Database portion of the offer/request/transfer protocol (untested).

api/net/sf/briar/api/db/DatabaseComponent.java

@@ -12,10 +12,14 @@ import net.sf.briar.api.protocol.Batch;
 import net.sf.briar.api.protocol.Group;
 import net.sf.briar.api.protocol.GroupId;
 import net.sf.briar.api.protocol.Message;
+import net.sf.briar.api.protocol.MessageId;
+import net.sf.briar.api.protocol.Offer;
 import net.sf.briar.api.protocol.Subscriptions;
 import net.sf.briar.api.protocol.Transports;
 import net.sf.briar.api.protocol.writers.AckWriter;
 import net.sf.briar.api.protocol.writers.BatchWriter;
+import net.sf.briar.api.protocol.writers.OfferWriter;
+import net.sf.briar.api.protocol.writers.RequestWriter;
 import net.sf.briar.api.protocol.writers.SubscriptionWriter;
 import net.sf.briar.api.protocol.writers.TransportWriter;
 
@@ -25,14 +29,14 @@ import net.sf.briar.api.protocol.writers.TransportWriter;
  */
 public interface DatabaseComponent {
 
-	static final long MEGABYTES = 1024L * 1024L;
+	static final int MEGABYTES = 1024 * 1024;
 
 	// FIXME: These should be configurable
 	static final long MIN_FREE_SPACE = 300L * MEGABYTES;
 	static final long CRITICAL_FREE_SPACE = 100L * MEGABYTES;
-	static final long MAX_BYTES_BETWEEN_SPACE_CHECKS = 5L * MEGABYTES;
+	static final int MAX_BYTES_BETWEEN_SPACE_CHECKS = 5 * MEGABYTES;
 	static final long MAX_MS_BETWEEN_SPACE_CHECKS = 60L * 1000L; // 1 min
-	static final long BYTES_PER_SWEEP = 5L * MEGABYTES;
+	static final int BYTES_PER_SWEEP = 5 * MEGABYTES;
 
 	/**
 	 * Opens the database.
@@ -67,6 +71,21 @@ public interface DatabaseComponent {
 	void generateBatch(ContactId c, BatchWriter b) throws DbException,
 	IOException;
 
+	/**
+	 * Generates a batch of messages for the given contact from the given
+	 * collection of requested messages, and returns the IDs of the messages
+	 * added to the bacth.
+	 */
+	Collection<MessageId> generateBatch(ContactId c, BatchWriter b,
+			Collection<MessageId> requested) throws DbException, IOException;
+
+	/**
+	 * Generates an offer for the given contact and returns the offered
+	 * message IDs.
+	 */
+	Collection<MessageId> generateOffer(ContactId c, OfferWriter h)
+	throws DbException, IOException;
+
 	/** Generates a subscription update for the given contact. */
 	void generateSubscriptions(ContactId c, SubscriptionWriter s) throws
 	DbException, IOException;
@@ -96,6 +115,13 @@ public interface DatabaseComponent {
 	/** Processes a batches of messages from the given contact. */
 	void receiveBatch(ContactId c, Batch b) throws DbException;
 
+	/**
+	 * Processes an offer from the given contact and generates a request for
+	 * any messages in the offer that have not been seen.
+	 */
+	void receiveOffer(ContactId c, Offer o, RequestWriter r) throws DbException,
+	IOException;
+
 	/** Processes a subscription update from the given contact. */
 	void receiveSubscriptions(ContactId c, Subscriptions s) throws DbException;
 

api/net/sf/briar/api/protocol/Offer.java

@@ -0,0 +1,16 @@
+package net.sf.briar.api.protocol;
+
+import java.util.Collection;
+
+/** A packet offering the recipient some messages. */
+public interface Offer {
+
+	/**
+	 * The maximum size of a serialised offer, excluding encryption and
+	 * authentication.
+	 */
+	static final int MAX_SIZE = (1024 * 1024) - 100;
+
+	/** Returns the message IDs contained in the offer. */
+	Collection<MessageId> getMessages();
+}

