#!/bin/bash
BASH_ARGV0=twopane.bash
echo -n "$0" >/proc/$BASHPID/comm
set -e
set -f
set -o pipefail
shopt -s lastpipe

PS4='+ \t$LINENO\t '

. read_chars.bash
. finally.bash

kill_tty_forward()
{
        read pid < "$TWOPANE"/tty_forward.pid
        /bin/kill "${1:-'-INT'}" -- -$pid 2>/dev/null
}

BOT_SIZE=8
BOT_TITLE=input

DO_RESTART=
DO_START=
DO_START=y
DO_RESTART=y

if [ $# = 0 ]
then
        TOP_CMD="$SHELL -i"
        if [ "$DO_RESTART" ]
        then
                SIG=INT
                BOT_CMD=PROMPT_COMMAND=prompt_command
                TOP_EXIT=restart
        elif [ "$DO_START" ]
        then
                BOT_CMD=start
                TOP_EXIT=quit
        else
                BOT_CMD=PROMPT_COMMAND=prompt_command
                TOP_EXIT=prompt
        fi
elif [ "$TWOPANE" -a "$*" = detach ]
then
        kill_tty_forward -TSTP
        exit
elif [ "$TWOPANE" -a "$*" = focus ]
then
        screen -X focus
        exit
elif [ "$TWOPANE" -a "$*" = quit ]
then
        screen -X quit
        exit
else
        TOP_CMD=$*
        BOT_CMD=start
        TOP_EXIT=quit
fi
TOP_TITLE="Command: $TOP_CMD"

TWOPANE=$(mktemp -d)
export TWOPANE TOP_CMD BOT_CMD TOP_TITLE BOT_TITLE BOT_SIZE
trap 'rm -r "$TWOPANE"' EXIT
# STY=$(systemd-escape -p "$TWOPANE")
systemd_escape_path()
{
        declare -n path="$1" result="$2"
        result=$(realpath "$path")
        result=${result//-/--}
        result=${result//\//-}
}
systemd_escape_path TWOPANE STY
STY=$0.$$.$STY

save_file()
{
        cat > "$TWOPANE"/"${1:?$0: Error: filename cannot be empty string}"
}

save_screenrc()
{
        save_file screenrc"${1:+.$1}"
}

save_screenrc <<'.'
# Disable keybindings.
unbindall
escape \0\0
# Disable messages.  This is needed for screen -X/-Q to work reasonably.
msgwait 0
msgminwait 0
# Try to disable blocking the terminal on ^S.
# Doesn't seem to work here.
# More is needed.  Xterm?
nonblock on

# Enable mouse focus
defmousetrack on
mousetrack on

caption string '%t'
layout new
split
focus bottom
resize $BOT_SIZE
screen -ln -t "$BOT_TITLE" 0 bash --noprofile --rcfile "$TWOPANE"/bashrc -i
layout save 0
.

with_screen_pane()
{
        (( $# > 1 )) || return
        declare -i PANE="$1" LOCK
        (( PANE > 0 )) || return
        (
                flock $LOCK
                "$2" "$1" "${@:3}"
        ) {LOCK}>"$TWOPANE"/pane$PANE
}

check_screen_pane()
{
        (( PANE > 0 )) || return
        screen -p "$PANE" -Q info >/dev/null
}

kill_screen_pane()
{
        (( PANE > 0 )) || return
        while check_screen_pane "$PANE"
        do
                screen -p "$PANE" -X kill
        done
}

start_screen_pane()
{
        (( PANE > 0 )) || return
        if check_screen_pane "$PANE"
        then
                i "pane already running: $PANE"
                return
        else
                i "starting pane: $PANE"
        fi
        shift
        if [ $# = 0 ]
        then
                set -f
                set -- ${TOP_CMD:-bash -i}
        fi
        TOP_TITLE="Command: ${*@Q}"
        screen -X focus top
        screen -X screen -ln -t "$TOP_TITLE" 1 "$@"
        start_screen_pane_subprocess
}

