Bug#1144635: zsh: broken by the FPATH environment variable defined by bash / lmod

Package: zsh
Version: 5.9-8+b23
Severity: serious
Tags: upstream

On a machine that has just been upgraded to trixie, I start zsh
via a bash login shell (run by sshd). The issue is that when it
is a login shell, bash defines the $FPATH environement variable
to '/usr/share/lmod/lmod/init/ksh_funcs' as shown by

  echo "FPATH = '$FPATH'" >&2

at the beginning of the ~/.bash_profile file, which outputs:

FPATH = '/usr/share/lmod/lmod/init/ksh_funcs'

This is possibly due to /usr/share/lmod/8.7.60/init/bash from the
lmod package, as mentioned at

  https://github.com/direnv/direnv/issues/858

(note that I'm not a direnv user... this was found by Google + AI).

Such a setting completely breaks zsh, which outputs

zsh:1: url-quote-magic: function definition file not found

each time I type a key (the key has no other effect, i.e. the
command line at the prompt remains empty, so I need to type Ctrl-D
to quit zsh).

It appears that instead of ignoring the $FPATH environment variable
on startup, zsh sets $fpath to $FPATH.

Note that the zshparam(1) man page says: "Some environment variables
are not imported for reasons of security or because they would
interfere with the correct operation of other shell features.",
which should be the case here.

-- Package-specific info:
Packages which depend, recommend, suggest or enhance a zsh package and hence may provide code meant to be sourced in .zshrc:

Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name           Version       Architecture Description
+++-==============-=============-============-===============================================================
ii  parallel       20240222+ds-2 all          build and execute command lines from standard input in parallel

Packages which provide vendor completions:

Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name                Version          Architecture Description
+++-===================-================-============-=======================================================================
ii  borgbackup          1.4.0-5          amd64        deduplicating and compressing backup program
ii  bubblewrap          0.11.0-2+deb13u1 amd64        utility for unprivileged chroot and namespace manipulation
ii  curl                8.14.1-2+deb13u4 amd64        command line tool for transferring data with URL syntax
ii  dpkg-dev            1.22.22          all          Debian package development tools
ii  git-annex           10.20250416-2    amd64        manage files with git, without checking their contents into git
ii  jupyter-core        5.7.2-5          all          Core common functionality of Jupyter projects (tools)
ii  legit               1.2.0.post0-5    all          Git extension to assist in manipulating branches
ii  mercurial-common    7.0.1-2          all          easy-to-use, scalable distributed version control system (common files)
ii  ninja-build         1.12.1-1         amd64        small build system closest in spirit to Make
ii  pass                1.7.4-7          all          lightweight directory-based password manager
ii  python3-argcomplete 3.6.2-1          all          bash/zsh tab completion for argparse (for Python 3)
ii  sqv                 1.3.0-3+b2       amd64        OpenPGP signature verification program from Sequoia
ii  systemd             257.13-1~deb13u1 amd64        system and service manager
ii  udev                257.13-1~deb13u1 amd64        /dev/ and hotplug management daemon
ii  wl-clipboard        2.2.1-2          amd64        command line interface to the wayland clipboard
ii  zathura             0.5.11-1         amd64        document viewer with a minimalistic interface

dpkg-query: no path found matching pattern /usr/share/zsh/vendor-functions/


-- System Information:
Debian Release: 13.6
  APT prefers stable-updates
  APT policy: (500, 'stable-updates'), (500, 'stable-security'), (500, 'stable')
Architecture: amd64 (x86_64)

Kernel: Linux 6.12.96+deb13-amd64 (SMP w/80 CPU threads; PREEMPT)
Locale: LANG=POSIX, LC_CTYPE=C.UTF-8 (charmap=UTF-8), LANGUAGE not set
Shell: /bin/sh linked to /usr/bin/dash
Init: systemd (via /run/systemd/system)
LSM: AppArmor: enabled

Versions of packages zsh depends on:
ii  debianutils  5.23.2
ii  libc6        2.41-12+deb13u3
ii  libcap2      1:2.75-10+deb13u1+b1
ii  libtinfo6    6.5+20250216-2
ii  zsh-common   5.9-8

Versions of packages zsh recommends:
ii  libgdbm6t64   1.24-2
ii  libncursesw6  6.5+20250216-2
ii  libpcre2-8-0  10.46-1~deb13u1

Versions of packages zsh suggests:
ii  zsh-doc  5.9-8

-- no debconf information