api/net/sf/briar/api/protocol/writers/OfferWriter.java

@@ -0,0 +1,18 @@
+package net.sf.briar.api.protocol.writers;
+
+import java.io.IOException;
+
+import net.sf.briar.api.protocol.MessageId;
+
+/** An interface for creating a have notification. */
+public interface OfferWriter {
+
+	/**
+	 * Attempts to add the given message ID to the offer and returns true if it
+	 * was added.
+	 */
+	boolean writeMessageId(MessageId m) throws IOException;
+
+	/** Finishes writing the offer. */
+	void finish() throws IOException;
+}

api/net/sf/briar/api/protocol/writers/RequestWriter.java

@@ -0,0 +1,11 @@
+package net.sf.briar.api.protocol.writers;
+
+import java.io.IOException;
+import java.util.BitSet;
+
+/** An interface for creating a request packet. */
+public interface RequestWriter {
+
+	/** Writes the contents of the request. */
+	void writeBitmap(BitSet b) throws IOException;
+}

components/net/sf/briar/db/Database.java

@@ -175,6 +175,16 @@ interface Database<T> {
 	 */
 	byte[] getMessage(T txn, MessageId m) throws DbException;
 
+	/**
+	 * Returns the message identified by the given ID, in raw format, or null
+	 * if the message is not present in the database or is not sendable to the
+	 * given contact.
+	 * <p>
+	 * Locking: contacts read, messages read, messageStatuses read.
+	 */
+	byte[] getMessageIfSendable(T txn, ContactId c, MessageId m)
+	throws DbException;
+
 	/**
 	 * Returns the IDs of all messages signed by the given author.
 	 * <p>
@@ -197,7 +207,7 @@ interface Database<T> {
 	 * <p>
 	 * Locking: messages read.
 	 */
-	Collection<MessageId> getOldMessages(T txn, long size) throws DbException;
+	Collection<MessageId> getOldMessages(T txn, int size) throws DbException;
 
 	/**
 	 * Returns the parent of the given message.
@@ -228,7 +238,7 @@ interface Database<T> {
 	 * <p>
 	 * Locking: contacts read, messages read, messageStatuses read.
 	 */
-	Collection<MessageId> getSendableMessages(T txn, ContactId c, int capacity)
+	Collection<MessageId> getSendableMessages(T txn, ContactId c, int size)
 	throws DbException;
 
 	/**
@@ -271,6 +281,8 @@ interface Database<T> {
 	/**
 	 * Marks the given batches received from the given contact as having been
 	 * acknowledged.
+	 * <p>
+	 * Locking: contacts read, messageStatuses write.
 	 */
 	void removeBatchesToAck(T txn, ContactId c, Collection<BatchId> sent)
 	throws DbException;
@@ -330,6 +342,18 @@ interface Database<T> {
 	void setStatus(T txn, ContactId c, MessageId m, Status s)
 	throws DbException;
 
+	/**
+	 * If the database contains the given message and it belongs to a group
+	 * that is visible to the given contact, sets the status of the message
+	 * with respect to the contact to Status.SEEN and returns true; otherwise
+	 * returns false.
+	 * <p>
+	 * Locking: contacts read, messages read, messageStatuses write,
+	 * subscriptions read.
+	 */
+	boolean setStatusSeenIfVisible(T txn, ContactId c, MessageId m)
+	throws DbException;
+
 	/**
 	 * Sets the subscriptions for the given contact, replacing any existing
 	 * subscriptions unless the existing subscriptions have a newer timestamp.

components/net/sf/briar/db/DatabaseComponentImpl.java

@@ -45,7 +45,7 @@ DatabaseCleaner.Callback {
 	 * Removes the oldest messages from the database, with a total size less
 	 * than or equal to the given size.
 	 */
-	protected abstract void expireMessages(long size) throws DbException;
+	protected abstract void expireMessages(int size) throws DbException;
 
