4 files changed, 128 insertions, 21 deletions

diff --git a/doc/emacs/emacs.texi b/doc/emacs/emacs.texi
index 5c8977c6b09..a3eb4225a75 100644
--- a/doc/emacs/emacs.texi
+++ b/doc/emacs/emacs.texi
@@ -1082,6 +1082,7 @@ Shell Command History
 
 Using Emacs as a Server
 
+* TCP Emacs server::     Listening to a TCP socket.
 * Invoking emacsclient:: Connecting to the Emacs server.
 * emacsclient Options::  Emacs client startup options.
 
diff --git a/doc/emacs/misc.texi b/doc/emacs/misc.texi
index bcc20a6db16..84681f2269a 100644
--- a/doc/emacs/misc.texi
+++ b/doc/emacs/misc.texi
@@ -1661,10 +1661,68 @@ expression @code{(+ 1 2)} on the @samp{foo} server, and returns
 signaled.)  Currently, this feature is mainly useful for developers.
 
 @menu
+* TCP Emacs server::     Listening to a TCP socket.
 * Invoking emacsclient:: Connecting to the Emacs server.
 * emacsclient Options::  Emacs client startup options.
 @end menu
 
+@node TCP Emacs server
+@subsection TCP Emacs server
+@cindex TCP Emacs server
+
+@vindex server-use-tcp
+  An Emacs server usually listens to connections on a local Unix
+domain socket.  Some operating systems, such as MS-Windows, do not
+support local sockets; in that case, the server uses TCP sockets
+instead.  In some cases it is useful to have the server listen on a
+TCP socket even if local sockets are supported, e.g., if you need to
+contact the Emacs server from a remote machine.  You can set
+@code{server-use-tcp} to non-@code{nil} to have Emacs listen on a TCP
+socket instead of a local socket.  This is the default if your OS does
+not support local sockets.
+
+@vindex server-host
+@vindex server-port
+  If the Emacs server is set to use TCP, it will by default listen to
+a random port on the localhost interface.  This can be changed to
+another interface and/or a fixed port using the variables
+@code{server-host} and @code{server-port}.
+
+@vindex server-auth-key
+  A TCP socket is not subject to file system permissions.  To retain
+some control over which users can talk to an Emacs server over TCP
+sockets, the @command{emacsclient} program must send an authorization
+key to the server.  This key is normally randomly generated by the
+Emacs server.  This is the recommended mode of operation.
+
+@findex server-generate-key
+  If needed, you can set the authorization key to a static value by
+setting the @code{server-auth-key} variable.  The key must consist of
+64 ASCII printable characters except for space (this means characters
+from @samp{!} to @samp{~}, or from decimal code 33 to 126).  You can
+use @kbd{M-x server-generate-key} to get a random key.
+
+@vindex server-auth-dir
+@cindex server file
+  When you start a TCP Emacs server, Emacs creates a @dfn{server file}
+containing the TCP information to be used by @command{emacsclient} to
+connect to the server.  The variable @code{server-auth-dir} specifies
+the directory containing the server file; by default, this is
+@file{~/.emacs.d/server/}.  In the absence of a local socket with file
+permissions, the permissions of this directory determine which users
+can have their @command{emacsclient} processes talk to the Emacs
+server.
+
+@vindex EMACS_SERVER_FILE@r{, environment variable}
+  To tell @command{emacsclient} to connect to the server over TCP with
+a specific server file, use the @samp{-f} or @samp{--server-file}
+option, or set the @env{EMACS_SERVER_FILE} environment variable
+(@pxref{emacsclient Options}).  If @code{server-auth-dir} is set to a
+non-standard value, @command{emacsclient} needs an absolute file name
+to the server file, as the default @code{server-auth-dir} is
+hard-coded in @command{emacsclient} to be used as the directory for
+resolving relative filenames.
+
 @node Invoking emacsclient
 @subsection Invoking @code{emacsclient}
 @cindex @code{emacsclient} invocation
@@ -1810,25 +1868,18 @@ evaluate, @emph{not} as a list of files to visit.
 
 @item -f @var{server-file}
 @itemx --server-file=@var{server-file}
-@cindex @env{EMACS_SERVER_FILE} environment variable
-Specify a @dfn{server file} for connecting to an Emacs server via TCP.
-
-An Emacs server usually uses a
-local socket to listen for connections.  Some operating systems,
-such as Microsoft Windows, do not support local sockets; in that case,
-the server communicates with @command{emacsclient} via TCP.
-
-@vindex server-auth-dir
-@cindex server file
-@vindex server-port
-When you start a TCP Emacs server, Emacs creates a @dfn{server file}
-containing the TCP information to be used by @command{emacsclient} to
-connect to the server.  The variable @code{server-auth-dir} specifies
-the directory containing the server file; by default, this is
-@file{~/.emacs.d/server/}.  To tell @command{emacsclient} to connect
-to the server over TCP with a specific server file, use the @samp{-f}
-or @samp{--server-file} option, or set the @env{EMACS_SERVER_FILE}
-environment variable.
+Specify a server file (@pxref{TCP Emacs server}) for connecting to an
+Emacs server via TCP.  Alternatively, you can set the
+@env{EMACS_SERVER_FILE} environment variable to point to the server
+file.
+
+An Emacs server usually uses a local socket to listen for connections,
+but also supports connections over TCP.  To connect to a TCP Emacs
+server, @command{emacsclient} needs to read a @dfn{server file}
+containing the connection details of the Emacs server.  The name of
+this file is specified with this option, either as a file name
+relative to @file{~/.emacs.d/server} or as an absolute file name.
+@xref{TCP Emacs server}.
 
