From 9226d4e9ee551ff7db365937949711ccd96611b4 Mon Sep 17 00:00:00 2001
From: r-vdp <ramses@well-founded.dev>
Date: Tue, 3 Dec 2024 11:35:02 +0100
Subject: [PATCH] nixos/filesystems: don't silently ignore label when device is
 set

Before this change, when both device and label were set, the label would
be silently ignored. This is especially problematic when the device is
set in another module, and it's not immediately obvious why the label is
not having any effect.

Additionally, some other modules use the device option to get the device
of a filesystem, but this option is not populated when only the label is
set, causing those modules to conclude that the filesystem does not have
a backing device.

With this change, we populate the device option using the label when the
label is set. This means that we get a clear error message when both the
device and label are set, and that the device is properly populated
whenever the label is used to specify the backing device.
---
 nixos/modules/tasks/filesystems.nix | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/nixos/modules/tasks/filesystems.nix b/nixos/modules/tasks/filesystems.nix
index 5c95cd3d451e..b7d18dfe4173 100644
--- a/nixos/modules/tasks/filesystems.nix
+++ b/nixos/modules/tasks/filesystems.nix
@@ -4,6 +4,8 @@ with lib;
 with utils;
 
 let
+  # https://wiki.archlinux.org/index.php/fstab#Filepath_spaces
+  escape = string: builtins.replaceStrings [ " " "\t" ] [ "\\040" "\\011" ] string;
 
   addCheckDesc = desc: elemType: check: types.addCheck elemType check
     // { description = "${elemType.description} (with check: ${desc})"; };
@@ -136,6 +138,8 @@ let
 
     };
 
+    config.device = lib.mkIf (config.label != null) "/dev/disk/by-label/${escape config.label}";
+
     config.options = let
       inInitrd = utils.fsNeededForBoot config;
     in mkMerge [
@@ -196,11 +200,8 @@ let
       ];
       isBindMount = fs: builtins.elem "bind" fs.options;
       skipCheck = fs: fs.noCheck || fs.device == "none" || builtins.elem fs.fsType fsToSkipCheck || isBindMount fs;
-      # https://wiki.archlinux.org/index.php/fstab#Filepath_spaces
-      escape = string: builtins.replaceStrings [ " " "\t" ] [ "\\040" "\\011" ] string;
     in fstabFileSystems: { }: concatMapStrings (fs:
       (if fs.device != null then escape fs.device
-         else if fs.label != null then "/dev/disk/by-label/${escape fs.label}"
          else throw "No device specified for mount point ‘${fs.mountPoint}’.")
       + " " + escape fs.mountPoint
       + " " + fs.fsType