Add support for *optional* session establishment

Don't perform XMPP session binding if the server announced the feature
as optional.

Fixes SMACK-622

smack-core/src/main/java/org/jivesoftware/smack/AbstractXMPPConnection.java

@@ -466,7 +466,10 @@ public abstract class AbstractXMPPConnection implements XMPPConnection {
         user = response.getJid();
         serviceName = XmppStringUtils.parseDomain(user);
 
-        if (hasFeature(Session.ELEMENT, Session.NAMESPACE) && !getConfiguration().isLegacySessionDisabled()) {
+        Session.Feature sessionFeature = getFeature(Session.ELEMENT, Session.NAMESPACE);
+        // Only bind the session if it's announced as stream feature by the server, is not optional and not disabled
+        // For more information see http://tools.ietf.org/html/draft-cridland-xmpp-session-01
+        if (sessionFeature != null && !sessionFeature.isOptional() && !getConfiguration().isLegacySessionDisabled()) {
             Session session = new Session();
             packetCollector = createPacketCollectorAndSend(new PacketIDFilter(session), session);
             packetCollector.nextResultOrThrow();
@@ -1137,7 +1140,7 @@ public abstract class AbstractXMPPConnection implements XMPPConnection {
                     streamFeature = Bind.Feature.INSTANCE;
                     break;
                 case Session.ELEMENT:
-                    streamFeature = Session.Feature.INSTANCE;
+                    streamFeature = PacketParserUtils.parseSessionFeature(parser);
                     break;
                 case RosterVer.ELEMENT:
                     if(namespace.equals(RosterVer.NAMESPACE)) {

smack-core/src/main/java/org/jivesoftware/smack/packet/Session.java

@@ -17,6 +17,8 @@
 
 package org.jivesoftware.smack.packet;
 
+import org.jivesoftware.smack.util.XmlStringBuilder;
+
 /**
  * IQ packet that will be sent to the server to establish a session.<p>
  *
@@ -44,9 +46,16 @@ public class Session extends SimpleIQ {
 
     public static class Feature implements PacketExtension {
 
-        public static final Session.Feature INSTANCE = new Feature();
+        public static final String OPTIONAL_ELEMENT = "optional";
 
-        private Feature() {
+        private final boolean optional;
+
+        public Feature(boolean optional) {
+            this.optional = optional;
+        }
+
+        public boolean isOptional() {
+            return optional;
         }
 
         @Override
@@ -60,8 +69,16 @@ public class Session extends SimpleIQ {
         }
 
         @Override
-        public String toXML() {
-            return SESSION;
+        public XmlStringBuilder toXML() {
+            XmlStringBuilder xml = new XmlStringBuilder(this);
+            if (optional) {
+                xml.rightAngleBracket();
+                xml.emptyElement(OPTIONAL_ELEMENT);
+                xml.closeElement(this);
+            } else {
+                xml.closeEmptyElement();
+            }
+            return xml;
         }
     }
 }

smack-core/src/main/java/org/jivesoftware/smack/util/PacketParserUtils.java

@@ -39,6 +39,7 @@ import org.jivesoftware.smack.packet.Message;
 import org.jivesoftware.smack.packet.Packet;
 import org.jivesoftware.smack.packet.PacketExtension;
 import org.jivesoftware.smack.packet.Presence;
+import org.jivesoftware.smack.packet.Session;
 import org.jivesoftware.smack.packet.StartTls;
 import org.jivesoftware.smack.packet.StreamError;
 import org.jivesoftware.smack.packet.XMPPError;
@@ -982,6 +983,32 @@ public class PacketParserUtils {
         return new StartTls(required);
     }
 
+    public static Session.Feature parseSessionFeature(XmlPullParser parser) throws XmlPullParserException, IOException {
+        ParserUtils.assertAtStartTag(parser);
+        final int initialDepth = parser.getDepth();
+        boolean optional = false;
+        if (!parser.isEmptyElementTag()) {
+        outerloop: while(true) {
+            int event = parser.next();
+            switch (event) {
+            case XmlPullParser.START_TAG:
+                String name = parser.getName();
+                switch (name) {
+                    case Session.Feature.OPTIONAL_ELEMENT:
+                        optional = true;
+                        break;
+                }
+                break;
+            case XmlPullParser.END_TAG:
+                if (parser.getDepth() == initialDepth) {
+                    break outerloop;
+                }
+            }
+        }
+        }
+        return new Session.Feature(optional);
+
+    }
     private static String getLanguageAttribute(XmlPullParser parser) {
     	for (int i = 0; i < parser.getAttributeCount(); i++) {
             String attributeName = parser.getAttributeName(i);