#!/bin/sh

set -eu

if [ "$#" -eq 0 ]; then

echo "Usage: $(basename "$0") [SSH_ARGS...] [user@]hostname"

echo

echo "Maintain an SSH connection or tunnel."

echo "No arguments provided. Exiting."

exit 0

fi

# POSIX arithmetic expressions do not support exponentiation,

# so define a function for integer exponentiation.

pow() {

set "$1" "$2" 1

while [ "$2" -gt 0 ]; do

set "$1" $(($2-1)) $(($1*$3))

done

echo "$3"

}

# Wait for a child process to complete and resume waiting after trap.

# Repeat wait builtin if it returns an exit status of 128 + SIGUSR1,

# which corresponds to wait returning early to handle that signal.

wait_continue_after_trap() {

while true; do

if wait "$1"; then

return

else

retval="$?"

fi

if [ "$retval" -eq "$((128 + $(kill -l | sed -n /USR1$/=) - 1))" ]; then

continue

else

return

fi

done

}

# SIGINT, SIGTERM: terminate child processes, then exit.

cleanup() {

if [ -n "${ssh_pid-}" ]; then

kill "${ssh_pid-}" || true

ssh_pid=

fi

if [ -n "${sleep_pid-}" ]; then

kill "${sleep_pid-}" || true

sleep_pid=

fi

wait

exit

}

trap cleanup INT TERM EXIT

# SIGUSR1: wakeup from sleep to retry ssh immediately.

wakeup() {

if [ -n "${sleep_pid-}" ]; then

echo "Received SIGUSR1. Retrying ssh immediately." >&2

tries=0

kill "${sleep_pid-}" || true; sleep_pid=

else

echo "Received SIGUSR1, but ssh is running. Ignoring." >&2

fi

}

trap wakeup USR1

tries=0

while true; do

# CLOCK_MONOTONIC would be resilient to administrator changes,

# but timestamp is good enough.

timestamp="$(date +%s)"

ssh -NT -F /etc/sidedoor/config "$@" & ssh_pid="$!"

if wait_continue_after_trap "$ssh_pid"; then

exit

fi

ssh_pid=

# If ssh exits quickly, retry with exponential backoff.

if [ "$(date +%s)" -lt "$((timestamp + 30))" ]; then

tries="$((tries + 1))"

timeout="$(pow 2 $tries)"

[ "$timeout" -gt 600 ] && timeout=600

echo "ssh exited too quickly. Retrying in $timeout seconds." >&2

sleep "$timeout" & sleep_pid="$!"

wait_continue_after_trap "$sleep_pid" || true

sleep_pid=

else

tries=0

fi

done