fix: allow desktop files without an Exec key

snapcraft/parts/desktop_file.py

@@ -77,10 +77,12 @@ def _parse_and_reformat_section_exec(self, section: str):
     def _parse_and_reformat_section(
         self, *, section: str, icon_path: str | None = None
     ):
-        if "Exec" not in self._parser[section]:
-            raise errors.DesktopFileError(self._filename, "missing 'Exec' key")
-
-        self._parse_and_reformat_section_exec(section)
+        # Exec is not required: desktop entries that are D-Bus- or
+        # systemd-activated (often with NoDisplay=true) are valid without it,
+        # per the Desktop Entry specification and desktop-file-validate. Only
+        # reformat the Exec line when it is present.
+        if "Exec" in self._parser[section]:
+            self._parse_and_reformat_section_exec(section)
 
         if "Icon" in self._parser[section]:
             icon = self._parser[section]["Icon"]

tests/unit/parts/test_desktop_file.py

@@ -207,16 +207,34 @@ def test_no_desktop_section(new_dir):
 
 
 def test_missing_exec_entry(new_dir):
+    """A desktop entry without an Exec key is written through unchanged.
+
+    Exec is not required for D-Bus- or systemd-activated entries (regression
+    test for https://github.com/canonical/snapcraft/issues/5799).
+    """
     with open("foo.desktop", "w") as desktop_file:
         print("[Desktop Entry]", file=desktop_file)
-        print("Icon=foo", file=desktop_file)
+        print("Type=Application", file=desktop_file)
+        print("Name=Foo", file=desktop_file)
+        print("NoDisplay=true", file=desktop_file)
 
     d = DesktopFile(
         snap_name="foo",
         app_name="foo",
         filename="foo.desktop",
         prime_dir=new_dir,
     )
+    d.write(gui_dir=new_dir)
 
-    with pytest.raises(errors.DesktopFileError):
-        d.write(gui_dir=new_dir)
+    written = new_dir / "foo.desktop"
+    assert written.exists()
+    with written.open() as desktop_file:
+        assert desktop_file.read() == dedent(
+            """\
+            [Desktop Entry]
+            Type=Application
+            Name=Foo
+            NoDisplay=true
+
+            """
+        )