build-fhs-userenv-bubblewrap: Preserve symlinks in /etc

[zhaofengli]

Description of changes

If the original file in /etc is a symlink, make it a symlink inside the sandbox as well. This PR is a more complete solution to #126234 (comment), replacing #145258.

If you bind-mount an individual file and the file is deleted then recreated, the mountpoint will still refer to the original file and cannot be further bind-mounted:

touch mp1 mp2
echo "original content" >file
sudo mount -o bind file mp1
cat mp1 # "original content"

# delete and recreate the file
rm file
echo "new content" >file
cat mp1 # "original content"

# the steam soldier runtime uses bwrap internally, causing another bind-mount
sudo mount -o bind mp1 mp2 # "mount(2) system call failed: No such file or directory."

Since both /etc/static and /run are bind-mounted already, we should just keep the symlinks inside the sandbox.

Tested in the same manner as in #126234 (comment). I can launch games after disconnecting and reconnecting to Wi-Fi with steam -steamdeck.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 22.11 Release Notes (or backporting 22.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
  • (Release notes changes) Ran nixos/doc/manual/md-to-db.sh to update generated release notes
• Fits CONTRIBUTING.md.

[queezle42]

This looks like a solid fix for #126234 and similar issues.

LGTM

[sg-qwt]

This patch has been sitting for a while. Do we know what's blocking this patch from merged?

[SuperSandro2000]

We can now drop https://github.com/NixOS/nixpkgs/blob/master/pkgs/applications/editors/vscode/generic.nix#L177-L179 right?

[sg-qwt]

I believe so. It'd also solve many other trick issues caused by missing symlink in sandbox (eg, wrong timezone in steam client)

[lheckemann]

I like this, some non-blocking nits though.

[hellwolf]

Back link a related fix in #286360