 	/**
 	 * Calculates and returns the sendability score of a message.

components/net/sf/briar/db/JdbcDatabase.java

@@ -751,6 +751,45 @@ abstract class JdbcDatabase implements Database<Connection> {
 		}
 	}
 
+	public byte[] getMessageIfSendable(Connection txn, ContactId c, MessageId m)
+	throws DbException {
+		PreparedStatement ps = null;
+		ResultSet rs = null;
+		try {
+			String sql = "SELECT size, raw FROM messages"
+				+ " JOIN contactSubscriptions"
+				+ " ON messages.groupId = contactSubscriptions.groupId"
+				+ " JOIN statuses ON messages.messageId = statuses.messageId"
+				+ " WHERE messages.messageId = ?"
+				+ " AND contactSubscriptions.contactId = ?"
+				+ " AND statuses.contactId = ? AND status = ?"
+				+ " AND sendability > ZERO()";
+			ps = txn.prepareStatement(sql);
+			ps.setBytes(1, m.getBytes());
+			ps.setInt(2, c.getInt());
+			ps.setInt(3, c.getInt());
+			ps.setShort(4, (short) Status.NEW.ordinal());
+			rs = ps.executeQuery();
+			byte[] raw = null;
+			if(rs.next()) {
+				int size = rs.getInt(1);
+				Blob b = rs.getBlob(2);
+				raw = b.getBytes(1, size);
+				assert raw.length == size;
+			}
+			boolean more = rs.next();
+			assert !more;
+			rs.close();
+			ps.close();
+			return raw;
+		} catch(SQLException e) {
+			tryToClose(rs);
+			tryToClose(ps);
+			tryToClose(txn);
+			throw new DbException(e);
+		}
+	}
+
 	public Collection<MessageId> getMessagesByAuthor(Connection txn, AuthorId a)
 	throws DbException {
 		PreparedStatement ps = null;
@@ -836,7 +875,7 @@ abstract class JdbcDatabase implements Database<Connection> {
 		}
 	}
 
-	public Collection<MessageId> getOldMessages(Connection txn, long capacity)
+	public Collection<MessageId> getOldMessages(Connection txn, int capacity)
 	throws DbException {
 		PreparedStatement ps = null;
 		ResultSet rs = null;
@@ -846,7 +885,7 @@ abstract class JdbcDatabase implements Database<Connection> {
 			ps = txn.prepareStatement(sql);
 			rs = ps.executeQuery();
 			Collection<MessageId> ids = new ArrayList<MessageId>();
-			long total = 0L;
+			int total = 0;
 			while(rs.next()) {
 				int size = rs.getInt(1);
 				if(total + size > capacity) break;
@@ -1356,6 +1395,44 @@ abstract class JdbcDatabase implements Database<Connection> {
 		}
 	}
 
+	public boolean setStatusSeenIfVisible(Connection txn, ContactId c,
+			MessageId m) throws DbException {
+		PreparedStatement ps = null;
+		ResultSet rs = null;
+		try {
+			String sql = "SELECT COUNT(messages.messageId) FROM messages"
+				+ " JOIN contactSubscriptions"
+				+ " ON messages.groupId = contactSubscriptions.groupId"
+				+ " WHERE messageId = ? AND contactId = ?";
+			ps = txn.prepareStatement(sql);
+			rs = ps.executeQuery();
+			boolean found = rs.next();
+			assert found;
+			int count = rs.getInt(1);
+			assert count <= 1;
+			boolean more = rs.next();
+			assert !more;
+			rs.close();
+			ps.close();
+			if(count == 0) return false;
+			sql = "UPDATE statuses SET status = ?"
+				+ " WHERE messageId = ? AND contactId = ?";
+			ps = txn.prepareStatement(sql);
+			ps.setShort(1, (short) Status.SEEN.ordinal());
+			ps.setBytes(2, m.getBytes());
+			ps.setInt(3, c.getInt());
+			int rowsAffected = ps.executeUpdate();
+			assert rowsAffected <= 1;
+			ps.close();
+			return true;
+		} catch(SQLException e) {
+			tryToClose(rs);
+			tryToClose(ps);
+			tryToClose(txn);
+			throw new DbException(e);
+		}
+	}
+
 	public void setSubscriptions(Connection txn, ContactId c,
 			Collection<Group> subs, long timestamp) throws DbException {
 		PreparedStatement ps = null;

