listen for remote commands.  See section
+I<REMOTE CONTROL GUESTFISH OVER A SOCKET> below.
+
 =item B<-m dev[:mountpoint]> | B<--mount dev[:mountpoint]>
 
 Mount the named partition or logical volume on the given mountpoint.
@@ -146,6 +157,11 @@ automatically launched.
 Disable autosync.  This is enabled by default.  See the discussion
 of autosync in the L<guestfs(3)> manpage.
 
+=item B<--remote[=pid]>
+
+Send remote commands to C<$GUESTFISH_PID> or C<pid>.  See section
+I<REMOTE CONTROL GUESTFISH OVER A SOCKET> below.
+
 =item B<-r> | B<--ro>
 
 This changes the C<-m> option so that mounts are done read-only
@@ -323,6 +339,58 @@ If you prefix a command with a I<-> character, then
that command will
 not cause guestfish to exit, even if that (one) command returns an
 error.
 
+=head1 REMOTE CONTROL GUESTFISH OVER A SOCKET
+
+Guestfish can be remote-controlled over a socket.  This is useful
+particularly in shell scripts where you want to make several different
+changes to a filesystem, but you don't want the overhead of starting
+up a guestfish process each time.
+
+Start a guestfish server process using:
+
+ eval `guestfish --listen`
+
+and then send it commands by doing:
+
+ guestfish --remote cmd [...]
+
+To cause the server to exit, send it the exit command:
+
+ guestfish --remote exit
+
+Note that the server will normally exit if there is an error in a
+command.  You can change this in the usual way.  See section I<EXIT ON
+ERROR BEHAVIOUR>.
+
+=head2 CONTROLLING MULTIPLE GUESTFISH PROCESSES
+
+The C<eval> statement sets the environment variable
C<$GUESTFISH_PID>,
+which is how the C<--remote> option knows where to send the commands.
+You can have several guestfish listener processes running using:
+
+ eval `guestfish --listen`
+ pid1=$GUESTFISH_PID
+ eval `guestfish --listen`
+ pid2=$GUESTFISH_PID
+ ...
+ guestfish --remote=$pid1 cmd
+ guestfish --remote=$pid2 cmd
+
+=head2 STANDARD OUTPUT DURING REMOTE CONTROL
+
+Because of limitations in the C<eval> statement, stdout from the
+listener is currently redirected to C</dev/null>.
+
+Stderr is unchanged.
+
+=head2 REMOTE CONTROL DETAILS
+
+Remote control happens over a Unix domain socket called
+C</tmp/.guestfish-$UID/socket-$PID>, where C<$UID> is the effective
+user ID of the process, and C<$PID> is the process ID of the server.
+
+Guestfish client and server versions must match exactly.
+
 =head1 GUESTFISH COMMANDS
 
 The commands in this section are guestfish convenience commands, in
@@ -451,6 +519,12 @@ can be useful for benchmarking operations.
 The C<edit> command uses C<$EDITOR> as the editor.  If not
 set, it uses C<vi>.
 
+=item GUESTFISH_PID
+
+Used with the I<--remote> option to specify the remote guestfish
+process to control.  See section I<REMOTE CONTROL GUESTFISH OVER A
+SOCKET>.
+
 =item HOME
 
 If compiled with GNU readline support, then the command history
diff --git a/regressions/Makefile.am b/regressions/Makefile.am
index cd575e0..0e0d520 100644
--- a/regressions/Makefile.am
+++ b/regressions/Makefile.am
@@ -29,6 +29,7 @@ TESTS = \
 	test-qemudie-midcommand.sh \
 	test-qemudie-killsub.sh \
 	test-qemudie-synch.sh \
+	test-remote.sh \
 	test-reopen.sh
 
 SKIPPED_TESTS = \
diff --git a/regressions/test-remote.sh b/regressions/test-remote.sh
new file mode 100755
index 0000000..f3c14b4
--- /dev/null
+++ b/regressions/test-remote.sh
@@ -0,0 +1,34 @@
+#!/bin/sh -
+# libguestfs
+# Copyright (C) 2009 Red Hat Inc.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 2 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
+
+# Test remote control of guestfish.
+
+set -e
+
+rm -f test.img
+
+eval `../fish/guestfish --listen`
+
+../fish/guestfish --remote alloc test.img 10M
+../fish/guestfish --remote run
+../fish/guestfish --remote sfdiskM /dev/sda ,
+../fish/guestfish --remote mkfs ext2 /dev/sda1
+../fish/guestfish --remote mount /dev/sda1 /
+../fish/guestfish --remote exit
+
+rm -f test.img
-- 
1.6.2.5