Add OverlayFs Multiple lower layers support

[augu5te]

Motivation for this change

OverlayFs Multiple lower layers is incorrectly addressed by the prefixing (add /mnt-root) step in stage-1-init.sh:

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

OverlayFs Multiple lower layers is incorrectly addressed by the prefixing (add /mnt-root) step in stage-1-init.sh:

Example:
optionsFiltered=lowerdir=/mnt1:/mnt2,upperdir=/mnt3,workdir=/mnt4

Actual prefix command (in nixos/modules/system/boot/stage-1-init.sh:):
echo "$optionsFiltered" | sed -E 's#\<(lowerdir|upperdir|workdir)=#\1=/mnt-root#g'

Obtained WRONG prefix:
lowerdir=/mnt-root/mnt1:/mnt2:/mnt3,upperdir=/mnt-root/mnt3,workdir=/mnt-root/mnt4

Proposed prefix command:
echo "$optionsFiltered" | sed -E 's#\<(lowerdir|upperdir|workdir)=#\1=/mnt-root#g; s#:#:/mnt-root#g'

Correct and EXPECTED prefix
lowerdir=/mnt-root/mnt1:/mnt-root/mnt2:/mnt-root/mnt3,upperdir=/mnt-root/mnt3,workdir=/mnt-root/mnt4

Added if [ "$fsType" = overlay ] to guard this prefixing step

@FRidh I'll apprecitate your opinion/review

[stale]

I marked this as stale due to inactivity. → More info

[augu5te]

This commit is really simple to review

[SuperSandro2000]

@cole-h @samueldr @flokli please take a look at this. I don't know enough about the boot stage to review this.

[cole-h]

I have no knowledge of overlayfs.

While it may be simple to review the contents of the change, it's not as simple to verify functionality. Do we have an overlayfs test in NixOS? If not, maybe it would be a good idea to write one? (Otherwise, a simple but explicit "I ran this and it works fine" would satisfy me, at least.)

[samueldr]

I'm missing a test, an example configuration or scenario to test this.

[augu5te]

One scenario is a cluster where the nodes are booting from the network with the base system being in ramdisk and an nfs shared nix store is added (this additional layer revealed this issue).

[RaitoBezarius]

Can a NixOS test be added to this so we can merge it?