components/net/sf/briar/db/ReadWriteLockDatabaseComponent.java

@@ -2,11 +2,10 @@ package net.sf.briar.db;
 
 import java.io.IOException;
 import java.util.ArrayList;
+import java.util.BitSet;
 import java.util.Collection;
-import java.util.HashSet;
 import java.util.Iterator;
 import java.util.Map;
-import java.util.Set;
 import java.util.concurrent.locks.ReentrantReadWriteLock;
 import java.util.logging.Level;
 import java.util.logging.Logger;
@@ -23,10 +22,13 @@ import net.sf.briar.api.protocol.Group;
 import net.sf.briar.api.protocol.GroupId;
 import net.sf.briar.api.protocol.Message;
 import net.sf.briar.api.protocol.MessageId;
+import net.sf.briar.api.protocol.Offer;
 import net.sf.briar.api.protocol.Subscriptions;
 import net.sf.briar.api.protocol.Transports;
 import net.sf.briar.api.protocol.writers.AckWriter;
 import net.sf.briar.api.protocol.writers.BatchWriter;
+import net.sf.briar.api.protocol.writers.OfferWriter;
+import net.sf.briar.api.protocol.writers.RequestWriter;
 import net.sf.briar.api.protocol.writers.SubscriptionWriter;
 import net.sf.briar.api.protocol.writers.TransportWriter;
 
@@ -64,7 +66,7 @@ class ReadWriteLockDatabaseComponent<Txn> extends DatabaseComponentImpl<Txn> {
 		super(db, cleaner);
 	}
 
-	protected void expireMessages(long size) throws DbException {
+	protected void expireMessages(int size) throws DbException {
 		contactLock.readLock().lock();
 		try {
 			messageLock.writeLock().lock();
@@ -287,7 +289,7 @@ class ReadWriteLockDatabaseComponent<Txn> extends DatabaseComponentImpl<Txn> {
 			if(!containsContact(c)) throw new NoSuchContactException();
 			messageLock.readLock().lock();
 			try {
-				Set<MessageId> sent;
+				Collection<MessageId> sent;
 				int bytesSent = 0;
 				messageStatusLock.readLock().lock();
 				try {
@@ -296,7 +298,7 @@ class ReadWriteLockDatabaseComponent<Txn> extends DatabaseComponentImpl<Txn> {
 						int capacity = b.getCapacity();
 						Iterator<MessageId> it =
 							db.getSendableMessages(txn, c, capacity).iterator();
-						sent = new HashSet<MessageId>();
+						sent = new ArrayList<MessageId>();
 						while(it.hasNext()) {
 							MessageId m = it.next();
 							byte[] message = db.getMessage(txn, m);
@@ -316,7 +318,7 @@ class ReadWriteLockDatabaseComponent<Txn> extends DatabaseComponentImpl<Txn> {
 					messageStatusLock.readLock().unlock();
 				}
 				BatchId id = b.finish();
-				// Record the contents of the batch, unless it was empty
+				// Record the contents of the batch, unless it's empty
 				if(sent.isEmpty()) return;
 				messageStatusLock.writeLock().lock();
 				try {
@@ -339,6 +341,104 @@ class ReadWriteLockDatabaseComponent<Txn> extends DatabaseComponentImpl<Txn> {
 		}
 	}
 
