ssh_control_socket.sh

# Call `setup_ssh_socket` to setup the control socket (function will return once
# the socket is ready to go), and `ssh_target` will connect using the control socket.
# Assumes TARGET_HOST variable is set.
# The connection is automatically closed when the script exists.
# TARGET_HOST="wolever.net"
# setup_ssh_control_socket
# ssh_target "hostname"

debug() {
    echo "DEBUG: $*"
}

_atexit_cmds=( )
_atexit_trap_handler() {
    for cmd in "${_atexit_cmds[@]}"; do
        eval "$cmd" || true
    done
}
trap _atexit_trap_handler EXIT

atexit() {
    _atexit_cmds[${#_atexit_cmds[@]}]="$1"
}

ssh_target() {
    ssh -S "$SSH_CONTROL_SOCKET" "$TARGET_HOST" "$@"
}

setup_ssh_control_socket() {
    SSH_CONTROL_SOCKET="/tmp/ssh-control-$TARGET_HOST-$$"
    local ssh_fifo="$SSH_CONTROL_SOCKET-fifo"
    mkfifo "$ssh_fifo"
    atexit "rm '$ssh_fifo'"

    local ssh_cmd="echo ready; while :; do sleep 100; done"
    ssh -nM -o ControlPath="$SSH_CONTROL_SOCKET" "$TARGET_HOST" "$ssh_cmd" > "$ssh_fifo" &
    SSH_CONTROL_PID="$!"
    atexit "kill $SSH_CONTROL_PID 2>/dev/null" EXIT
    debug "ssh control started at $SSH_CONTROL_PID"

    debug "waiting for ssh connection..."
    local ssh_line
    read -t10 ssh_line < "$ssh_fifo" || {
        echo "error: ssh connect timeout"
        return 1
    }
    [[ "$ssh_line" != "ready" ]] && {
        echo "error: invalid line from SSH: $ssh_line"
        return 1
    }
    kill -INFO "$SSH_CONTROL_PID" 2> /dev/null || {
        echo "error: could not establish SSH control channel"
        return 1
    }
    debug "SSH control channel to $TARGET_HOST started at $SSH_CONTROL_SOCKET"
}