## Remoteusage without password-free login requirement

This is alternative to [[remoteusage|remoteusage]] where password-free
login is not a requirement. See [[remoteusage|remoteusage]] page for
other requirements and general information.

This solution uses one pre-made ssh connection where the client is put
into "master" mode (-M) for connection sharing. The wrapper script then
uses the control socket created by this pre-made ssh connection for
its own connection. As long as master ssh connection is live, slave
can use it. Disconnecting master all future attempts to connect
from the script will fail.

It is possible to use this solution without any changes to
`$HOME/.ssh/config`.

At the end of this document there is information for some possible ways
how master ssh connection can be done.

## The script

Write the following code to a file, for example `remote-notmuch.sh`.

	#!/bin/bash

	set -euf

	# To trace execution, uncomment next line:
	#exec 6>>remote-errors; BASH_XTRACEFD=6; echo -- >&6; set -x

	: ${REMOTE_NOTMUCH_SSHCTRL_SOCK:=master-notmuch@remote:22}
	: ${REMOTE_NOTMUCH_COMMAND:=notmuch}

	readonly REMOTE_NOTMUCH_SSHCTRL_SOCK REMOTE_NOTMUCH_COMMAND

	SSH_CONTROL_ARGS='-oControlMaster=no -S ~'/.ssh/$REMOTE_NOTMUCH_SSHCTRL_SOCK
	readonly SSH_CONTROL_ARGS

	printf -v ARGS '%q ' "$@" # bash feature
	readonly ARGS

	if ssh -q $SSH_CONTROL_ARGS 0.1 "$REMOTE_NOTMUCH_COMMAND" $ARGS
	then exit 0
	else ev=$?
	fi

	# continuing here in case ssh exited with nonzero value

	case $* in
	 'config get user.primary_email') echo 'nobody@nowhere.invalid'; exit 0 ;;
	 'config get user.name') echo 'nobody'; exit 0 ;;
	 'count'*'--batch'*) while read line; do echo 1; done; exit 0 ;;
	 'count'*) echo 1; exit 0 ;;
	 'search-tags'*) echo 'errors'; exit 0 ;;
	 'search'*'--output=tags'*) echo 'errors'; exit 0 ;;
	esac

	# fallback exit handler; print only to stderr...
	exec >&2

	if ssh $SSH_CONTROL_ARGS -O check 0.1
	then
	 echo " Control socket is alive but something exited with status $ev"
	 exit $ev
	fi

	echo " See`sed '1d;2d;s/.//;q' "$0"` for help"
	exit $ev
	#eof

Note the `0.1` in ssh command line. It is used to avoid any opportunistic
behaviour ssh might do; for example if control socket is not alive ssh
would attempt to do its own ssh connection to remote ssh server. As
address `0.1` is invalid this attempt will fail early.

## Test

Easiest way to test this script is to run the pre-made ssh connection
using the following command line:

        ssh -M -S '~'/.ssh/master-notmuch@remote:22 [user@]remotehost sleep 600

(replace `[user@]remotehost` with your login info). Doing this the
above wrapper script can be run unmodified. After the above command has
been run on **one terminal**, enter `chmod +x remote-notmuch.sh` in
**another terminal** and then test the script with

        ./remote-notmuch.sh help

Note that the '~' in the ssh command line above is inside single quotes
for a reason. In this case shell never expand it to `$HOME` -- ssh does
it by not reading `$HOME` but checking the real user home directory
from `/etc/passwd`.  For security purposes this is just how it should
be.

## Tune

The path `'~'/.ssh/master-notmuch@remote:22` might look too generic to be
used as is as the control socket after initial testing (but it can
be used). It is presented as a template for what could be configured
to `$HOME/.ssh/config`. For example:

        Host *
            ControlPath ~/.ssh/master-%h@%p:%r

is a good entry to be written in `$HOME/.ssh/config`;
[[remoteusage|remoteusage]] uses the same. Now, let's say you'd
make your pre-made ssh connection with command

        ssh -M robin@example.org

There are 3 options how to handle this with `./nottoomuch-remote.bash`:

1) Edit `./nottoomuch-remote.bash` and change `REMOTE_NOTMUCH_SSHCTRL_SOCK`
   to contain the new value (being *master-robin@example.org:22* in this
   case)

2) Make symlink:
   `ln -sfT master-robin@example.org:22 ~/.ssh/master-notmuch@remote:22`

