changed exit code for 'dc' in Ubuntu Bionic

[Wladimir Mutel]

in Ubuntu Xenial :
$ lsb_release -rdc ; echo 1 1 + p q | dc ; echo $?
Description: Ubuntu 16.04.4 LTS
Release: 16.04
Codename: xenial
2
0

in Ubuntu Bionic :
$ lsb_release -rdc ; echo 1 1 + p q | dc ; echo $?
Description: Ubuntu 18.04.1 LTS
Release: 18.04
Codename: bionic
2
1

This change causes some scripts using 'dc' to fail, most importantly, 'dphys-swapfile' which for some ironic reason uses 'dc' instead of 'echo $((...))' for its calculations.

[Wladimir Mutel (mwg)]

same is reproduced when you run 'dc', enter into it some expression ending with 'p q', and examining its exit code with 'echo $?' afterwards

[Axel Beckert (xtaran)]

Hi Wladimir,

Wladimir Mutel wrote:
> in Ubuntu Xenial :
> $ lsb_release -rdc ; echo 1 1 + p q | dc ; echo $?
> Description: Ubuntu 16.04.4 LTS
> Release: 16.04
> Codename: xenial
> 2
> 0
>
> in Ubuntu Bionic :
> $ lsb_release -rdc ; echo 1 1 + p q | dc ; echo $?
> Description: Ubuntu 18.04.1 LTS
> Release: 18.04
> Codename: bionic
> 2
> 1
>
> This change causes some scripts using 'dc' to fail, most importantly,
> 'dphys-swapfile'

Thanks for that bug report. This might be the cause for
https://bugs.debian.org/906767 which I'm currently trying to fix.

> which for some ironic reason uses 'dc' instead of 'echo
> $((...))' for its calculations.

That's upstream's decision. And since I know that upstream also runs
rather old Unices, I assume it's because $(( ... )) might not be
available in all shells. According to
https://en.wikipedia.org/wiki/Comparison_of_command_shells#cite_ref-57
it should though be available in all relevant shells since 1992.

The relevant change in dc seems to be from 1.3 to 1.4:

    Detects and reports on any output errors; exits non-success.

The fix seems simple: Drop the "q":

~ → echo 1 1 + p q | dc ; echo $?
2
1
~ → echo 1 1 + p | dc ; echo $?
2
0

Will have to check if that's indeed the cause for
https://bugs.debian.org/906767, but will do a new upload to Debian
Unstable in either case.

Someone else will need to take care of a regression fix for bionic
though.

  Regards, Axel
--
 ,''`. | Axel Beckert <email address hidden>, https://people.debian.org/~abe/
: :' : | Debian Developer, ftp.ch.debian.org Admin
`. `' | 4096R: 2517 B724 C5F6 CA99 5329 6E61 2FF9 CD59 6126 16B5
  `- | 1024D: F067 EA27 26B9 C3FC 1486 202E C09E 1D89 9593 0EDE

[Launchpad Janitor (janitor)]

This bug was fixed in the package dphys-swapfile - 20100506-5

---------------
dphys-swapfile (20100506-5) unstable; urgency=high

  * Add patch to fix the setup subcommand exiting prematurely with more
    recent dc versions. (Closes: #906767, LP: #1788681) Also update
    check-diskspace-first.patch which uses the same syntax. Thanks to
    eHenry Berg for the initial bug report, Davide Prina for additional
    debugging help, and Wladimir Mutel for finding the actual culprit.

 -- Axel Beckert <email address hidden> Thu, 23 Aug 2018 23:29:41 +0200

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Paul Graydon (twirrim)]

Fix has only been released for Cosmic. Given the package is basically non-functional on Bionic, is there a plan to backport the fix? There doesn't seem much point in keeping it in the repository otherwise.

[Wladimir Mutel (mwg)]

4 years passed, no movements detected in Bionic ...

[DanglingPointer (ferncasado)]

Holy necromancy! Is this still a bug? Is this affecting latest LTS 22.04?

[DanglingPointer (ferncasado)]

$ lsb_release -rdc ; echo 1 1 + p q | dc ; echo $?
Description: Ubuntu 22.04.1 LTS
Release: 22.04
Codename: jammy
2
1

[Wladimir Mutel (mwg)]

The bug is not in bc/dc (which documented their change of behavior).
It is in dphys-swapfile script which used to exit 'dc' by 'q' command.
The proper fix, in my view, would still be to use arithmetic substitution $((...)) provided by most modern shells, and make dphys-swapfile independent from 'dc' at all.