Revert "xmpp-parsers: Remove optional child from sm element"

This reverts commit 0a1b291574 once again.

This is currently required to connect to Prosody, otherwise the parser
will choke on the extra undocumented child.

I still wonder if required (4e0cd5d96b)
would be useful to support then, as it might also be found in the wild.

parsers/ChangeLog

@@ -109,6 +109,8 @@ XXXX-YY-ZZ RELEASER <admin@example.com>
       - JSON Containers (XEP-0335) (!546)
       - Verifying HTTP Requests via XMPP (XEP-0070)
     * Improvements:
+      - Add support for undocumented `<optional/> in XEP-0198 feature
+        advertisment, for compatibility with servers in the wild.
       - Add support application-specific error conditions in XEP-0198
       - Keep unsupported vCard elements as `minidom::Element`, so that they
         get serialized back instead of being dropped.  We now also test for

parsers/src/sasl2.rs

@@ -269,7 +269,7 @@ mod tests {
 
         let inline = auth.inline.unwrap();
         assert_eq!(inline.bind2.unwrap().inline_features.len(), 0);
-        assert_eq!(inline.sm.unwrap(), StreamManagement);
+        assert_eq!(inline.sm.unwrap(), StreamManagement { optional: false });
         assert_eq!(inline.payloads.len(), 0);
     }
 

parsers/src/sm.rs

@@ -136,7 +136,13 @@ pub struct Resumed {
 /// Represents availability of Stream Management in `<stream:features/>`.
 #[derive(FromXml, AsXml, PartialEq, Debug, Clone)]
 #[xml(namespace = ns::SM, name = "sm")]
-pub struct StreamManagement;
+pub struct StreamManagement {
+    /// Undocumented `<optional/>` flag.
+    // TODO: Remove this flag once servers in the wild have been updated to not send it, as it is
+    // completely undocumented in XEP-0198, only appearing in the XML Schema before 1.6.3.
+    #[xml(flag)]
+    pub optional: bool,
+}
 
 /// Application-specific error condition to use when the peer acknowledges
 /// more stanzas than the local side has sent.
@@ -215,7 +221,7 @@ mod tests {
         assert_size!(R, 0);
         assert_size!(Resume, 16);
         assert_size!(Resumed, 16);
-        assert_size!(StreamManagement, 0);
+        assert_size!(StreamManagement, 1);
         assert_size!(HandledCountTooHigh, 8);
     }
 
@@ -230,7 +236,7 @@ mod tests {
         assert_size!(R, 0);
         assert_size!(Resume, 32);
         assert_size!(Resumed, 32);
-        assert_size!(StreamManagement, 0);
+        assert_size!(StreamManagement, 1);
         assert_size!(HandledCountTooHigh, 8);
     }