dpkg: conffile difference visualizer subprocess returned error exit status 127

This really looks like an environment issue:

Setting up smartmontools (7.1-1build1) ...

Configuration file '/etc/default/smartmontools'
 ==> Modified (by you or by a script) since installation.
 ==> Package distributor has shipped an updated version.
   What would you like to do about it ? Your options are:
    Y or I : install the package maintainer's version
    N or O : keep your currently-installed version
      D : show the differences between the versions
      Z : start a shell to examine the situation
 The default action is to keep your current version.
*** smartmontools (Y/I/N/O/D/Z) [default=N] ? d
sh: 1: pager: not found
diff: standard output: Broken pipe
dpkg: error processing package smartmontools (--configure):
 conffile difference visualizer subprocess returned error exit status 127

Due to missing /bin/pager:

(c)rafaeldtinoco@devel:~$ which pager
(c)rafaeldtinoco@devel:~$ ls -lah /bin/pager
lrwxrwxrwx 1 root root 23 Apr 24 21:39 /bin/pager -> /etc/alternatives/pager
(c)rafaeldtinoco@devel:~$ ls -lah /etc/alternatives/pager
lrwxrwxrwx 1 root root 13 Apr 24 21:39 /etc/alternatives/pager -> /usr/bin/less

Try checking your system for pager:

(c)rafaeldtinoco@devel:~$ sudo update-alternatives --query pager
Name: pager
Link: /usr/bin/pager
Slaves:
 pager.1.gz /usr/share/man/man1/pager.1.gz
Status: auto
Best: /usr/bin/less
Value: /usr/bin/less

Alternative: /bin/more
Priority: 50
Slaves:
 pager.1.gz /usr/share/man/man1/more.1.gz

Alternative: /usr/bin/less
Priority: 77
Slaves:
 pager.1.gz /usr/share/man/man1/less.1.gz

and fixing it!

Hum,

This might be something else... chances for 2 bugs:

https://bugs.launchpad.net/ubuntu/+source/smartmontools/+bug/1874953 (this)
https://bugs.launchpad.net/ubuntu/+source/smartmontools/+bug/1875010 (similar)

not to have pager is lower than just 1.

I'm marking the other bug as a duplicate of this.

Looks like "D" option is not working, two bugs with problems...

Need to double check.

probably related to:

less (551-1) sid; urgency=low
  * move binaries back to /usr/bin (closes: #500092)

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

Hi! I agree, this:

less (551-1) sid; urgency=low
  * move binaries back to /usr/bin (closes: #500092)

is almost certainly related. If during the upgrade process the new 'less' package is unpacked but not configured yet, there's a window where the pager alternative points to the wrong location.

I can't see an easy way to fix this. What the linked Debian bug suggests, that is: make dpkg fallback to more(1) if $PAGER is not available, is probably the right balance between making dpkg solid and not adding an overly complex logic for what basically is a one-off failure.

We must not change the location of binaries from /bin to /usr/bin or vice-versa, inside the deb.

It doesn't matter on freshly installed systems that have usr-merge, but breaks upgraded systems.

This must be reverted.

Thinking about it.

Can we add update-alternatives to preinst/upgrade, similar to how it is done in postinst/configure?

The minimal change is probably moving

update-alternatives --quiet --remove pager /bin/less

from postinst to preinst. The pager alternative will fallback to 'more' at least until the new alternative is set up. Strictly speaking it is probably the most correct thing to do, as 'pager' is never left wrongly configured.

However there are several packages calling `update-alternatives --install` from preinst [1]. That's probably OK too.

[1] https://codesearch.debian.net/search?q=path%3Adebian%2Fpreinst+update-alternatives&literal=1

This bug was fixed in the package less - 551-1ubuntu1

---------------
less (551-1ubuntu1) groovy; urgency=medium

  * On upgrade from packages that shipped less in both /bin and /usr/bin,
    call update-alternatives in preinst, such that after unpacking,
    /usr/bin/pager remains operational. This should resolve dpkg failing
    to invoke pager by dpkg. LP: #1874953

 -- Dimitri John Ledkov <email address hidden> Fri, 05 Jun 2020 16:18:51 +0100

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

I filed a less bug in Debian: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=962590

Hello Craig, or anyone else affected,

Accepted less into focal-proposed. The package will build now and be available at https://launchpad.net/ubuntu/+source/less/551-1ubuntu0.1 in a few hours, and then in the -proposed repository.

Please help us by testing this new package. See https://wiki.ubuntu.com/Testing/EnableProposed for documentation on how to enable and use -proposed. Your feedback will aid us getting this update out to other Ubuntu users.

