Pager breaks terminal

[stierhofj]

Describe the bug
Latest version 9.4.2, after upgrading from 9.1.2, module avail causes the terminal to break. The issue is not clear but it seems that the pipe opening less causes this.

The symptom is that aver ml avail the list of modules is show, but after ~1 second delay the paging with less gives double entries on the page. It looks like every line is displayed twice.

Exiting with less now breaks the shell display completely, that is, no input

Interestingly, if one closes less quickly enough (before the ~1 sec) the terminal does stay intact.

To Reproduce
I am not 100%. We had lmod version 9.1.2 installed and I upgraded. The resulting behavior is as described. No other changes were done (except that I set LMOD_PAGER to empty to prevent this issue).

Expected behavior
Normal pager output, no broken shell afterwards

Desktop (please complete the following information):

• OS: Linux
• Linux distribution: Ubuntu 24
• Lmod Version: 9.4.2
• For Lmod Versions 8.7.7+, please run "module --miniConfig" and
  include the output here.

Modules based on Lua: Version 9.4.2 2026-09-09 19:38 +02:00
    https://lmod.readthedocs.io

Lmod Version: 9.4.2
Lua Version:  5.4
MODULEPATH:   /data/software/.modulefiles/Ubuntu-24.04/Misc:/data/software/.modulefiles/Ubuntu-24.04/Util:/data/software/.modulefiles/Ubuntu-24.04/Science:/data/software/.modulefiles/Ubuntu-24.04/Computing:/data/software/.modulefiles/Ubuntu-24.04/Programming:/data/software/.modulefiles/Ubuntu-24.04/Development:/data/software/.modulefiles/Ubuntu-24.04/Core:/home/stierhof/.modules:/data/software/.modulefiles/Core:/data/software/.modulefiles/Guix:/data/software/.modulefiles/Science:/data/software/.modulefiles/Development:/data/software/.modulefiles/Programming:/data/software/.modulefiles/Util:/data/software/.modulefiles/Extensions/Ubuntu-24.04/Science/heasoft/6.37:/data/software/.modulefiles/Extensions/Ubuntu-24.04/Programming/slang/2.3
OS Name:      Ubuntu 24.04.3 LTS



Changes from Default Configuration
----------------------------------

Name                         Where Set  Default                                                 Value
----                         ---------  -------                                                 -----
LFS_VERSION                  D          1.6.3                                                   1.9.0
LMOD_ALLOW_TCL_MFILES        C          yes                                                     no
LMOD_AVAIL_STYLE             E          <system>                                                remeis
LMOD_CACHED_LOADS            Other      no                                                      yes
LMOD_FAST_TCL_INTERP         C          yes                                                     no
LMOD_MODULEPATH_INIT         C          /data/software/Core/lmod/lmod/init/.modulespath         /data/software/.modulefiles/.modulepaths
LMOD_PACKAGE_PATH            D          nil                                                     /data/software/.modulefiles
LMOD_PAGER                   E          less                                                    no
LMOD_SITEPACKAGE_LOCATION    Other      /data/software/Core/lmod/9.4.2/libexec/SitePackage.lua  /data/software/.modulefiles/SitePackage.lua
LMOD_SYSTEM_DEFAULT_MODULES  E          __unknown__                                             .system:stierhof
LMOD_TCLSH                   C          tclsh                                                   /usr/bin/tclsh
LMOD_USING_FAST_TCL_INTERP   Other      yes                                                     no
MODULEPATH_ROOT              E                                                                  /data/software/.modulefiles/
PATH_TO_LUA                  C          lua                                                     /data/software/Core/lua/5.4.4/bin/lua


Where Set -> D: default, E: environment, C: configuration
             lmod_cfg: lmod_config.lua SitePkg: SitePackage StdPkg: StandardPackage
             S1, S2 ... from LMOD_SITEPACKAGE_PREPEND
             Other: Set somewhere outside of normal locations

Additional context
Please excuse the somewhat vague description. If there is any additional information I should provide, let me know.

[rtmclay]

I am unable to reproduce this issue. I have a ubuntu 24.04.4 running and it works fine. I am wondering what version of less you have running. I'm running less 590. Here are my pager options:

% module --version                    

Modules based on Lua: Version 9.4.2 (9.4.2-1-g217cdbe02) 2026-09-13 07:22 -06:00
    https://lmod.readthedocs.io

% module --config 2>&1 | grep -i pager

Pager (LMOD_PAGER)                                         /bin/less
Pager Options (LMOD_PAGER_OPTS)                            -XqMREF

Also I wonder if you have the same issue with paging anything. For example how does the following work:

% module show settarg

[stierhofj]

I run the same less version (590).

LMOD_PAGER='/bin/less' LMOD_PAGER_OPTS='-XqMREF' module show settarg

does work as expected.

LMOD_PAGER='/bin/less' LMOD_PAGER_OPTS='-XqMREF' module avail

does not.

This got me thinking that the avail_hook is somehow messing this up. But turning it off had no effect. For completeness

-- We change appearence a bit such that the modulefiles represent
-- the "real" software structure
local SOFTWARE = os.getenv('SOFTWARE')
function avail_hook (t)
  local target = pathJoin(SOFTWARE, '.modulefiles')
  local replace = SOFTWARE
  for k,v in pairs(t) do
    if (k:find("^" .. target)) then
      t[k] = t[k]:gsub(target, replace)
    end
  end
end

hook.register("avail", avail_hook)

Is there anything else that hooks into avail?

[mrcawood]

@stierhofj Thanks for the follow-up. module show working with the same less and the same LMOD_PAGER_OPTS pointed away from a general pager setup problem. module avail is the command that walks the module tree.

During that walk Lmod replaces os.exit, so one modulefile cannot abort the scan. If a modulefile calls LmodWarning while that is in effect, Lmod also asks for the terminal width. That probe forks a child to run stty or tput, and the child was calling os.exit(0). Because os.exit is disabled for the scan, the child did not stop. It kept running, and so did a second child started for tput. All three then printed module avail. With less in front, that shows up as a doubled listing and a broken terminal after you quit. The probe waits up to about a second, which matches the delay you saw. Quitting less before that second is up restores the terminal first. module show never enters this walk, so it stayed fine. Setting LMOD_PAGER empty only avoids the pager. It does not stop the extra copies.

The fix is on branch IS859-avail-termwidth (commit ff3cdd076). The width-probe child now stops with _exit, which does not go through the os.exit hook. Modulefiles still cannot abort a spider scan. With that change, an avail listing that was printed three times is printed once, including a second module avail after the cache has been built.

Could you try that branch with the pager turned back on?

git fetch https://github.com/TACC/Lmod.git IS859-avail-termwidth
git checkout IS859-avail-termwidth
# build and install as you usually do, then:
unset LMOD_TERM_WIDTH
export LMOD_PAGER=/bin/less
export LMOD_PAGER_OPTS=-XqMREF
module avail

Quit less normally and check that the shell still echoes. On 9.4.2, export LMOD_TERM_WIDTH=$(tput cols) is a workaround that skips the probe. It would be useful to know whether that also stops the break for you on 9.4.2, and whether any module prints a warning during module avail.