README.multi-tty 15 KB
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			-*- coding: utf-8; -*-
GOAL
----

The ultimate goal of this branch is to implement support for opening
multiple, different tty devices and simultaneous X and tty frames from
a single Emacs session.

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Some use cases:

Emacs is notoriously slow at startup, so most people use another
editor or emacsclient for quick editing jobs from the console.
Unfortunately, emacsclient was very awkward to use, because it did not
support opening a new Emacs frame on the current virtual console.
Now, with multi-tty support, it can do that.  (Emacsclient starts up
faster than vi!)

Some Gnus users (including me) run Gnus in an X frame in its own Emacs
instance, which they typically leave running for weeks.  It would be
nice if they could connect to this instance from a remote ssh session
and check their messages without opening a remote X frame or resorting
to gnus-slave.


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WHO IS DOING IT
---------------

I'm Károly Lőrentey.  My address: lorentey@elte.hu.

Patches or suggestions are welcome!

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Retrieving the latest version of the branch:
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	tla register-archive lorentey@elte.hu--2004 http://lorentey.web.elte.hu/arch/2004/
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	tla get lorentey@elte.hu--2004/emacs--multi-tty <directory>
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(I use a recent arch development snapshot, but any of the released
versions of arch will do fine, I think.)

If you don't have arch, the branch has a homepage from which you can
download conventional patches against Emacs CVS HEAD:
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	http://lorentey.web.elte.hu/project/emacs.html
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STATUS
------

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Multi-tty support is stable, I think most of the problems were fixed.
(It still needs testing on other architectures, though.)  Please let
me know if you find any bugs in it.  Emacsclient has been extended to
support opening a new terminal frame.
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To try it out, compile and run the multi-tty branch with the following
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commands:
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   	mkdir +build
	cd +build
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	../configure
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	make bootstrap
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	src/emacs -nw
	M-x server-start
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and then (from a shell prompt on another terminal) start emacsclient
with
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	lib-src/emacsclient -t /optional/file/names...
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You'll hopefully have two fully working, independent frames on
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separate terminals. The new frame is closed automatically when you
have finished editing the specified files (C-x #), but delete-frame
(C-x 5 0) also works.  Of course, you can create frames on more than
two tty devices.
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Creating new frames on the same tty with C-x 5 2 works, and they
behave the same way as in previous Emacs versions.  If you exit emacs,
all terminals should be restored to their previous states.
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This is work in progress, and probably full of bugs.  You should
always run emacs from gdb, so that you'll have a live instance to
debug if something goes wrong.  Please send me your reports.
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Problems:
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	* Suspending Emacs is disabled if there are multiple tty
	  devices.  Also, there is no way to suspend emacsclient. This
	  will be fixed.

	* X support is (I hope) working, but at the moment there are
	  problems with simultaneous X and tty devices, so don't do
	  that - start a separate Emacs with -nw and run the server
	  there.

	* Mac, Windows and DOS support is broken, probably doesn't
	  even compile -- this will be solved later.

	* Only tested on my GNU/Linux box.
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NEWS
----

For the NEWS file:

** Support for multiple terminal devices has been added.  You can
   specify a terminal device (`tty' parameter) and a terminal type
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   (`tty-type' parameter) to `make-terminal-frame'. `tty' must be a
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   terminal device created by the updated emacsclient, or there will
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   be problems with terminal input and window resizes.  
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   You can test for the presence of multiple terminal support by
   testing for the `multi-tty' feature.

** A make-frame-on-tty function has been added to make it easier to
   create frames on new terminals.
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** New functions: frame-tty-name, frame-tty-type for accessing
   terminal parameters, and delete-tty for closing the terminal
   device.

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** Emacsclient has been extended to support opening a new terminal
   frame.
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CHANGELOG
---------

See arch logs.

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THINGS TO DO
------------

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** Something with (maybe) multi-keyboard support broke function keys
   and arrows on ttys during X+tty combo sessions.  Debug this.
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** Fix faces on tty frames during X-tty combo sessions.  There is an
   init_frame_faces call in init_sys_modes, see if there is a problem
   with it.
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** During an X-tty combo session, a (message "Hello") from a tty frame
   goes to the X frame.  Fix this.

** Find out the best way to support suspending Emacs with multiple
   ttys.  My guess: disable it on the controlling tty, but from other
   ttys pass it on to emacsclient somehow.  (It is (I hope) trivial to
   extend emacsclient to handle suspend/resume.  A `kill -STOP' almost
   works right now.)

** Move baud_rate to tty_output.

** Do tty output through term_hooks, like graphical display backends.

** Implement support for starting an interactive Emacs session without
   an initial frame.  (The user would connect to it and open frames
   later, with emacsclient.)  Not necessarily a good idea.

