Add rendezvous poller.

bramble-api/src/main/java/org/briarproject/bramble/api/plugin/duplex/DuplexPlugin.java

@@ -52,8 +52,10 @@ public interface DuplexPlugin extends Plugin {
 	/**
 	 * Creates and returns an endpoint that uses the given key material to
 	 * rendezvous with a pending contact, and the given connection handler to
-	 * handle incoming connections.
+	 * handle incoming connections. Returns null if an endpoint cannot be
+	 * created.
 	 */
+	@Nullable
 	RendezvousEndpoint createRendezvousEndpoint(KeyMaterialSource k,
 			ConnectionHandler incoming);
 }

bramble-api/src/main/java/org/briarproject/bramble/api/rendezvous/RendezvousConstants.java

@@ -1,6 +1,7 @@
 package org.briarproject.bramble.api.rendezvous;
 
 import static java.util.concurrent.TimeUnit.DAYS;
+import static java.util.concurrent.TimeUnit.MINUTES;
 
 public interface RendezvousConstants {
 
@@ -14,6 +15,11 @@ public interface RendezvousConstants {
 	 */
 	long RENDEZVOUS_TIMEOUT_MS = DAYS.toMillis(2);
 
+	/**
+	 * How often to try to rendezvous with pending contacts.
+	 */
+	long POLLING_INTERVAL_MS = MINUTES.toMillis(1);
+
 	/**
 	 * Label for deriving the rendezvous key from the handshake key pairs.
 	 */

bramble-api/src/main/java/org/briarproject/bramble/api/rendezvous/event/RendezvousFailedEvent.java

@@ -0,0 +1,25 @@
+package org.briarproject.bramble.api.rendezvous.event;
+
+import org.briarproject.bramble.api.contact.PendingContactId;
+import org.briarproject.bramble.api.event.Event;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import javax.annotation.concurrent.Immutable;
+
+/**
+ * An event that is broadcast when a rendezvous with a pending contact fails.
+ */
+@Immutable
+@NotNullByDefault
+public class RendezvousFailedEvent extends Event {
+
+	private final PendingContactId pendingContactId;
+
+	public RendezvousFailedEvent(PendingContactId pendingContactId) {
+		this.pendingContactId = pendingContactId;
+	}
+
+	public PendingContactId getPendingContactId() {
+		return pendingContactId;
+	}
+}