3) `REMOTE_NOTMUCH_SSHCTRL_SOCK` can be used via environment; like:

      REMOTE_NOTMUCH_SSHCTRL_SOCK=master-robin@example.org:22 ./nottoomuch-remote.bash help

## Configure Emacs on the client computer ##

Add something like the following functions to your Emacs (general(*) or
notmuch specific) configuration files:

    ;; this should work as backend function when copied verbatim
    (defun user/notmuch-remote-setup (sockname)
      (setq notmuch-command "/path/to/nottoomuch-remote.bash")
      (setenv "REMOTE_NOTMUCH_SSHCTRL_SOCK" sockname)
      ;; If you use Fcc, you may want to do something like this on the client,
      ;; to Bcc mails to yourself (if not, remove in your implementation):
      (setq notmuch-fcc-dirs nil)
      (add-hook 'message-header-setup-hook
                (lambda () (insert (format "Bcc: %s <%s>\n"
                                           (notmuch-user-name)
                                           (notmuch-user-primary-email))))))

     ;; this is just an example to configure using "default" master socket
     (defun user/notmuch-remote-default ()
       (interactive)
       (user/notmuch-remote-setup "master-notmuch@remote:22")

     ;; usage example2: set USER & HOST1 according to your remote...
     (defun user/notmuch-remote-at-HOST1 ()
       (interactive)
       (user/notmuch-remote-setup "master-USER@HOST1:22")

     ;; ... you probably got the point now -- add relevant funcs to your config
     (defun user/notmuch-remote-at-HOST2 ()
       (interactive)
       (user/notmuch-remote-setup "master-USER@HOST2:22")

... and if you want to activate your remote by default just call
`(user/notmuch-remote-setup "master-USER@HOST:22")` without function call
wrapper.

(*) general most likely being ~/.emacs

### Yet another possibility -- script to start emacs

Instead of adding new configurations you could also write a special
script which starts and configures emacs suitable for remote usage.
Copy the following "template" to a new name e.g. in `$HOME/bin/`,
edit the value for `master-USER@HOST3:22` and perhaps add more
post eval-after-load notmuch configs there.

    #!/bin/sh
    :; exec "${EMACS:-emacs}" --debug-init --load "$0" "$@"; exit

    (setenv "REMOTE_NOTMUCH_SSHCTRL_SOCK" "master-USER@HOST3:22")

    (eval-after-load "notmuch"
      (lambda ()
        (setq notmuch-command (concat (file-name-directory load-file-name)
                                      "remote-notmuch.sh"))
        ;; place for more post-notmuch-load emacs configs if any
        ))
    (notmuch)

    ;; Local Variables:
    ;; mode: emacs-lisp
    ;; End:

## Creating master connection

**(Note: all the examples below use the default master socket written in**
`./nottoomuch-remote.bash` **for initial test easiness; remove/change the**
`-S '~'/.ssh/master-notmuch@remote:22` **in case you don't need it.)**

As mentioned so many times, using this solution requires one pre-made
ssh connection in "master" mode. The simplest way is to dedicate one
terminal for the connection with shell access to the remote machine:

        ssh -M -S '~'/.ssh/master-notmuch@remote:22 [user@]remotehost

One possibility is to have this dedicated terminal in a way that the
connection has (for example 1 hour) timeout:

        ssh -M -S '~'/.ssh/master-notmuch@remote:22 [user@]remotehost sleep 3600

The above holds the terminal. The next alternative puts the command in
background:

        ssh -f -M -S '~'/.ssh/master-notmuch@remote:22 [user@]remotehost sleep 3600

If you don't want this to timeout so soon, use a longer sleep, like 99999999
(8 9:s, 1157 days, a bit more than 3 years).

A more "exotic" solution would be to make a shell script running on remote
machine, checking/inotifying when new mail arrives. When mail arrives it
could send message back to local host, where a graphical client (to be written)
pops up on display providing info about received mail (and exiting this
graphical client connection to remote host is terminated).

## Troubleshooting

If you experience strange output when using from emacs first attempt to just
run

        ./remote-notmuch.sh help

from command line and observe output. If it looks as it should be next uncomment
the line

        #exec 6>>remote-errors; BASH_XTRACEFD=6; echo -- >&6; set -x

in `./remote-notmuch.sh` and attempt to use it from emacs again -- and then
examine the contents of `remote-errors` in the working directory emacs was
started.