Query message IDs rather than metadata when only IDs are needed.

bramble-api/src/main/java/org/briarproject/bramble/api/client/ClientHelper.java

@@ -17,6 +17,7 @@ import org.briarproject.bramble.api.sync.Message;
 import org.briarproject.bramble.api.sync.MessageId;
 
 import java.security.GeneralSecurityException;
+import java.util.Collection;
 import java.util.Map;
 
 @NotNullByDefault
@@ -51,9 +52,11 @@ public interface ClientHelper {
 	BdfDictionary getGroupMetadataAsDictionary(Transaction txn, GroupId g)
 			throws DbException, FormatException;
 
+	Collection<MessageId> getMessageIds(Transaction txn, GroupId g,
+			BdfDictionary query) throws DbException, FormatException;
+
 	BdfDictionary getMessageMetadataAsDictionary(MessageId m)
-			throws DbException,
-			FormatException;
+			throws DbException, FormatException;
 
 	BdfDictionary getMessageMetadataAsDictionary(Transaction txn, MessageId m)
 			throws DbException, FormatException;

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

@@ -288,6 +288,16 @@ public interface DatabaseComponent extends TransactionManager {
 	Collection<MessageId> getMessageIds(Transaction txn, GroupId g)
 			throws DbException;
 
+	/**
+	 * Returns the IDs of any delivered messages in the given group with
+	 * metadata that matches all entries in the given query. If the query is
+	 * empty, the IDs of all delivered messages are returned.
+	 * <p/>
+	 * Read-only.
+	 */
+	Collection<MessageId> getMessageIds(Transaction txn, GroupId g,
+			Metadata query) throws DbException;
+
 	/**
 	 * Returns the IDs of any messages that need to be validated.
 	 * <p/>