StanzaDroppedListener for XEP-0198 resumption failures

If a stream resume fails, smack will re-send all queued stanzas after a
reconnect. However, it does not make sense to re-send them:

* IQs / IQ responses have probably timed out
* MUC messages and PMs will be rejected as you haven't rejoined yet
* regular messages should be amended with a <delay> element

This patch adds a StanzaDroppedListener interface to the
XMPPTCPConnection. If at least one StanzaDroppedListener is provided,
all queued messages will be drained into the StanzaDroppedListener(s).
Otherwise, the original behavior of attempting to transmit them will be
followed.

Discussion: https://discourse.igniterealtime.org/t/xep-0198-resume-failure-reconnect-resending-of-muc-messages/83510/3

Signed-off-by: Georg Lukas <georg@op-co.de>

smack-tcp/src/main/java/org/jivesoftware/smack/tcp/XMPPTCPConnection.java

@@ -283,6 +283,15 @@ public class XMPPTCPConnection extends AbstractXMPPConnection {
      */
     private final Collection<StanzaListener> stanzaAcknowledgedListeners = new ConcurrentLinkedQueue<>();
 
+    /**
+     * These listeners are invoked for every stanza that got dropped.
+     * <p>
+     * We use a {@link ConcurrentLinkedQueue} here in order to allow the listeners to remove
+     * themselves after they have been invoked.
+     * </p>
+     */
+    private final Collection<StanzaListener> stanzaDroppedListeners = new ConcurrentLinkedQueue<>();
+
     /**
      * This listeners are invoked for a acknowledged stanza that has the given stanza ID. They will
      * only be invoked once and automatically removed after that.
@@ -447,9 +456,24 @@ public class XMPPTCPConnection extends AbstractXMPPConnection {
                 }
             }
         }
-        // (Re-)send the stanzas *after* we tried to enable SM
+        // Inform client about failed resumption if possible, resend stanzas otherwise
-        for (Stanza stanza : previouslyUnackedStanzas) {
+        // Process the stanzas synchronously so a client can re-queue them for transmission
-            sendStanzaInternal(stanza);
+        // before it is informed about connection success
+        if (!stanzaDroppedListeners.isEmpty()) {
+            for (Stanza stanza : previouslyUnackedStanzas) {
+                for (StanzaListener listener : stanzaDroppedListeners) {
+                    try {
+                        listener.processStanza(stanza);
+                    }
+                    catch (InterruptedException | NotConnectedException | NotLoggedInException e) {
+                        LOGGER.log(Level.FINER, "StanzaDroppedListener received exception", e);
+                    }
+                }
+            }
+        } else {
+            for (Stanza stanza : previouslyUnackedStanzas) {
+                sendStanzaInternal(stanza);
+            }
         }
 
         afterSuccessfulLogin(false);
@@ -1787,6 +1811,32 @@ public class XMPPTCPConnection extends AbstractXMPPConnection {
         stanzaAcknowledgedListeners.clear();
     }
 
+    /**
+     * Add a Stanza dropped listener.
+     * <p>
+     * Those listeners will be invoked every time a Stanza has been dropped due to a failed SM resume. They will not get
+     * automatically removed. If at least one StanzaDroppedListener is configured, no attempt will be made to retransmit
+     * the Stanzas.
+     * </p>
+     *
+     * @param listener the listener to add.
+     * @since 4.3.3
+     */
+    public void addStanzaDroppedListener(StanzaListener listener) {
+        stanzaDroppedListeners.add(listener);
+    }
+
+    /**
+     * Remove the given Stanza dropped listener.
+     *
+     * @param listener the listener.
+     * @return true if the listener was removed.
+     * @since 4.3.3
+     */
+    public boolean removeStanzaDroppedListener(StanzaListener listener) {
+        return stanzaDroppedListeners.remove(listener);
+    }
+
     /**
      * Add a new Stanza ID acknowledged listener for the given ID.
      * <p>