+	public Collection<MessageId> generateBatch(ContactId c, BatchWriter b,
+			Collection<MessageId> requested) throws DbException, IOException {
+		contactLock.readLock().lock();
+		try {
+			if(!containsContact(c)) throw new NoSuchContactException();
+			messageLock.readLock().lock();
+			try {
+				Collection<MessageId> sent;
+				messageStatusLock.readLock().lock();
+				try{
+					Txn txn = db.startTransaction();
+					try {
+						sent = new ArrayList<MessageId>();
+						int bytesSent = 0;
+						for(MessageId m : requested) {
+							byte[] message = db.getMessageIfSendable(txn, c, m);
+							if(b == null) continue; // Expired or not sendable
+							if(!b.writeMessage(message)) break;
+							bytesSent += message.length;
+							sent.add(m);
+						}
+						db.commitTransaction(txn);
+					} catch(DbException e) {
+						db.abortTransaction(txn);
+						throw e;
+					} catch(IOException e) {
+						db.abortTransaction(txn);
+						throw e;
+					}
+				} finally {
+					messageStatusLock.readLock().unlock();
+				}
+				BatchId id = b.finish();
+				// Record the contents of the batch, unless it's empty
+				if(sent.isEmpty()) return sent;
+				messageStatusLock.writeLock().lock();
+				try {
+					Txn txn = db.startTransaction();
+					try {
+						db.addOutstandingBatch(txn, c, id, sent);
+						db.commitTransaction(txn);
+						return sent;
+					} catch(DbException e) {
+						db.abortTransaction(txn);
+						throw e;
+					}
+				} finally {
+					messageStatusLock.writeLock().unlock();
+				}
+			} finally {
+				messageLock.readLock().unlock();
+			}
+		} finally {
+			contactLock.readLock().unlock();
+		}
+	}
+
+	public Collection<MessageId> generateOffer(ContactId c, OfferWriter o)
+	throws DbException, IOException {
+		contactLock.readLock().lock();
+		try {
+			if(!containsContact(c)) throw new NoSuchContactException();
+			messageLock.readLock().lock();
+			try {
+				messageStatusLock.readLock().lock();
+				try {
+					Txn txn = db.startTransaction();
+					try {
+						Collection<MessageId> sendable =
+							db.getSendableMessages(txn, c, Integer.MAX_VALUE);
+						Iterator<MessageId> it = sendable.iterator();
+						Collection<MessageId> sent = new ArrayList<MessageId>();
+						while(it.hasNext()) {
+							MessageId m = it.next();
+							if(!o.writeMessageId(m)) break;
+							sent.add(m);
+						}
+						o.finish();
+						db.commitTransaction(txn);
+						return sent;
+					} catch(DbException e) {
+						db.abortTransaction(txn);
+						throw e;
+					} catch(IOException e) {
+						db.abortTransaction(txn);
+						throw e;
+					}
+				} finally {
+					messageStatusLock.readLock().unlock();
+				}
+			} finally {
+				messageLock.readLock().unlock();
+			}
+		} finally {
+			contactLock.readLock().unlock();
+		}
+	}
+
 	public void generateSubscriptions(ContactId c, SubscriptionWriter s)
 	throws DbException, IOException {
 		contactLock.readLock().lock();
@@ -565,6 +665,50 @@ class ReadWriteLockDatabaseComponent<Txn> extends DatabaseComponentImpl<Txn> {
 		}
 	}
 
