pkgs-lib.formats.xml: init (#342633)

nixos/doc/manual/development/settings-options.section.md

···
     and returning a set with TOML-specific attributes `type` and
     `generate` as specified [below](#pkgs-formats-result).
 
+`pkgs.formats.xml` { format ? "badgerfish", withHeader ? true}
+
+:   A function taking an attribute set with values
+    and returning a set with XML-specific attributes `type` and
+    `generate` as specified [below](#pkgs-formats-result).
+
+    `format`
+
+    :   Input format. Because XML can not be translated one-to-one, we have to use intermediate formats. Possible values:
+      - `"badgerfish"`: Uses [badgerfish](http://www.sklar.com/badgerfish/) conversion.
+
+    `withHeader`
+
+    :   Outputs the xml with header.
+
 `pkgs.formats.cdn` { }
 
 :   A function taking an empty attribute set (for future extensibility)

pkgs/pkgs-lib/formats.nix

···
     '') {};
   };
 
+  xml =
+    {
+      format ? "badgerfish",
+      withHeader ? true,
+    }:
+    if format == "badgerfish" then
+      {
+        type = let
+          valueType = nullOr (oneOf [
+            bool
+            int
+            float
+            str
+            path
+            (attrsOf valueType)
+            (listOf valueType)
+          ]) // {
+            description = "XML value";
+          };
+        in valueType;
+
+        generate =
+          name: value:
+          pkgs.callPackage (
+            {
+              runCommand,
+              python3,
+              libxml2Python,
+            }:
+            runCommand name
+              {
+                nativeBuildInputs = [
+                  python3
+                  python3.pkgs.xmltodict
+                  libxml2Python
+                ];
+                value = builtins.toJSON value;
+                pythonGen = ''
+                  import json
+                  import os
+                  import xmltodict
+
+                  with open(os.environ["valuePath"], "r") as f:
+                      print(xmltodict.unparse(json.load(f), full_document=${toString withHeader}, pretty=True, indent=" " * 2))
+                '';
+                passAsFile = [
+                  "value"
+                  "pythonGen"
+                ];
+                preferLocalBuild = true;
+              }
+              ''
+                python3 "$pythonGenPath" > $out
+                xmllint $out > /dev/null
+              ''
+          ) { };
+      }
+    else
+      throw "pkgs.formats.xml: Unknown format: ${format}";
+
 }

pkgs/pkgs-lib/tests/formats.nix

···
     '';
   };
 
+  badgerfishToXmlGenerate = shouldPass {
+    format = formats.xml { };
+    input = {
+      root = {
+        "@id" = "123";
+        "@class" = "example";
+        child1 = {
+          "@name" = "child1Name";
+          "#text" = "text node";
+        };
+        child2 = {
+          grandchild = "This is a grandchild text node.";
+        };
+        nulltest = null;
+      };
+    };
+    expected = ''
+      <?xml version="1.0" encoding="utf-8"?>
+      <root class="example" id="123">
+        <child1 name="child1Name">text node</child1>
+        <child2>
+          <grandchild>This is a grandchild text node.</grandchild>
+        </child2>
+        <nulltest></nulltest>
+      </root>
+    '';
+  };
 }