Re: feature request: new builtin `defer`, scope delayed eval

[Koichi Murase]

2022年10月8日(土) 12:04 Cynthia Coan <cynthia@coan.dev>:
> [...]
>
> Otherwise, I think we can perhaps reword this into two smaller
> features: "function local trap signals",

The existing RETURN trap is exactly the trap that can be used to clean
up resources local to functions and is already ``function-local''
unless the user changes its behavior by setting `declare -ft', `set
-T', or `shopt -s extdebug'. One thing to note is that the RETURN trap
should be cleaned up inside the RETURN trap itself by running `trap -
RETURN', or otherwise the RETURN trap is erroneously considered by
Bash to be set for the caller function.

> and "options to safely
> append/prepend to a trap" (for some definition of 'safe', perhaps this
> just means separating by ';'?

Currently, there is no way to register multiple handlers to a trap in
a safe way, but if you can accept the approach by `;' or newline, you
can just do it by getting the current trap string with `trap -p
RETURN' and then overwrite the trap with the adjusted trap string. If
you don't want to write such a code every time, you can write a shell
function to perform that. In that case, be careful that the function
needs to be marked with the trace attribute by ``declare -ft
<function-name>''. Also, the RETURN trap needs to be suppressed for
such a function itself. I haven't really tested it but something like
this should work as you expect:

function defer {
  local command=$1 trap
  eval "trap=($(trap -p RETURN))"
  if [[ $trap ]]; then
    trap[2]=$command$'\n'${trap[2]}
  else
    trap=(trap -- "$command" RETURN)
  fi
  trap[2]='if [[ $FUNCNAME != defer ]]; then '${trap[2]}$'\ntrap - RETURN\nfi'
  "${trap[@]}"
}
declare -ft defer

The above `defer' function effectively "eval"s the first argument
through `trap', so you need to specify in the following way:

defer 'rm -r "${tmp_dir}"'
defer 'set +e'
defer 'set +o pipefail'

Another thing to note is that the RETURN trap will be skipped when
Bash receives SIGINT, so if you want to properly perform the clean up
even when your Bash program receives SIGINT, you need to additionally
set the SIGINT handler in a proper way.

--
Koichi