+	public void receiveOffer(ContactId c, Offer o, RequestWriter r)
+	throws DbException, IOException {
+		contactLock.readLock().lock();
+		try {
+			if(!containsContact(c)) throw new NoSuchContactException();
+			messageLock.readLock().lock();
+			try {
+				messageStatusLock.writeLock().lock();
+				try {
+					subscriptionLock.readLock().lock();
+					try {
+						BitSet request;
+						Txn txn = db.startTransaction();
+						try {
+							Collection<MessageId> offered = o.getMessages();
+							request = new BitSet(offered.size());
+							Iterator<MessageId> it = offered.iterator();
+							for(int i = 0; it.hasNext(); i++) {
+								// If the message is not in the database or if
+								// it is not visible to the contact, request it
+								MessageId m = it.next();
+								if(!db.setStatusSeenIfVisible(txn, c, m))
+									request.set(i);
+							}
+							db.commitTransaction(txn);
+						} catch(DbException e) {
+							db.abortTransaction(txn);
+							throw e;
+						}
+						r.writeBitmap(request);
+					} finally {
+						subscriptionLock.readLock().unlock();
+					}
+				} finally {
+					messageStatusLock.writeLock().unlock();
+				}
+			} finally {
+				messageLock.readLock().unlock();
+			}
+		} finally {
+			contactLock.readLock().unlock();
+		}
+	}
+
 	public void receiveSubscriptions(ContactId c, Subscriptions s)
 	throws DbException {
 		// Update the contact's subscriptions

components/net/sf/briar/db/SynchronizedDatabaseComponent.java

@@ -2,11 +2,10 @@ package net.sf.briar.db;
 
 import java.io.IOException;
 import java.util.ArrayList;
+import java.util.BitSet;
 import java.util.Collection;
-import java.util.HashSet;
 import java.util.Iterator;
 import java.util.Map;
-import java.util.Set;
 import java.util.logging.Level;
 import java.util.logging.Logger;
 
@@ -22,10 +21,13 @@ import net.sf.briar.api.protocol.Group;
 import net.sf.briar.api.protocol.GroupId;
 import net.sf.briar.api.protocol.Message;
 import net.sf.briar.api.protocol.MessageId;
+import net.sf.briar.api.protocol.Offer;
 import net.sf.briar.api.protocol.Subscriptions;
 import net.sf.briar.api.protocol.Transports;
 import net.sf.briar.api.protocol.writers.AckWriter;
 import net.sf.briar.api.protocol.writers.BatchWriter;
+import net.sf.briar.api.protocol.writers.OfferWriter;
+import net.sf.briar.api.protocol.writers.RequestWriter;
 import net.sf.briar.api.protocol.writers.SubscriptionWriter;
 import net.sf.briar.api.protocol.writers.TransportWriter;
 
@@ -57,7 +59,7 @@ class SynchronizedDatabaseComponent<Txn> extends DatabaseComponentImpl<Txn> {
 		super(db, cleaner);
 	}
 
-	protected void expireMessages(long size) throws DbException {
+	protected void expireMessages(int size) throws DbException {
 		synchronized(contactLock) {
 			synchronized(messageLock) {
 				synchronized(messageStatusLock) {
@@ -220,7 +222,7 @@ class SynchronizedDatabaseComponent<Txn> extends DatabaseComponentImpl<Txn> {
 						int capacity = b.getCapacity();
 						Iterator<MessageId> it =
 							db.getSendableMessages(txn, c, capacity).iterator();
-						Set<MessageId> sent = new HashSet<MessageId>();
+						Collection<MessageId> sent = new ArrayList<MessageId>();
 						int bytesSent = 0;
 						while(it.hasNext()) {
 							MessageId m = it.next();
@@ -230,7 +232,7 @@ class SynchronizedDatabaseComponent<Txn> extends DatabaseComponentImpl<Txn> {
 							sent.add(m);
 						}
 						BatchId id = b.finish();
-						// Record the contents of the batch, unless it was empty
+						// Record the contents of the batch, unless it's empty
 						if(!sent.isEmpty())
 							db.addOutstandingBatch(txn, c, id, sent);
 						db.commitTransaction(txn);
@@ -246,6 +248,73 @@ class SynchronizedDatabaseComponent<Txn> extends DatabaseComponentImpl<Txn> {
 		}
 	}
 
