From mboxrd@z Thu Jan 1 00:00:00 1970 From: Vitaly Sinilin Subject: [BUG] Redirection bug in subshell's EXIT trap Date: Thu, 19 Oct 2017 20:44:10 +0300 Message-ID: <20171019174410.GA22649@ghost.hq.kp4.ru> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Received: from ghost.kp4.ru ([188.143.137.101]:64235 "EHLO ghost.kp4.ru" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753131AbdJSSKr (ORCPT ); Thu, 19 Oct 2017 14:10:47 -0400 Received: from vs by ghost.kp4.ru with local (Exim 4.84) (envelope-from ) id 1e5Era-0006PD-U3 for dash@vger.kernel.org; Thu, 19 Oct 2017 20:44:10 +0300 Content-Disposition: inline Sender: dash-owner@vger.kernel.org List-Id: dash@vger.kernel.org To: dash@vger.kernel.org Hi, I've recently written a quite unusual script and faced a strange dash behavior that seems not POSIX-ly correct to me. Here is the script with a lot of debugging extra output. (I am sorry about posting a pretty lengthy script, but I feel like a shorten version will look just insane without the context.) #!/bin/dash # # This helper script takes a terminal from stdin to interact with user # and prints uuid of the selected entity to stdout that is not # necessarily connected to the same terminal. # # It's meant to be run as part of command substitution, e.g. # # uuid=$(pickthing "$@") # die() { echo "$@" >&2 exit 2 } getch() ( # need a subshell here, so round braces instead of curly ones dump_self_fds trap 'dump_self_fds; stty -cbreak echo' EXIT stty cbreak -echo head -c 1 dump_self_fds ) dump_self_fds() { { read void; read void; read header pid; } < /proc/self/status printf "%d:" $pid for fd in 0 1 2; do printf " %s" "$(readlink /proc/$pid/fd/$fd)" done echo } >&2 test -t 0 || die "Standard input is not a terminal" exec 3>&1 # 3 will be initial stdout exec 1>$(tty) # switch stdout to terminal exec 4<&0 # 4 will be initial stdin (for use in pipelines) exec 0<&- # Even when run as sh, bash doesn't interrupt script execution if SIGINT # is received during execution of a foreground pipeline (dash does). So # we need to explicitly instruct it to do so. # https://www.cons.org/cracauer/sigint.html trap 'trap - INT; kill -INT $$' INT ask() { echo "Take $2? (y/n)" while :; do #key=$(exec 0<&4; dump_self_fds; getch) # works in dash key=$(dump_self_fds; getch <&4) # looks better but # doesn't work in dash case $key in [yY]) echo $1 >&3; return 0;; [nN]) break;; esac done return 1 } lsthings() { # just a stub instead of a real command cat <<-"EOF" df66e01c-1dcf-487c-aedc-a8b1c1859b49 item a fff43fed-7560-48f7-9b38-c5509d74693f item b EOF } dump_self_fds lsthings "$@" | { while read uuid info; do dump_self_fds ask "$uuid" "$info" && exit 0 done; exit 1; } || ask "$*" "$* (initial literal value)" # End of script N.B. dump_self_fds() dumps PID and file descriptors 0, 1 and 2. It is very Linux-specific. So there is no point in trying this script in other environments. N.B. This script will mess up your terminal settings. Use 'reset' or 'stty echo' after run to fix the terminal. This script involves 3 levels of subshells. [1] pipeline with a while loop. `--[2] command substitution in ask() key=$(...) `--[3] getch() body Let's see what happens when the script is run (my comments inlined): $ ./pickthing test 18406: pipe:[4898025] /dev/pts/1 /dev/pts/1 # toplevel shell 18412: pipe:[4898032] /dev/pts/1 /dev/pts/1 # [1] Take item a? (y/n) 18417: pipe:[4898032] /dev/pts/1 /dev/pts/1 # [2] got stdin from [1] 18421: /dev/pts/1 /dev/pts/1 /dev/pts/1 # [3] got FD#4 as stdin # I answered n here 18421: /dev/pts/1 /dev/pts/1 /dev/pts/1 # [3] last line 18421: pipe:[4898032] /dev/pts/1 /dev/pts/1 # [3] EXIT trap. WTF??? stty: standard input: Inappropriate ioctl for device # Since I answered no, second iteration happened. 18412: pipe:[4898032] /dev/pts/1 /dev/pts/1 # [1] Take item b? (y/n) 18439: pipe:[4898032] /dev/pts/1 /dev/pts/1 # [2] 18443: /dev/pts/1 /dev/pts/1 /dev/pts/1 # [3] # I answered n here 18443: /dev/pts/1 /dev/pts/1 /dev/pts/1 # [3] 18443: pipe:[4898032] /dev/pts/1 /dev/pts/1 # [3] WTF??? stty: standard input: Inappropriate ioctl for device Take test (initial literal value)? (y/n) # It's the last line ask, so no pipeline here anymore. 18461: pipe:[4898099] /dev/pts/1 /dev/pts/1 # [2] 18465: /dev/pts/1 /dev/pts/1 /dev/pts/1 # [3] 18465: /dev/pts/1 /dev/pts/1 /dev/pts/1 # [3] 18465: pipe:[4898118] /dev/pts/1 /dev/pts/1 # [3] WTF??? stty: standard input: Bad file descriptor # WTF?!?!? The problem here is that although getch is run in a subshell and it got FD#4 as stdin it somehow has parent's stdin in its EXIT trap. POSIX says: "The environment in which the shell executes a trap on EXIT shall be identical to the environment immediately after the last command executed before the trap on EXIT was taken." As you can see the last command of getch is dump_self_fds and stdin is OK at that time. In bash this script works as expected. For dash I had to invent a workaround (commented out line key=$(...)). -- Thanks, Vitaly Sinilin (I am not subscribed to the list, so please CC me.)