** Fix input from raw ttys (again).

** Fix Mac support (I can't do this myself).

** Fix W32 support (I can't do this myself).

** Fix DOS support (I can't do this myself).

** Do a grep on XXX and ?? for more issues.

** Understand Emacs's low-level input system (it seems complicated) :-)

** What does interrupt_input do?  I tried to disable it for raw
   secondary tty support, but it does not seem to do anything useful.
   (Update: Look again. X unconditionally enables this, maybe that's
   why raw terminal support is broken again.  I really do need to
   understand input.)

** Make sure C-g goes to the right frame.  This is hard, as SIGINT
   doesn't have a tty parameter. :-(

** I have seen a case when Emacs with multiple ttys fell into a loop
   eating 100% of CPU time.  Strace showed this loop:

	getpid()                                = 30284
	kill(30284, SIGIO)                      = 0
	--- SIGIO (I/O possible) @ 0 (0) ---
	ioctl(6, FIONREAD, [0])                 = -1 EIO (Input/output error)
	ioctl(5, FIONREAD, [0])                 = -1 EIO (Input/output error)
	ioctl(0, FIONREAD, [0])                 = 0
	sigreturn()                             = ? (mask now [])
	gettimeofday({1072842297, 747760}, NULL) = 0
	gettimeofday({1072842297, 747806}, NULL) = 0
	select(9, [0 3 5 6], NULL, NULL, {0, 0}) = 2 (in [5 6], left {0, 0})
	select(9, [0 3 5 6], NULL, NULL, {0, 0}) = 2 (in [5 6], left {0, 0})
	gettimeofday({1072842297, 748245}, NULL) = 0

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   I have seen something similar with a single X frame, but have not
   been able to reproduce it for debugging.
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** Define a output_initial value for output_method for the initial
   frame that is dumped with Emacs.  Checking for this frame (e.g. in
   cmd_error_internal) is ugly.

** emacsclient -t from an Emacs term buffer does not work, complains
   about face problems.  This can even lock up Emacs (if the recursive
   frame sets single_kboard).
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** Clean up the source of emacsclient.  It is a mess.

** C-x 5 2, C-x 5 o, C-x 5 0 on an emacsclient frame unexpectedly
   exits emacsclient.  This is a result of trying to be clever with
   delete-frame-functions.

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DIARY OF CHANGES
----------------

(ex-TODO items with explanations.)
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-- Introduce a new struct for terminal devices.  
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   (Done, see struct tty_output.  The list of members is not yet
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   complete.)

-- Change the bootstrap procedure to initialize tty_list.

   (Done, but needs review.)

-- Change make-terminal-frame to support specifying another tty.

   (Done, new frame parameters: `tty' and `tty-type'.)

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-- Implement support for reading from multiple terminals.

   (Done, read_avail_input tries to read from each terminal, until one
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   succeeds.  MULTI_KBOARD is not used.  Secondary terminals don't send
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   SIGIO!)
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   (Update: They do, now.)

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   (Update2: After enabling X, they don't.)

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-- other-frame should cycle through the frames on the `current'
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   terminal only.  
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   (Done, by trivially modifiying next_frame and prev_frame.)
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-- Support different terminal sizes.
   
   (Done, no problem.)

-- Make sure terminal resizes are handled gracefully.  (Could be
   problematic.)

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   (Done.  We don't get automatic SIGWINCH for additional ttys,
   though.)
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-- Extend emacsclient to automatically open a new tty when it connects
   to Emacs.
   
   (Done.  It's an ugly hack, needs more work.)

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-- Redisplay must refresh the topmost frame on *all* terminals, not
   just the initial terminal.
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   (Done, but introduced an ugly redisplay problems.  Ugh.)
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-- Fix redisplay problems.

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   (Done; it turned out that the entire Wcm structure must be moved
   inside tty_output.  Why didn't I catch this earlier?)
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-- Provide a way for emacsclient to tell Emacs that the tty has been
   resized.

   (Done, simply forward the SIGWINCH signal.)

-- Each keypress should automatically select the frame corresponding
   to the terminal that it was coming from.  This means that Emacs
   must know from which terminal the last keyboard event came from.

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   (Done, it was quite simple, the input event system already
   supported multiple frames.)
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-- Fix SIGIO issue with secondary terminals.

   (Done, emacsclient signals Emacs after writing to the proxy pseudo
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   terminal.  Note that this means that multi-tty does not work with
   raw ttys!)
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   (Update: This is bullshit.  There is a read_input_waiting function,
   extend that somehow.)

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   (Update of update: The first update was not right either, extending
   read_input_waiting was not necessary.  Secondary ttys do seem to
   send signals on input.)

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   (Update^3: Not any more.)

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-- Make make-terminal-frame look up the `tty' and `tty-type' frame
   parameters from the currently selected terminal before the global
   default.

   (Done.)

-- Put all cached terminal escape sequences into struct tty_output.
   Currently, they are still stored in global variables, so we don't
   really support multiple terminal types.

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   (Done.  It was not fun.)
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-- Implement sane error handling after initialization.  (Currently
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   emacs exits if you specify a bad terminal type.)  The helpful error
   messages must still be provided when Emacs starts.

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   (Done.)