 @item -n
 @itemx --no-wait
@@ -1872,6 +1923,37 @@ On MS-Windows, @samp{-t} behaves just like @samp{-c} if the Emacs
 server is using the graphical display, but if the Emacs server is
 running on a text terminal, it creates a new frame in the current text
 terminal.
+
+@item -T @var{tramp-prefix}
+@itemx --tramp-prefix=@var{tramp-prefix}
+Set the prefix to add to filenames for Emacs to locate files on remote
+machines using TRAMP (@pxref{Top, The Tramp Manual,, tramp, The Tramp
+Manual}).  This is mostly useful in combination with using the Emacs
+server over TCP (@pxref{TCP Emacs server}).  By ssh-forwarding the
+listening port and making the @var{server-file} available on a remote
+machine, programs on the remote machine can use @command{emacsclient}
+as the value for the @env{EDITOR} and similar environment variables,
+but instead of talking to an Emacs server on the remote machine, the
+files will be visited in the local Emacs session using TRAMP.
+
+@vindex EMACSCLIENT_TRAMP@r{, environment variable}
+Setting the environment variable @env{EMACSCLIENT_TRAMP} has the same
+effect as using the @samp{-T} option.  If both are specified, the
+command-line option takes precedence.
+
+For example, assume two hosts, @samp{local} and @samp{remote}, and
+that the local Emacs listens on tcp port 12345.  Assume further that
+@file{/home} is on a shared file system, so that the server file
+@file{~/.emacs.d/server/server} is readable on both hosts.
+
+@example
+local$ ssh -R12345:localhost:12345 remote
+remote$ export EDITOR="emacsclient \
+        --server-file=server \
+        --tramp=/ssh:remote:"
+remote$ $EDITOR /tmp/foo.txt #Should open in local emacs.
+@end example
+
 @end table
 
   The new graphical or text terminal frames created by the @samp{-c}
diff --git a/etc/NEWS b/etc/NEWS
index 4121c44b0c2..340718ecbc4 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -344,6 +344,14 @@ for DNS-querying functions 'nslookup-host', 'dns-lookup-host',
 and 'run-dig'.  Each function now accepts an optional name server
 argument interactively (with a prefix argument) and non-interactively.
 
++++
+** Emacsclient has a new option -T/--tramp.
+This helps with using a local Emacs session as the server for a remote
+emacsclient.  With appropriate setup, one can now set the EDITOR
+environment variable on a remote machine to emacsclient, and
+use the local Emacs to edit remote files via Tramp.  See the node
+"emacsclient Options" in the user manual for the details.
+
 
 * Editing Changes in Emacs 26.1
 
diff --git a/lib-src/emacsclient.c b/lib-src/emacsclient.c
index 7b735dfb05d..c21ee6bd395 100644
--- a/lib-src/emacsclient.c
+++ b/lib-src/emacsclient.c
@@ -149,6 +149,9 @@ const char *socket_name = NULL;
 /* If non-NULL, the filename of the authentication file.  */
 const char *server_file = NULL;
 
+/* If non-NULL, the tramp prefix emacs must use to find the files.  */
+const char *tramp_prefix = NULL;
+
 /* PID of the Emacs server process.  */
 int emacs_pid = 0;
 
@@ -178,6 +181,7 @@ struct option longopts[] =
   { "server-file",      required_argument, NULL, 'f' },
   { "display",  required_argument, NULL, 'd' },
   { "parent-id", required_argument, NULL, 'p' },
+  { "tramp",    required_argument, NULL, 'T' },
   { 0, 0, 0, 0 }
 };
 
@@ -468,14 +472,15 @@ static void
 decode_options (int argc, char **argv)
 {
   alternate_editor = egetenv ("ALTERNATE_EDITOR");
+  tramp_prefix = egetenv ("EMACSCLIENT_TRAMP");
 
   while (1)
     {
       int opt = getopt_long_only (argc, argv,
 #ifndef NO_SOCKETS_IN_FILE_SYSTEM
-                             "VHnequa:s:f:d:F:tc",
+                             "VHnequa:s:f:d:F:tcT:",
 #else
-                             "VHnequa:f:d:F:tc",
+                             "VHnequa:f:d:F:tcT:",
 #endif
                              longopts, 0);
 
@@ -554,6 +559,10 @@ decode_options (int argc, char **argv)
           frame_parameters = optarg;
           break;
 
+        case 'T':
+          tramp_prefix = optarg;
+          break;
+
         default:
           message (true, "Try '%s --help' for more information\n", progname);
           exit (EXIT_FAILURE);
@@ -654,6 +663,9 @@ The following OPTIONS are accepted:\n\
                         Editor to fallback to if the server is not running\n"
 "                       If EDITOR is the empty string, start Emacs in daemon\n\
                         mode and try connecting again\n"
+"-T PREFIX, --tramp=PREFIX\n\
+                        PREFIX to prepend to filenames sent by emacsclient\n\
+                        for locating files remotely via Tramp\n"
 "\n\
 Report bugs with M-x report-emacs-bug.\n");
   exit (EXIT_SUCCESS);
@@ -1687,6 +1699,8 @@ main (int argc, char **argv)
         }
     }
   send_to_emacs (emacs_socket, "-dir ");
+  if (tramp_prefix)
+    quote_argument (emacs_socket, tramp_prefix);
   quote_argument (emacs_socket, cwd);
   send_to_emacs (emacs_socket, "/");
   send_to_emacs (emacs_socket, " ");
@@ -1791,6 +1805,8 @@ main (int argc, char **argv)
 #endif
 
           send_to_emacs (emacs_socket, "-file ");
+          if (tramp_prefix && file_name_absolute_p (argv[i]))
+            quote_argument (emacs_socket, tramp_prefix);
           quote_argument (emacs_socket, argv[i]);
           send_to_emacs (emacs_socket, " ");
         }