Broadcast message IDs to request instead of storing them.

bramble-api/src/main/java/org/briarproject/bramble/api/Consumable.java

@@ -0,0 +1,25 @@
+package org.briarproject.bramble.api;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import java.util.concurrent.atomic.AtomicReference;
+
+import javax.annotation.Nullable;
+
+/**
+ * An item that can be consumed once.
+ */
+@NotNullByDefault
+public class Consumable<T> {
+
+	private final AtomicReference<T> reference;
+
+	public Consumable(T item) {
+		reference = new AtomicReference<>(item);
+	}
+
+	@Nullable
+	public T consume() {
+		return reference.getAndSet(null);
+	}
+}

bramble-api/src/main/java/org/briarproject/bramble/api/db/DatabaseComponent.java

@@ -181,14 +181,6 @@ public interface DatabaseComponent extends TransactionManager {
 	Offer generateOffer(Transaction txn, ContactId c, int maxMessages,
 			int maxLatency, boolean small) throws DbException;
 
-	/**
-	 * Returns a request for the given contact, or null if there are no
-	 * messages to request.
-	 */
-	@Nullable
-	Request generateRequest(Transaction txn, ContactId c, int maxMessages)
-			throws DbException;
-
 	/**
 	 * Returns a batch of messages for the given contact, with a total length
 	 * less than or equal to the given length, for transmission over a

bramble-api/src/main/java/org/briarproject/bramble/api/sync/event/MessageToRequestEvent.java

@@ -1,9 +1,14 @@
 package org.briarproject.bramble.api.sync.event;
 
+import org.briarproject.bramble.api.Consumable;
 import org.briarproject.bramble.api.contact.ContactId;
 import org.briarproject.bramble.api.event.Event;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.sync.MessageId;
 
+import java.util.Collection;
+
+import javax.annotation.Nullable;
 import javax.annotation.concurrent.Immutable;
 
 /**
@@ -15,12 +20,20 @@ import javax.annotation.concurrent.Immutable;
 public class MessageToRequestEvent extends Event {
 
 	private final ContactId contactId;
+	private final Consumable<Collection<MessageId>> ids;
 
-	public MessageToRequestEvent(ContactId contactId) {
+	public MessageToRequestEvent(ContactId contactId,
+			Collection<MessageId> ids) {
 		this.contactId = contactId;
+		this.ids = new Consumable<>(ids);
 	}
 
 	public ContactId getContactId() {
 		return contactId;
 	}
+
+	@Nullable
+	public Collection<MessageId> consumeIds() {
+		return ids.consume();
+	}
 }