Add optional label to volumes

This makes volumes trivial to identify on the guest by using
`/dev/disks/by-label`.

Author: ereslibre

checks/default.nix

@@ -62,6 +62,7 @@ let
         microvm.writableStoreOverlay = "/nix/.rw-store";
         microvm.volumes = [ {
           image = "nix-store-overlay.img";
+          label = "nix-store";
           mountPoint = config.microvm.writableStoreOverlay;
           size = 128;
         } ];

checks/startup-shutdown.nix

@@ -15,6 +15,7 @@ let
         };
         microvm = {
           volumes = [ {
+            label = "var";
             mountPoint = "/var";
             image = "var.img";
             size = 32;

lib/default.nix

@@ -30,21 +30,35 @@ rec {
 
   createVolumesScript = pkgs: pkgs.lib.concatMapStringsSep "\n" (
     { image
+    , label
     , size ? throw "Specify a size for volume ${image} or use autoCreate = false"
     , fsType ? defaultFsType
     , autoCreate ? true
     , ...
-    }: nixpkgs-lib.optionalString autoCreate ''
-      PATH=$PATH:${with pkgs; lib.makeBinPath [ coreutils util-linux e2fsprogs ]}
+    }: pkgs.lib.warnIf
+      (label != null && !autoCreate) "Volume is not automatically labeled unless autoCreate is true. Volume has to be labeled manually, otherwise it will not be identified"
+      (let labelOption =
+             if autoCreate then
+               (if builtins.elem fsType ["ext2" "ext3" "ext4" "xfs"] then "-L"
+                else if fsType == "vfat" then "-n"
+                else (pkgs.lib.warnIf (label != null)
+                  "Will not label volume ${label} with filesystem type ${fsType}. Open an issue on the microvm.nix project to request a fix."
+                  null))
+             else null;
+           labelArgument =
+             if (labelOption != null && label != null) then "${labelOption} '${label}'"
+             else "";
+      in (nixpkgs-lib.optionalString autoCreate ''
+      PATH=$PATH:${with pkgs.buildPackages; lib.makeBinPath [ coreutils util-linux e2fsprogs ]}
 
       if [ ! -e '${image}' ]; then
         touch '${image}'
         # Mark NOCOW
         chattr +C '${image}' || true
         fallocate -l${toString size}MiB '${image}'
-        mkfs.${fsType} '${image}'
+        mkfs.${fsType} ${pkgs.lib.optionalString (label != null) "-L '${label}'"} '${image}'
       fi
-    '');
+    ''));
 
   buildRunner = import ./runner.nix;
 

nixos-modules/microvm/mounts.nix

@@ -94,11 +94,16 @@ lib.mkIf config.microvm.guest.enable {
     };
   } (
     # Volumes
-    builtins.foldl' (result: { mountPoint, letter, fsType ? defaultFsType, ... }:
+    builtins.foldl' (result: { label, mountPoint, letter, fsType ? defaultFsType, ... }:
       result // lib.optionalAttrs (mountPoint != null) {
         "${mountPoint}" = {
           inherit fsType;
-          device = "/dev/vd${letter}";
+          # Prioritize identifying a device by label if provided. This
+          # minimizes the risk of misidentifying a device.
+          device = if label != null then
+            "/dev/disk/by-label/${label}"
+          else
+            "/dev/vd${letter}";
         } // lib.optionalAttrs (mountPoint == config.microvm.writableStoreOverlay) {
           neededForBoot = true;
         };

nixos-modules/microvm/options.nix

@@ -180,6 +180,11 @@ in
             type = str;
             description = "Path to disk image on the host";
           };
+          label = mkOption {
+            type = nullOr str;
+            default = null;
+            description = "Label of the volume, if any. Only applicable if autoCreate is true; otherwise labeling of the volume must be done manually";
+          };
           mountPoint = mkOption {
             type = nullOr path;
             description = "If and where to mount the volume inside the container";