Here we test what happens if there is a signal for the program to terminate when we are
running with dune exec. Signals are dispatched from the operating system. It is usually
impossible for a program to recover after recieving such a signal.

For dune exec -w, we are indifferent to what the process we are running is actually doing
since it shouldn't affect dune's other functions.

  $ DONE_FLAG=_build/done_flag

  $ cat > dune-project <<EOF
  > (lang dune 3.18)
  > EOF

  $ cat > dune <<EOF
  > (executable
  >  (name foo)
  >  (libraries unix))
  > EOF

  $ cat > touch.ml <<EOF
  > let touch path =
  >  let fd = Unix.openfile path [ Unix.O_CREAT ] 0o777 in
  >  Unix.close fd
  > ;;
  > EOF

This first program will signal itself with a KILL signal.:
  $ cat > foo.ml <<EOF
  > let () =
  >   Touch.touch "$DONE_FLAG";
  >   print_endline "about to be killed";
  >   let pid = Unix.getpid () in
  >   Unix.kill pid Sys.sigkill
  > ;;
  > EOF

  $ LOG_FILE=_build/log_file
  $ mkdir _build

When reaching a signal like SEGV dune exec -w will exit.
  $ dune exec -w ./foo.exe 2> >(tee "$LOG_FILE" >&2) &
  about to be killed
  Command got signal KILL.
  Had 1 error, waiting for filesystem changes...
  fixed signal
  Success, waiting for filesystem changes...
  $ PID=$!
  $ ./wait-for-file.sh $DONE_FLAG

Waiting for KILL signal...
  $ tail -f "$LOG_FILE" | while read line; do
  >   echo "$line" | grep 'KILL' && break
  > done
  Command got signal KILL.

We can now start a new build by modifying the original program and removing the segfault.
This rebuilds successfully as indicated by the above output.
  $ cat > foo.ml <<EOF
  > let () =
  >   Touch.touch "$DONE_FLAG";
  >   print_endline "fixed signal";
  > ;;
  > EOF

  $ ./wait-for-file.sh $DONE_FLAG

  $ kill $PID
  $ wait $PID
  [130]