Merge branch '589-when-a-message-is-shared-share-its-transitive-dependencies' into 'master'

When a message is shared, share its transitive dependencies

Like other recursive operations on the dependency graph, this is
not done in a single transaction to prevent an attacker from creating
arbitrary large transactions.

So at startup, the `ValidationManager` finds and resumes any
unfinished operations, by looking for unshared messages with shared
dependents.

Closes #589

See merge request !325

briar-api/src/org/briarproject/api/clients/ClientHelper.java

@@ -68,12 +68,6 @@ public interface ClientHelper {
 	void mergeMessageMetadata(Transaction txn, MessageId m,
 			BdfDictionary metadata) throws DbException, FormatException;
 
-	/**
-	 * Marks the given message as shared or unshared with other contacts.
-	 */
-	void setMessageShared(Transaction txn, MessageId m, boolean shared)
-			throws DbException;
-
 	byte[] toByteArray(BdfDictionary dictionary) throws FormatException;
 
 	byte[] toByteArray(BdfList list) throws FormatException;

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

@@ -258,6 +258,15 @@ public interface DatabaseComponent {
 	Collection<MessageId> getPendingMessages(Transaction txn, ClientId c)
 			throws DbException;
 
+	/**
+	 * Returns the IDs of any messages from the given client
+	 * that have a shared dependent, but are still not shared themselves.
+	 * <p/>
+	 * Read-only.
+	 */
+	Collection<MessageId> getMessagesToShare(Transaction txn,
+			ClientId c) throws DbException;
+
 	/**
 	 * Returns the message with the given ID, in serialised form, or null if
 	 * the message has been deleted.
@@ -456,10 +465,9 @@ public interface DatabaseComponent {
 			throws DbException;
 
 	/**
-	 * Marks the given message as shared or unshared.
+	 * Marks the given message as shared.
 	 */
-	void setMessageShared(Transaction txn, MessageId m, boolean shared)
-			throws DbException;
+	void setMessageShared(Transaction txn, MessageId m) throws DbException;
 
 	/**
 	 * Sets the validation and delivery state of the given message.

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

@@ -55,8 +55,9 @@ public interface ValidationManager {
 
 		/**
 		 * Called once for each incoming message that passes validation.
+		 * @return whether or not this message should be shared
 		 */
-		void incomingMessage(Transaction txn, Message m, Metadata meta)
+		boolean incomingMessage(Transaction txn, Message m, Metadata meta)
 				throws DbException;
 	}
 }

briar-core/src/org/briarproject/blogs/BlogManagerImpl.java

@@ -176,7 +176,7 @@ class BlogManagerImpl extends BdfIncomingMessageHook implements BlogManager,
 	}
 
 	@Override
-	protected void incomingMessage(Transaction txn, Message m, BdfList list,
+	protected boolean incomingMessage(Transaction txn, Message m, BdfList list,
 			BdfDictionary meta) throws DbException, FormatException {
 
 		GroupId groupId = m.getGroupId();
@@ -196,13 +196,14 @@ class BlogManagerImpl extends BdfIncomingMessageHook implements BlogManager,
 					throw new FormatException();
 				}
 			}
-			// share dependencies recursively - TODO remove with #598
-			share(txn, h);
 
 			// broadcast event about new post or comment
 			BlogPostAddedEvent event =
 					new BlogPostAddedEvent(groupId, h, false);
 			txn.attach(event);
+
+			// shares message and its dependencies
+			return true;
 		} else if (type == WRAPPED_COMMENT) {
 			// Check that the original message ID in the dependency's metadata
 			// matches the original parent ID of the wrapped comment
@@ -216,6 +217,8 @@ class BlogManagerImpl extends BdfIncomingMessageHook implements BlogManager,
 				throw new FormatException();
 			}
 		}
+		// don't share message until parent arrives
+		return false;
 	}
 
 	@Override
@@ -672,14 +675,4 @@ class BlogManagerImpl extends BdfIncomingMessageHook implements BlogManager,
 		Long longType = d.getLong(KEY_TYPE);
 		return MessageType.valueOf(longType.intValue());
 	}
-
-	// TODO remove when implementing #589
-	@Deprecated
-	private void share(Transaction txn, BlogPostHeader h) throws DbException {
-		clientHelper.setMessageShared(txn, h.getId(), true);
-		if (h instanceof BlogCommentHeader) {
-			BlogPostHeader h2 = ((BlogCommentHeader) h).getParent();
-			share(txn, h2);
-		}
-	}
 }

briar-core/src/org/briarproject/clients/BdfIncomingMessageHook.java

@@ -28,14 +28,14 @@ public abstract class BdfIncomingMessageHook implements IncomingMessageHook,
 		this.metadataParser = metadataParser;
 	}
 