-- Implement terminal deletion, i.e., deleting local frames, closing
   the tty device and restoring its previous state without exiting
   Emacs.
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   (Done, but at the moment only called when an error happens during
   initialization.  There is a memory corruption error around this
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   somewhere.)  (Update: now it is fully enabled.)
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-- Implement automatic deletion of terminals when the last frame on
   that terminal is closed.

   (Done.)

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-- Restore tty screen after closing the terminal.

   (Done, we do the same as Emacs 21.2 for all terminals.)

-- 'TERM=dumb src/emacs' does not restore the terminal state.
   
   (Done.)
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-- C-g should work on secondary terminals.

   (Done, but the binding is not configurable.)

-- Deal with SIGHUP in Emacs and in emacsclient.  (After this, the
   server-frames may be removed from server.el.)

   (Done, nothing to do.  It seems that Emacs does not receive SIGHUP
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   from secondary ttys, which is actually a good thing.)  (Update: I
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   think it would be a bad idea to remove server-frames.)
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-- Change emacsclient/server.el to support the -t argument better,
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   i.e. automatically close the socket when the frame is closed.

   (Seems to be working OK.)

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-- Fix mysterious memory corruption error with tty deletion.  To
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   trigger it, try the following shell command:

	while true; do TERM=no-such-terminal-definition emacsclient -h; done

   Emacs usually dumps core after a few dozen iterations.  (The bug
   seems to be related to the xfree()ing or bzero()ing of
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   tty_output.Wcm.  Maybe there are outside references to struct Wcm?
   Why were these vars collected into a struct before multi-tty
   support?)
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   (Done.  Whew.  It turned out that the problem had nothing to do
   with hypothetical external references to Wcm, or any other
   tty_output component; it was simply that delete_tty closed the
   filehandles of secondary ttys twice, resulting in fclose doubly
   free()ing memory.  Utterly trivial matter.  I love the C's memory
   management, it puts hair on your chest.)

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-- Support raw secondary terminals.  (Note that SIGIO works only on
   the controlling terminal.) Hint: extend read_input_waiting() for
   multiple ttys and hopefully this will be fixed.
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   (Done, it seems to have been working already for some time.  It
   seems F_SETOWN does work, after all.  Not sure what made it fail
   earlier, but it seems to be fixed (there were several changes
   around request_sigio, maybe one of them did it).
   read_input_waiting() is only used in sys_select(), don't change
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   it.)  (Update: After adding X support, it's broken again.)
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-- Find out why does Emacs abort when it wants to close its
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   controlling tty.  Hint: chan_process[] array.  Hey, maybe
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   noninterrupt-IO would work, too?  Update: no, there is no process
   for stdin/out.
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   (Done.  Added add/delete_keyboard_wait_descriptor to
   term_init/delete_tty.  The hint was right, in a way.)
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-- Issue with SIGIO: it needs to be disabled during redisplay.  See if
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   fcntl() kernel behaviour could be emulated by emacsclient.

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   (Done.  Simply disabled the SIGIO emulation hack in emacsclient.)
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   (Update: it was added back.)
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-- server.el: There are issues with saving files in buffers of closed
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   clients.  Try editing a file with emacsclient -f, and (without
   saving it) do a delete-frame.  The frame is closed without
   question, and a surprising confirmation prompt appears in another
   frame.

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   (Done.  delete-frame now asks for confirmation if it still has
   pending buffers, and modified buffers don't seem to be deleted.)

-- emacsclient.el, server.el: Handle eval or file open errors when
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   doing -t.
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   (Done.)
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-- Make parts of struct tty_output accessible from Lisp.  The device
   name and the type is sufficient.
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   (Done, see frame-tty-name and frame-tty-type.)

-- Export delete_tty to the Lisp environment, for emacsclient.
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   (Done, see delete-tty.)

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-- Get rid of the accessor macros in termchar.h, or define macros for
   all members.

   (Done.)

-- Move device-specific parameters (like costs) commonly used by
   device backends to a common, device-dependent structure.

   (Done.  See struct display_method in termhooks.h.)

-- Fix X support.

   (Done.  Well, it seems to be working.)

-- Allow simultaneous X and tty frames.  (Handling input could be
   tricky.  Or maybe not.)

   (Done.  Allowed, that is.  It is currently extremely unstable, to
   the point of being unusable.  The rif variable causes constant
   core dumps.  Handling input is indeed tricky.)
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-- Rewrite multi-tty input in terms of MULTI_KBOARD.
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   (Done.  In fact, there was no need to rewrite anything, I just
   added a kboard member to tty_display_info, and initialized the
   frame's kboard from there.)
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-- Fix rif issue with X-tty combo sessions.  IMHO the best thing to do
   is to get rid of that global variable (and use the value value in
   display_method, which is guaranteed to be correct).

   (Done, did exactly that.  Core dumps during combo sessions became
   much rarer.  In fact, I have not yet met a single one.)

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-- Add multi-tty support to talk.el.

   (Done.)

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