support for include filters

- Added a new option for `file` sources, which allows to specify a list
  of `include` and `exclude` filters
- Added a new option for `file` sources, which allows to disable the
  `--delete-excluded` behaviour.

README.md

@@ -157,6 +157,17 @@ Supported attributes:
   manual](https://download.samba.org/pub/rsync/rsync.html) for further
   information.
 
+* `filters` (optional)
+  List of filters that should be passed to rsync. Filters are specified as
+  attribute sets with the attributes `type` and `pattern`. Supported filter
+  types are `include` and `exclude`. This allows for more advanced
+  configurations.
+
+* `deleteExcluded` (optional)
+  boolean that controls whether the excluded directories should be deleted
+  if they exist on the target. This is passed to the `--delete-excluded` option
+  of rsync. Defaults to `true`.
+
 
 ### `git`
 

lib/types/populate.nix

@@ -54,6 +54,18 @@
     };
   });
 
+  filter = lib.types.submodule {
+    options = {
+      type = lib.mkOption {
+        type = lib.types.enum ["include" "exclude"];
+        default = "exclude";
+      };
+      pattern = lib.mkOption {
+        type = lib.types.str;
+      };
+    };
+  };
+
   source-types = {
     derivation = lib.types.submodule {
       options = {
@@ -76,6 +88,28 @@
           default = [];
           example = [".git"];
         };
+        filters = lib.mkOption {
+          type = lib.types.listOf filter;
+          default = [];
+          example = [
+            {
+              type = "include";
+              pattern = "*.nix";
+            }
+            {
+              type = "include";
+              pattern = "*/";
+            }
+            {
+              type = "exclude";
+              pattern = "*";
+            }
+          ];
+        };
+        deleteExcluded = lib.mkOption {
+          default = true;
+          type = lib.types.bool;
+        };
       };
     };
     git = lib.types.submodule {

pkgs/populate/default.nix

@@ -45,7 +45,7 @@ let
   '';
 
   pop.file = target: source: let
-    configAttrs = ["useChecksum" "exclude"];
+    configAttrs = ["useChecksum" "exclude" "filters" "deleteExcluded"];
     config = filterAttrs (name: _: elem name configAttrs) source;
   in
     rsync' target config (quote source.path);
@@ -161,9 +161,12 @@ let
         ${concatMapStringsSep " "
           (pattern: /* sh */ "--exclude ${quote pattern}")
           (config.exclude or [])} \
+        ${concatMapStringsSep " "
+          (filter: /* sh */ "--${filter.type} ${quote filter.pattern}")
+          (config.filters or [])} \
         -e ${quote (ssh' target)} \
         -vFrlptD \
-        --delete-excluded \
+        ${optionalString (config.deleteExcluded or true) /* sh */ "--delete-excluded"} \
         "$source_path" \
         ${quote (
           optionalString (!isLocalTarget target) (