Add option to disable toplevel symlink in microvm

This removes the last reference to uncompressed store content when using erofs.

Author: SuperSandro2000

doc/src/conventions.md

@@ -14,7 +14,7 @@ MicroVM deployments using the information on this page.
 | `microvm.devices.*.path`     | `share/microvm/pci-devices`            | `[email protected]`      | PCI devices that must be bound to the **vfio-pci** driver on the host                         |
 | `microvm.shares.*.source`    | `share/microvm/virtiofs/${tag}/source` | `[email protected]`        | Source directory of a **virtiofs** instance by tag                                            |
 | `microvm.shares.*.socket`    | `share/microvm/virtiofs/${tag}/socket` | `[email protected]`        | **virtiofsd** socket path by tag                                                              |
-|                              | `share/microvm/system`                 |                                     | `config.system.build.toplevel` symlink, used for comparing versions when running `microvm -l` |
+| `microvm.systemSymlink`      | `share/microvm/system`                 |                                     | `config.system.build.toplevel` symlink, used for comparing versions when running `microvm -l` |
 
 
 ## Generating custom operating system hypervisor packages

lib/runner.nix

@@ -72,7 +72,9 @@ pkgs.buildPackages.runCommand "microvm-${microvmConfig.hypervisor}-${hostName}"
   '') (builtins.attrNames binScriptPkgs)}
 
   mkdir -p $out/share/microvm
+  ${lib.optionalString microvmConfig.systemSymlink ''
   ln -s ${toplevel} $out/share/microvm/system
+  ''}
 
   echo vnet_hdr > $out/share/microvm/tap-flags
   ${lib.optionalString tapMultiQueue ''

nixos-modules/microvm/options.nix

@@ -602,6 +602,15 @@ in
       description = "Flags to pass to gensquashfs";
       default = [ "-c" "zstd" "-j" "$NIX_BUILD_CORES" ];
     };
+
+    systemSymlink = mkOption {
+      type = types.bool;
+      default = !config.microvm.storeOnDisk;
+      description = ''
+        Whether to inclcude a symlink of `config.system.build.toplevel` to `share/microvm/system`.
+        This is required for commands like `microvm -l` to function but removes reference to the uncompressed store content when using a disk image for the nix store.
+      '';
+    };
   };
 
   config = lib.mkMerge [ {