Expose transactions through DatabaseComponent interface.

briar-api/src/org/briarproject/api/db/DatabaseComponent.java

@@ -39,6 +39,21 @@ public interface DatabaseComponent {
 	/** Waits for any open transactions to finish and closes the database. */
 	void close() throws DbException, IOException;
 
+	/** Starts a new transaction and returns an object representing it. */
+	Transaction startTransaction() throws DbException;
+
+	/**
+	 * Aborts the given transaction - no changes made during the transaction
+	 * will be applied to the database.
+	 */
+	void abortTransaction(Transaction txn);
+
+	/**
+	 * Commits the given transaction - all changes made during the transaction
+	 * will be applied to the database.
+	 */
+	void commitTransaction(Transaction txn) throws DbException;
+
 	/**
 	 * Stores a contact associated with the given local and remote pseudonyms,
 	 * and returns an ID for the contact.

briar-api/src/org/briarproject/api/db/Transaction.java

@@ -0,0 +1,15 @@
+package org.briarproject.api.db;
+
+/** A wrapper around a database transaction. */
+public class Transaction {
+
+	private final Object txn;
+
+	public Transaction(Object txn) {
+		this.txn = txn;
+	}
+
+	public Object unbox() {
+		return txn;
+	}
+}