manifest_xml: - Added doc and testing of unknown tags/elements.

Added this test to verify that older versions of repo can handle
"<superproject" element. Tested by adding "<iankaz" unknown element.

Tested the code with the following commands.

$ ./run_tests tests/test_manifest_xml.py
$ ./run_tests -v

Bug: https://crbug.com/gerrit/13709
Tested-by: Raman Tenneti <rtenneti@google.com>
Change-Id: I858d56f38cefcfcd14474efdd631a5a940c3ce47
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/293482
Reviewed-by: Mike Frysinger <vapier@google.com>

docs/manifest-format.md

@@ -109,6 +109,12 @@ following DTD:
 ]>
 ```
 
+For compatibility purposes across repo releases, all unknown elements are
+silently ignored.  However, repo reserves all possible names for itself for
+future use.  If you want to use custom elements, the `x-*` namespace is
+reserved for that purpose, and repo guarantees to never allocate any
+corresponding names.
+
 A description of the elements and their attributes follows.
 
 

tests/test_manifest_xml.py

@@ -280,6 +280,27 @@ class XmlManifestTests(unittest.TestCase):
         '<superproject name="superproject"/>' +
         '</manifest>')
 
+  def test_unknown_tags(self):
+    """Check superproject settings."""
+    manifest = self.getXmlManifest("""
+<manifest>
+  <remote name="test-remote" fetch="http://localhost" />
+  <default remote="test-remote" revision="refs/heads/main" />
+  <superproject name="superproject"/>
+  <iankaz value="unknown (possible) future tags are ignored"/>
+  <x-custom-tag>X tags are always ignored</x-custom-tag>
+</manifest>
+""")
+    self.assertEqual(manifest.superproject['name'], 'superproject')
+    self.assertEqual(manifest.superproject['remote'].name, 'test-remote')
+    self.assertEqual(
+        manifest.ToXml().toxml(),
+        '<?xml version="1.0" ?><manifest>' +
+        '<remote name="test-remote" fetch="http://localhost"/>' +
+        '<default remote="test-remote" revision="refs/heads/main"/>' +
+        '<superproject name="superproject"/>' +
+        '</manifest>')
+
   def test_project_group(self):
     """Check project group settings."""
     manifest = self.getXmlManifest("""