nix-env/query: don't use RAII for closing xml tag

I could instead add the right braces or manually ensure the
XMLOpenElement destructor is run at the right time. Or I could just
write what I actually want to happen.

Fixes #1137.

Change-Id: Ie1f157de1632571fb0c21e26d8e3cae56a6a6964

lix/legacy/nix-env.cc

@@ -1137,7 +1137,7 @@ static void opQuery(Globals & globals, Strings opFlags, Strings opArgs)
         Table table;
         std::ostringstream xmlStream;
         XMLWriter xml(true, xmlStream);
-        XMLOpenElement xmlRoot(xml, "items");
+        xml.openElement("items");
 
         for (auto & i : elems) {
             try {
@@ -1366,6 +1366,9 @@ static void opQuery(Globals & globals, Strings opFlags, Strings opArgs)
             }
         }
 
+        // </items>
+        xml.closeElement();
+
         if (!xmlOutput) {
             pager << formatTable(table);
         } else {

tests/functional2/commands/test_env_query_xml.py

@@ -0,0 +1,22 @@
+from xml.etree import ElementTree
+
+from testlib.fixtures.file_helper import with_files
+from testlib.fixtures.nix import Nix
+from testlib.utils import get_global_asset_pack
+
+
+@with_files(get_global_asset_pack("dependencies"))
+def test_correct_xml(nix: Nix):
+    """Test that nix-env --query --xml produces valid XML."""
+    query_output = (
+        nix.nix_env(
+            ["--query", "--available", "--attr-path", "--xml", "--file", "./dependencies.nix"]
+        )
+        .run()
+        .ok()
+        .stdout_plain
+    )
+
+    # Should parse without raising.
+    parsed = ElementTree.fromstring(query_output)
+    assert parsed.tag == "items"