Merge release-21.05 into staging-next-21.05

nixos/modules/installer/cd-dvd/iso-image.nix

@@ -644,7 +644,11 @@ in
       ];
 
     fileSystems."/" =
-      { fsType = "tmpfs";
+      # This module is often over-layed onto an existing host config
+      # that defines `/`. We use mkOverride 60 to override standard
+      # values, but at the same time leave room for mkForce values
+      # targeted at the image build.
+      { fsType = mkOverride 60 "tmpfs";
         options = [ "mode=0755" ];
       };
 

nixos/modules/installer/netboot/netboot.nix

@@ -30,7 +30,11 @@ with lib;
           else [ pkgs.grub2 pkgs.syslinux ]);
 
     fileSystems."/" =
-      { fsType = "tmpfs";
+      # This module is often over-layed onto an existing host config
+      # that defines `/`. We use mkOverride 60 to override standard
+      # values, but at the same time leave room for mkForce values
+      # targeted at the image build.
+      { fsType = mkOverride 60 "tmpfs";
         options = [ "mode=0755" ];
       };
 

nixos/modules/services/misc/nix-daemon.nix

@@ -458,7 +458,7 @@ in
                 description = "The flake reference to which <option>from></option> is to be rewritten.";
               };
               flake = mkOption {
-                type = types.unspecified;
+                type = types.nullOr types.attrs;
                 default = null;
                 example = literalExample "nixpkgs";
                 description = ''