If this package fixes the bug for you, please add a comment to this bug, mentioning the version of the package you tested, what testing has been performed on the package and change the tag from verification-needed-focal to verification-done-focal. If it does not fix the bug for you, please add a comment stating that, and change the tag to verification-failed-focal. In either case, without details of your testing we will not be able to proceed.

Further information regarding the verification process can be found at https://wiki.ubuntu.com/QATeam/PerformingSRUVerification . Thank you in advance for helping!

N.B. The updated package will be released to -updates after the bug(s) fixed by this package have been verified and the package has been in -proposed for a minimum of 7 days.

All autopkgtests for the newly accepted less (551-1ubuntu0.1) for focal have finished running.
The following regressions have been reported in tests triggered by the package:

mercurial/5.3.1-1ubuntu1 (s390x)

Please visit the excuses page listed below and investigate the failures, proceeding afterwards as per the StableReleaseUpdates policy regarding autopkgtest regressions [1].

https://people.canonical.com/~ubuntu-archive/proposed-migration/focal/update_excuses.html#less

[1] https://wiki.ubuntu.com/StableReleaseUpdates#Autopkgtest_Regressions

Thank you!

The version of less from focal-proposed resolved the issue and I'm now able to view the diff.

Configuration file '/etc/default/smartmontools'
 ==> Modified (by you or by a script) since installation.
 ==> Package distributor has shipped an updated version.
   What would you like to do about it ? Your options are:
    Y or I : install the package maintainer's version
    N or O : keep your currently-installed version
      D : show the differences between the versions
      Z : start a shell to examine the situation
 The default action is to keep your current version.
*** smartmontools (Y/I/N/O/D/Z) [default=N] ? d
--- /etc/default/smartmontools 2020-07-01 15:02:20.945403642 -0700
+++ /etc/default/smartmontools.dpkg-new 2019-10-09 17:32:47.000000000 -0700
@@ -4,9 +4,6 @@
 # List of devices you want to explicitly enable S.M.A.R.T. for
 # Not needed (and not recommended) if the device is monitored by smartd
 #enable_smart="/dev/hda /dev/hdb"
-enable_smart="/dev/vda"
-# uncomment to start smartd on system startup
-#start_smartd=yes

 # uncomment to pass additional options to smartd on startup
 #smartd_opts="--interval=1800"

Configuration file '/etc/default/smartmontools'
 ==> Modified (by you or by a script) since installation.
 ==> Package distributor has shipped an updated version.
   What would you like to do about it ? Your options are:
    Y or I : install the package maintainer's version
    N or O : keep your currently-installed version
      D : show the differences between the versions
      Z : start a shell to examine the situation
 The default action is to keep your current version.
*** smartmontools (Y/I/N/O/D/Z) [default=N] ? z
Useful environment variables:
 - DPKG_SHELL_REASON
 - DPKG_CONFFILE_OLD
 - DPKG_CONFFILE_NEW
Type 'exit' when you're done.
root@clean-xenial-amd64:/# apt-cache policy less
less:
  Installed: 551-1ubuntu0.1
  Candidate: 551-1ubuntu0.1
  Version table:
 *** 551-1ubuntu0.1 500
        500 http://ubuntu.osuosl.org/ubuntu focal-proposed/main amd64 Packages
        100 /var/lib/dpkg/status
     551-1 500
        500 http://ubuntu.osuosl.org/ubuntu focal/main amd64 Packages

All autopkgtests for the newly accepted less (551-1ubuntu0.1) for focal have finished running.
The following regressions have been reported in tests triggered by the package:

mercurial/5.3.1-1ubuntu1 (s390x)

Please visit the excuses page listed below and investigate the failures, proceeding afterwards as per the StableReleaseUpdates policy regarding autopkgtest regressions [1].

https://people.canonical.com/~ubuntu-archive/proposed-migration/focal/update_excuses.html#less

[1] https://wiki.ubuntu.com/StableReleaseUpdates#Autopkgtest_Regressions

Thank you!

The verification of the Stable Release Update for less has completed successfully and the package is now being released to -updates. Subsequently, the Ubuntu Stable Release Updates Team is being unsubscribed and will not receive messages about this bug report. In the event that you encounter a regression using the package from -updates please report a new bug using ubuntu-bug and tag the bug report regression-update so we can easily find any regressions.

This bug was fixed in the package less - 551-1ubuntu0.1

---------------
less (551-1ubuntu0.1) focal; urgency=medium

  * On upgrade from packages that shipped less in both /bin and /usr/bin,
    call update-alternatives in preinst, such that after unpacking,
    /usr/bin/pager remains operational. This should resolve dpkg failing
    to invoke pager by dpkg. LP: #1874953

 -- Brian Murray <email address hidden> Wed, 01 Jul 2020 16:25:13 -0700