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Re: Bizarre interaction bug involving bash w/ lastpipe + Almquist 'wait'

From: Harald van Dijk
Subject: Re: Bizarre interaction bug involving bash w/ lastpipe + Almquist 'wait'
Date: Tue, 18 Feb 2020 21:59:37 +0000
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:73.0) Gecko/20100101 Thunderbird/73.0

On 18/02/2020 16:46, Denys Vlasenko wrote:
On Sat, Feb 8, 2020 at 7:41 PM Harald van Dijk <address@hidden> wrote:
If NetBSD sh
manages to avoid this pattern, and assuming NetBSD sh is not still
susceptible to one of those race conditions

Please let us know what you discovered.

Okay, please take a look. I hope I managed to avoid race conditions in the test shell script.


  while test "$i" -lt 100000
    printf "%d\r" "$i"
    "$@" test2.sh
    i=$((i + 1))


  trap 'kill $!; exit 0' TERM
  { kill $$; exec sleep 1000; } &
  wait $!

To run:

  sh test1.sh $shell

For instance:

  sh test1.sh busybox ash

test1.sh will repeatedly run test2.sh and increment and print a counter variable to display progress.

test2.sh will immediately exit, in a complicated way, if all goes well. It may sleep for 1000s or fail to clean up its background process if something goes wrong.

On my system, I see:

  bash 5.0.11        - sleeps after a while
  bosh 2019-11-11    - sleeps after a while
  busybox 1.31.1 ash - ok
  dash      - ok
  dash (current)     - sleeps immediately
  fbsh 12.1          - ok *
  gwsh (current)     - leaves subprocesses
  ksh 93v            - sleeps after a while
  ksh 2020.0.0       - sleeps after a while
  mksh 57            - sleeps after a while
  nbsh (current)     - sleeps after a while *
  pdksh 5.2.14       - leaves subprocesses + sleeps after a while
  posh 0.13.1        - ok
  yash               - ok
  zsh                - sleeps after a while

* Because of the way I was running FreeBSD sh and NetBSD sh on qemu, I could not easily check what happens to the subprocesses.

I think that confirms that NetBSD sh does have a problem with a race condition, but that many shells have that same problem. It also tells me that there is another different problem in my shell that I should look at.

Harald van Dijk

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