Introduce a MessageContext class for more flexibility

This change will allow to pass message dependencies from the client
validators to the ValidationManager.

briar-api/src/org/briarproject/api/sync/BaseMessageContext.java

@@ -0,0 +1,17 @@
+package org.briarproject.api.sync;
+
+import java.util.Collection;
+
+public abstract class BaseMessageContext {
+
+	private final Collection<MessageId> dependencies;
+
+	public BaseMessageContext(Collection<MessageId> dependencies) {
+		this.dependencies = dependencies;
+	}
+
+	public Collection<MessageId> getDependencies() {
+		return dependencies;
+	}
+
+}

briar-api/src/org/briarproject/api/sync/InvalidMessageException.java

@@ -0,0 +1,20 @@
+package org.briarproject.api.sync;
+
+import java.io.IOException;
+
+/** An exception that indicates an invalid message. */
+public class InvalidMessageException extends IOException {
+
+	public InvalidMessageException() {
+		super();
+	}
+
+	public InvalidMessageException(String str) {
+		super(str);
+	}
+
+	public InvalidMessageException(Throwable t) {
+		super(t);
+	}
+
+}

briar-api/src/org/briarproject/api/sync/MessageContext.java

@@ -0,0 +1,26 @@
+package org.briarproject.api.sync;
+
+import org.briarproject.api.db.Metadata;
+
+import java.util.Collection;
+
+public class MessageContext extends BaseMessageContext {
+
+	private final Metadata metadata;
+
+	public MessageContext(Metadata metadata,
+			Collection<MessageId> dependencies) {
+
+		super(dependencies);
+		this.metadata = metadata;
+	}
+
+	public MessageContext(Metadata metadata) {
+		this(metadata, null);
+	}
+
+	public Metadata getMetadata() {
+		return metadata;
+	}
+
+}

briar-api/src/org/briarproject/api/sync/ValidationManager.java

@@ -44,10 +44,11 @@ public interface ValidationManager {
 	interface MessageValidator {
 
 		/**
-		 * Validates the given message and returns its metadata if the message
-		 * is valid, or null if the message is invalid.
+		 * Validates the given message and returns its metadata and
+		 * dependencies.
 		 */
-		Metadata validateMessage(Message m, Group g);
+		MessageContext validateMessage(Message m, Group g)
+				throws InvalidMessageException;
 	}
 
 	interface IncomingMessageHook {