+	public Collection<MessageId> generateBatch(ContactId c, BatchWriter b,
+			Collection<MessageId> requested) throws DbException, IOException {
+		synchronized(contactLock) {
+			if(!containsContact(c)) throw new NoSuchContactException();
+			synchronized(messageLock) {
+				synchronized(messageStatusLock) {
+					Txn txn = db.startTransaction();
+					try {
+						Collection<MessageId> sent = new ArrayList<MessageId>();
+						int bytesSent = 0;
+						for(MessageId m : requested) {
+							byte[] message = db.getMessageIfSendable(txn, c, m);
+							if(b == null) continue; // Expired or not sendable
+							if(!b.writeMessage(message)) break;
+							bytesSent += message.length;
+							sent.add(m);
+						}
+						BatchId id = b.finish();
+						// Record the contents of the batch, unless it's empty
+						if(!sent.isEmpty())
+							db.addOutstandingBatch(txn, c, id, sent);
+						db.commitTransaction(txn);
+						return sent;
+					} catch(DbException e) {
+						db.abortTransaction(txn);
+						throw e;
+					} catch(IOException e) {
+						db.abortTransaction(txn);
+						throw e;
+					}
+				}
+			}
+		}
+	}
+
+	public Collection<MessageId> generateOffer(ContactId c, OfferWriter o)
+	throws DbException, IOException {
+		synchronized(contactLock) {
+			if(!containsContact(c)) throw new NoSuchContactException();
+			synchronized(messageLock) {
+				synchronized(messageStatusLock) {
+					Txn txn = db.startTransaction();
+					try {
+						Collection<MessageId> sendable =
+							db.getSendableMessages(txn, c, Integer.MAX_VALUE);
+						Iterator<MessageId> it = sendable.iterator();
+						Collection<MessageId> sent = new ArrayList<MessageId>();
+						while(it.hasNext()) {
+							MessageId m = it.next();
+							if(!o.writeMessageId(m)) break;
+							sent.add(m);
+						}
+						o.finish();
+						db.commitTransaction(txn);
+						return sent;
+					} catch(DbException e) {
+						db.abortTransaction(txn);
+						throw e;
+					} catch(IOException e) {
+						db.abortTransaction(txn);
+						throw e;
+					}
+				}
+			}
+		}
+	}
+
 	public void generateSubscriptions(ContactId c, SubscriptionWriter s)
 	throws DbException, IOException {
 		synchronized(contactLock) {
@@ -421,6 +490,38 @@ class SynchronizedDatabaseComponent<Txn> extends DatabaseComponentImpl<Txn> {
 		}
 	}
 
+	public void receiveOffer(ContactId c, Offer o, RequestWriter r)
+	throws DbException, IOException {
+		synchronized(contactLock) {
+			if(!containsContact(c)) throw new NoSuchContactException();
+			synchronized(messageLock) {
+				synchronized(messageStatusLock) {
+					synchronized(subscriptionLock) {
+						BitSet request;
+						Txn txn = db.startTransaction();
+						try {
+							Collection<MessageId> offered = o.getMessages();
+							request = new BitSet(offered.size());
+							Iterator<MessageId> it = offered.iterator();
+							for(int i = 0; it.hasNext(); i++) {
+								// If the message is not in the database or if
+								// it is not visible to the contact, request it
+								MessageId m = it.next();
+								if(!db.setStatusSeenIfVisible(txn, c, m))
+									request.set(i);
+							}
+							db.commitTransaction(txn);
+						} catch(DbException e) {
+							db.abortTransaction(txn);
+							throw e;
+						}
+						r.writeBitmap(request);
+					}
+				}
+			}
+		}
+	}
+
 	public void receiveSubscriptions(ContactId c, Subscriptions s)
 	throws DbException {
 		// Update the contact's subscriptions