system: add patches module

modules/module-list.nix

@@ -23,6 +23,7 @@
   ./system/etc.nix
   ./system/keyboard.nix
   ./system/launchd.nix
+  ./system/patches.nix
   ./system/shells.nix
   ./system/version.nix
   ./time

modules/system/activation-scripts.nix

@@ -57,6 +57,7 @@ in
       ${cfg.activationScripts.users.text}
       ${cfg.activationScripts.nix.text}
       ${cfg.activationScripts.applications.text}
+      ${cfg.activationScripts.patches.text}
       ${cfg.activationScripts.etc.text}
       ${cfg.activationScripts.defaults.text}
       ${cfg.activationScripts.launchd.text}

modules/system/default.nix

@@ -86,6 +86,7 @@ in
         mkdir -p $out/darwin
         cp -f ${../../CHANGELOG} $out/darwin-changes
 
+        ln -s ${cfg.build.patches}/patches $out/patches
         ln -s ${cfg.build.etc}/etc $out/etc
         ln -s ${cfg.path} $out/sw
 

modules/system/patches.nix

@@ -0,0 +1,64 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+  cfg = config.system;
+in
+
+{
+  options = {
+
+    system.patches = mkOption {
+      type = types.listOf types.path;
+      default = [];
+      example = literalExample ''
+        [
+          (pkgs.writeText "bashrc.patch" ''''
+            --- a/etc/bashrc
+            +++ b/etc/bashrc
+            @@ -8,3 +8,5 @@
+             shopt -s checkwinsize
+
+             [ -r "/etc/bashrc_$TERM_PROGRAM" ] && . "/etc/bashrc_$TERM_PROGRAM"
+            +
+            +if test -e /etc/static/bashrc; then . /etc/static/bashrc; fi
+          '''')
+        ]
+      '';
+      description = ''
+        Set of patches to apply to <filename>/</filename>.
+
+        Useful for safely changing system files.  Unlike the etc module this
+        won't remove or modify files with unexpected content.
+      '';
+    };
+
+  };
+
+  config = {
+
+    system.build.patches = pkgs.runCommandNoCC "patches"
+      { preferLocalBuild = true; }
+      ''
+        mkdir -p $out/patches
+        cd $out/patches
+        ${concatMapStringsSep "\n" (f: "ln -s '${f}' $(basename '${f}')") cfg.patches}
+      '';
+
+    system.activationScripts.patches.text = ''
+      # Applying patches to /.
+      echo "applying patches..." >&2
+
+      f=$(readlink /run/current-system/patches)
+      if ! test "$f" = ${config.system.build.patches}/patches; then
+          for f in $(find /run/current-system/patches/ -type l); do
+              patch --reverse --backup -d / -p1 < "$f" >/dev/null || true
+          done
+
+          ${concatMapStringsSep "\n" (f: "patch --forward --backup -d / -p1 < '${f}' || true") cfg.patches}
+      fi
+    '';
+
+  };
+}