start_screen_pane_subprocess()
{
        (( PANE > 0 )) || return
        subproc_command=(
                exec -a top-pane-tty-forward
                        socat
                                UNIX-LISTEN:"$TWOPANE"/socket
                                STDIN,cfmakeraw,opost=1,onlcr=1!!STDOUT
        )
        screen -p "$PANE" -X exec .!. bash -c "${subproc_command[*]}"
}

restart_pane()
{
        (( PANE > 0 )) || return
        kill_screen_pane
        start_screen_pane "$@"
}

restart_top_pane()
{
        with_screen_pane 1 restart_pane "$@"
}

# connect_coproc() -- start a coprocess and connect it to copied file
# descriptors named {NAME}_IN and {NAME}_OUT where {NAME} is the
# supplied first argument or default "COPROC".  {NAME} is the name of
# the created bash coprocess.

# The copied file descriptors (unlike the original coprocess
# file descriptors, in ${NAME[0]} and ${NAME[1]}) can be
# passed to external processes (e.g.: socat(1) instances) and
# used in subshells.

# The created coprocess is silently disowned from job control before
# bash can make noise about it.
connect_coproc()
{
        [ $# -gt 0 ] || set -- COPROC
        declare -n coproc="$1"
        shift
        [ $# -gt 0 ] || set -- "${!coproc}"
        declare -n std0="${!coproc}_IN"
        declare -n std1="${!coproc}_OUT"
        declare -n std2="${!coproc}_ERR"
        declare -n pid="${!coproc}_PID"
        if ! [ "$pid" ]
        then
                i "coproc starting: ${!coproc}"
                {
                        coproc "${!coproc}" \
                        {
                                "$@"
                        } 2>&$std2 {std2}>&-
                        unset std2
                        disown "%coproc ${!coproc} "
                } {std2}>&2 2>/dev/null
        else
                i "coproc already running: ${!coproc}"
        fi
        {
                exec {std0}<&0 {std1}>&1
        } <&${coproc[0]} >&${coproc[1]}
}

disconnect_coproc()
{
        [ $# -gt 0 ] || set -- COPROC
        declare -n coproc="$1"
        declare -n std0="${!coproc}_IN"
        declare -n std1="${!coproc}_OUT"
        declare -n  pid="${!coproc}_PID"
        if [ "$std0" -o "$std1" ]
        then
                exec {std0}<&- {std1}>&-
        fi
        if [ "$pid" ]
        then
                kill "$pid" 2>/dev/null
                wait -f "$pid" 2>/dev/null
        fi
        unset coproc std0 std1 pid
}

connect()
{
        if ! [ "$TOP_PANE_PID" ]
        then
                disconnect_sink ECHOSEND
                connect_coproc TOP_PANE
        fi
        connect_sink ECHOSEND echo_sender
}

disconnect()
{
        : "${TOP_PANE_IN@A} ${TOP_PANE_OUT@A} ${TOP_PANE_PID@A} ${TOP_PANE[@]@A}"
        disconnect_coproc TOP_PANE
        : "${ECHOSEND@A} ${ECHOSEND_PID@A}"
        disconnect_sink ECHOSEND
}

sendc()
{
        [ "${TOP_PANE[0]}" ] || connect
        printf '%s' "$*" >&${TOP_PANE_OUT?Internal error}
}

send()
{
        [ "${TOP_PANE[0]}" ] || connect
        printf '%s\n' "$*" >&${TOP_PANE_OUT?Internal error}
}

tty_forward()
{
        printf '%d\n' "$BASHPID" > "$TWOPANE"/tty_forward.pid
        trap 'kill -INT $$' INT
        read-tty
}

TOP_PANE()
{
        (exec -a bottom-pane-tty-forward socat - UNIX-CONNECT:"$TWOPANE"/socket,forever)
        case "$TOP_EXIT" in
                restart )
                        x kill_tty_forward -STOP
                        x kill -INT $$
                ;;
                quit )
                        quit
                ;;
                prompt | * )
                        x kill_tty_forward -STOP
                        focus bottom
                ;;
        esac
}

connect_sink()
{
        (( $# > 0 )) || set -- SINK
        declare -n fd="$1"
        declare -n pid="${!fd}_PID"
        if [ "$pid" ]
        then
                i "sink already running: ${!fd}"
                return
        fi
        i "sink starting: ${!fd}"
        exec {fd}> >("${@:2}")
        pid=$!
}

disconnect_sink()
{
        (( $# > 0 )) || set -- SINK
        declare -n fd="$1"
        [ "$fd" ] || return
        declare -n pid="${!fd}_PID"
        i "sink stopping: ${!fd}"
        exec {fd}>&-
        kill $pid 2>/dev/null
        unset fd pid
}

quiet_bg()
{
        local STDERR
        {
                eval "$(printf '%q ' "$@") & 2>&\$STDERR {STDERR}>&-"
        } {STDERR}>&2 2>/dev/null
}

