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#! /bin/sh
#
# [sh -x] spawn [command [argument ...]]
#
# export id to create&destroy or reuse the working directory //proc/$id/.
# this feature is for debug only and marked as deprecated, so don't rely
# on it too hard.
#
spawn() {
set -euf
# establish working subdirectory in //proc. we're mking only
# transient dirs, i.e. if we mkdir, then we also defer rmdir.
if test -n "${id-}"; then
: "using id=[32;1m$id[m from env"
wd=$pd/$id
if ! test -d $wd; then
: "make transient [32;1m$wd/[m"
mkdir $wd
defer rmdir $wd
elif ! test `ls $wd | wc -l` = 0; then
: "[31;1m$wd/[;31m is not empty![m"
exit 23
else
: "reuse existing [32;1m$wd/[m"
fi
else
id=`cd $pd && mktemp -d XXXXXXXXXXXXXXXX`
wd=$pd/$id
defer rmdir $wd
: "made transient [32;1m$wd/[m"
fi
# change to //proc working directory
cwd="$PWD"
cd $wd
#defer cd $cwd # no need for this, b/c at that time we're aldeady dead
# create named pipes for the child process's stdio
mkfifo 0 1 2
defer rm 0 1 2
# spawn child process
( : "in [32;1m$PWD/[m spawn [32m${*-"[35;1mnothing"}[m"
exec 0>&- 1>&- 2>&- 0<>0 1<>1 2<>2
cd "$cwd"
exec "$@") &
pid=$!
# setup a trap to kill the child process if this (parent) process dies
defer kill $pid
# store misc. info.
ln -snf $cwd cwd
echo $id >id
echo $$ >ppid
echo $pid >pid
defer rm cwd id pid ppid
# wait for the child process's
set +e
wait $pid
code=$?
set -e
# the child is already dead
cancel kill $pid
# return the same way wait did
(exit $code)
}
#
# defer [command [argument ...]]
#
# Defer execution of a command. Deferred commands are executed in LIFO
# order immediately before the script terminates. See (golang's defer
# statement for more information how this should work).
#
defer() {
defer="$*${defer+
$defer}"
}
#
# cancel [command [argument ...]]
#
# Cancel a deferred command. The arguments have to match exactly a
# prior defer call or else chaos and mayhem shall haunt thee and shi-
#
cancel() {
defer="`echo "$defer" | grep -Fxv "$*"`"
}
# setup //proc directory
pd=/tmp/krebs/proc
mkdir -p $pd
test -w $pd
# setup deferred execution and spawn command
trap 'eval "${defer-}"; defer=' EXIT INT TERM
spawn "$@"
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