appstream: support legacy ids without desktop suffix

If no launchables are found and component id does not have `.desktop`
suffix, use the component id + .desktop as possible desktop file id.

This is not required per the spec, but some applications such
as Foliate use this and other tools such as Flatpak support it.

LP: #1778546

snapcraft/extractors/appstream.py

@@ -87,8 +87,11 @@ def extract(relpath: str, *, workdir: str) -> ExtractedMetadata:
     desktop_file_ids = _get_desktop_file_ids_from_nodes(dom.findall("launchable"))
     # if there are no launchables, use the appstream id to take into
     # account the legacy appstream definitions
-    if common_id and common_id.endswith(".desktop") and not desktop_file_ids:
-        desktop_file_ids.append(common_id)
+    if common_id and not desktop_file_ids:
+        if common_id.endswith(".desktop"):
+            desktop_file_ids.append(common_id)
+        else:
+            desktop_file_ids.append(common_id + ".desktop")
 
     for desktop_file_id in desktop_file_ids:
         desktop_file_path = _desktop_file_id_to_path(desktop_file_id, workdir=workdir)

tests/unit/extractors/test_appstream.py

@@ -557,6 +557,26 @@ def test_appstream_with_launchable(self):
             extracted.get_desktop_file_paths(), Equals([self.desktop_file_path])
         )
 
+    def test_appstream_no_desktop_suffix(self):
+        with open("foo.metainfo.xml", "w") as f:
+            f.write(
+                textwrap.dedent(
+                    """\
+                <?xml version="1.0" encoding="UTF-8"?>
+                <component type="desktop">
+                  <id>com.example.test-app</id>
+                </component>"""
+                )
+            )
+
+        _create_desktop_file(self.desktop_file_path)
+
+        extracted = appstream.extract("foo.metainfo.xml", workdir=".")
+
+        self.assertThat(
+            extracted.get_desktop_file_paths(), Equals([self.desktop_file_path])
+        )
+
 
 class AppstreamMultipleLaunchableTestCase(unit.TestCase):
     def test_appstream_with_multiple_launchables(self):