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

[Martijn Dekker]

Op 07-02-20 om 12:19 schreef Walter Harms:

IMHO is the bug on bash side. ash can assume to get an "healthy"
environment from the caller. You can simply not fix everything that
can possible go wrong.


That is a rather fallacious argument. Of course you cannot fix 
*everything* that could possibly go wrong. You can certainly fix *this* 
thing, though. I know, because every non-Almquist shell does it.


These days, no program can realistically assume a "healthy" environment. 
Computers have become unimaginably complex machines, built on thousands 
of interdependent abstraction layers, each as fallible as the humans 
that designed and implemented them. So "unhealthy" environments happen 
all the time, due to all sorts of unforeseen causes.


It's well past time to accept that the 1980s are behind us. In 2020, 
systems have to be programmed robustly and defensively.



Obviously it should not segfault but so far i understand it is bsd as
that does, not busybox ash.


True. But instead, it simply gets stuck forever, with no message or 
other indicator of what went wrong. How is that better?


(Going slightly off-topic below...)

Segfaulting is actually a good thing: it's one form of failing reliably. 
And failing reliably is vastly better than what often happens instead, 
especially in shell scripts: subtle breakage, which can take a lot of 
detective work to trace, and in some cases can cause serious damage due 
to the program functioning inconsistently and incorrectly (instead of 
not at all).


Failing reliably is something the shell is ATROCIOUSLY bad at, and it's 
one of the first things modernish aims to fix.


- Martijn

--
modernish -- harness the shell
https://github.com/modernish/modernish

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

[Walter Harms]

IMHO is the bug on bash side. ash can assume to get an "healthy" environment 
from the caller. You can simply not fix everything that can possible go wrong.

Obviously it should not segfault but so far i understand it is bsd as that 
does, not busybox ash.

re,
 wh

Von: busybox  im Auftrag von Harald van Dijk 

Gesendet: Donnerstag, 6. Februar 2020 20:29
An: Martijn Dekker; DASH shell mailing list; busybox; Bug reports for the GNU 
Bourne Again SHell; Robert Elz; Jilles Tjoelker
Betreff: Re: Bizarre interaction bug involving bash w/ lastpipe + Almquist 
'wait'

On 06/02/2020 16:12, Martijn Dekker wrote:
> This is probably the strangest bug (or maybe pair of bugs) I've run into
> in nearly five years of breaking shells by developing modernish.
>
> I've traced it to an interaction between bash >= 4.2 (i.e.: bash with
> shopt -s lastpipe) and variants of the Almquist shell, at least: dash,
> gwsh, Busybox ash, FreeBSD sh, and NetBSD 9.0rc2 sh.
>
> Symptom: if 'return' is invoked on bash in the last element of a pipe
> executed in the main shell environment, then if you subsequently 'exec'
> an Almquist shell variant so that it has the same PID, its 'wait'
> builtin breaks.
>
> I can consistently reproduce this on Linux, macOS, FreeBSD, NetBSD
> 9.0rc2, OpenBSD, and Solaris.
>
> To reproduce this, you need bash >= 4.2, some Almquist shell variant,
> and these two test scripts:
>
> ---begin test.bash---
> fn() {
>  : | return
> }
> shopt -s lastpipe || exit
> fn
> exec "${1:-dash}" test.ash
> ---end test.bash---
>
> ---begin test.ash---
> echo '*ash-begin'
> : &
> echo '*ash-middle'
> wait "$!"
> echo '*ash-end'
> ---end test.ash---
>
> When executing test.bash with dash, gwsh, Busybox ash, or FreeBSD sh,
> then test.ash simply waits forever on executing 'wait "$!"'.

Nice test. bash leaves the process in a state where SIGCHLD is blocked,
and the various ash-based shells do not unblock it. Because of that,
they do not pick up on the fact that the child process has terminated. I
would consider this a bug both in bash and in the ash-based shells.

Cheers,
Harald van Dijk
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