# echo_sender() reads from two inputs and writes to two outputs.
#
# Read from both:
#       read from stdin
#       read from $TOP_PANE_IN
# Write to both:
#       write to stdout
#       write to $TOP_PANE_OUT
# I.e.:
#       read from /dev/tty
#       read from the socat-coproc-forwarded /dev/tty in the top pane.
#       write to /dev/tty
#       write to the socat-coproc-forwarded /dev/tty in the top pane.
echo_sender()
{
        (exec -a echo_sender socat - fd:${TOP_PANE_IN?$0: Internal error}!!-) |
        tee >(write-tty) >&${TOP_PANE_OUT?$0: Internal error}
}

restart_tty_forward()
{
        detach
        with_screen_pane 1 start_screen_pane "$@"
        focus bottom
        connect
        i 'starting tty_forward'
        quiet_bg tty_forward >&$ECHOSEND
        echo $! > "$TWOPANE"/tty_forward.pid
}

attach()
{
        if ! jobs %tty_forward &>/dev/null
        then
                restart_tty_forward "$@"
        fi
        focus top
        { %tty_forward; } &>/dev/null
        focus bottom
}

detach()
{
        {
                kill -INT %tty_forward
                disown %tty_forward
        } &>/dev/null
}

restart() { attach "$@"; }
start() { attach "$@"; }
foreground() { attach "$@"; }
twopane() { attach "$@"; }
forward() { attach "$@"; }

quit()
{
        screen -X quit
}

focus()
{
        screen -X focus "$@"
}

resize()
{
        screen -X resize "$@"
}

SIGUSR1()
{
        i "SIGUSR1: ${BASH_COMMAND@A}"
}

SIGCHLD()
{
        i "SIGCHILD: ${BASH_COMMAND@A}"
}

prompt_command()
{
        [ "$TOP_EXIT" = 'restart' ] || return 0

        if [ "$SIG" = INT ]
        then
                finally 'unset SIG; start' detach
                return
        fi

        local job
        if      ! { job=$(jobs %tty_forward 2>/dev/null) && [ "$job" ]; }
        then
                finally 'start' :
                return
        fi

        read _ jobstatus _ <<< "${job?Internal error: line $LINENO}"
        case "${jobstatus?Internal error: line $LINENO}" in
                Running )
                        jobs -x finally 'attach' kill -CONT %tty_forward
                ;;
                Terminated | Interrupt )
                        finally 'start' :
                ;;
                Stopped )
                ;;
        esac
}

SIGINT()
{
        i INT "${BASH_COMMAND@A}"
        kill $TOP_PANE_PID $ECHOSEND_PID 2>/dev/null
        SIG=INT
        PROMPT_COMMAND=prompt_command
}

our_bashrc_main()
{
        BASH_ARGV0=twopane
        echo -n "$0" >/proc/$BASHPID/comm
        set -m
        set -f
        set -o pipefail
        trap 'SIGUSR1' USR1
        # trap 'SIGCHLD' CHLD
        trap 'SIGINT' INT
        trap '! [ "$DEBUG" ] || read -p "Exit> "; quit' EXIT
        export PS1="$BOT_TITLE\\\$ "
}

save_file bashrc <<.
${TOP_CMD@A}
${BOT_CMD@A}
${TOP_TITLE@A}
${BOT_TITLE@A}
${TOP_EXIT@A}
$(declare -f)
our_bashrc_main
$BOT_CMD
.

main()
{
        screen -c "$TWOPANE"/screenrc -m -S "$STY" -ln
}

main "$@"