-	protected abstract void incomingMessage(Transaction txn, Message m,
+	protected abstract boolean incomingMessage(Transaction txn, Message m,
 			BdfList body, BdfDictionary meta) throws DbException,
 			FormatException;
 
 	@Override
-	public void incomingMessage(Transaction txn, Message m, Metadata meta)
+	public boolean incomingMessage(Transaction txn, Message m, Metadata meta)
 			throws DbException {
-		incomingMessage(txn, m, meta, MESSAGE_HEADER_LENGTH);
+		return incomingMessage(txn, m, meta, MESSAGE_HEADER_LENGTH);
 	}
 
 	@Override
@@ -44,14 +44,14 @@ public abstract class BdfIncomingMessageHook implements IncomingMessageHook,
 		incomingMessage(txn, q, meta, QUEUE_MESSAGE_HEADER_LENGTH);
 	}
 
-	private void incomingMessage(Transaction txn, Message m, Metadata meta,
+	private boolean incomingMessage(Transaction txn, Message m, Metadata meta,
 			int headerLength) throws DbException {
 		try {
 			byte[] raw = m.getRaw();
 			BdfList body = clientHelper.toList(raw, headerLength,
 					raw.length - headerLength);
 			BdfDictionary metaDictionary = metadataParser.parse(meta);
-			incomingMessage(txn, m, body, metaDictionary);
+			return incomingMessage(txn, m, body, metaDictionary);
 		} catch (FormatException e) {
 			throw new DbException(e);
 		}

briar-core/src/org/briarproject/clients/ClientHelperImpl.java

@@ -245,12 +245,6 @@ class ClientHelperImpl implements ClientHelper {
 		db.mergeMessageMetadata(txn, m, metadataEncoder.encode(metadata));
 	}
 
-	@Override
-	public void setMessageShared(Transaction txn, MessageId m, boolean shared)
-			throws DbException {
-		db.setMessageShared(txn, m, shared);
-	}
-
 	@Override
 	public byte[] toByteArray(BdfDictionary dictionary) throws FormatException {
 		ByteArrayOutputStream out = new ByteArrayOutputStream();

briar-core/src/org/briarproject/clients/MessageQueueManagerImpl.java

@@ -188,12 +188,12 @@ class MessageQueueManagerImpl implements MessageQueueManager {
 
 		private final IncomingQueueMessageHook delegate;
 
-		DelegatingIncomingMessageHook(IncomingQueueMessageHook delegate) {
+		private DelegatingIncomingMessageHook(IncomingQueueMessageHook delegate) {
 			this.delegate = delegate;
 		}
 
 		@Override
-		public void incomingMessage(Transaction txn, Message m, Metadata meta)
+		public boolean incomingMessage(Transaction txn, Message m, Metadata meta)
 				throws DbException {
 			long queuePosition = ByteUtils.readUint64(m.getRaw(),
 					MESSAGE_HEADER_LENGTH);
@@ -239,6 +239,9 @@ class MessageQueueManagerImpl implements MessageQueueManager {
 					delegate.incomingMessage(txn, q, meta);
 				}
 			}
+			// message queues are only useful for groups with two members
+			// so messages don't need to be shared
+			return false;
 		}
 	}
 }

briar-core/src/org/briarproject/db/Database.java

@@ -434,6 +434,15 @@ interface Database<T> {
 	Collection<MessageId> getPendingMessages(T txn, ClientId c)
 			throws DbException;
 
+	/**
+	 * Returns the IDs of any messages from the given client
+	 * that have a shared dependent, but are still not shared themselves.
+	 * <p/>
+	 * Read-only.
+	 */
+	Collection<MessageId> getMessagesToShare(T txn, ClientId c)
+			throws DbException;
+
 	/**
 	 * Returns the message with the given ID, in serialised form, or null if
 	 * the message has been deleted.
@@ -599,10 +608,9 @@ interface Database<T> {
 			throws DbException;
 
 	/**
-	 * Marks the given message as shared or unshared.
+	 * Marks the given message as shared.
 	 */
-	void setMessageShared(T txn, MessageId m, boolean shared)
-			throws DbException;
+	void setMessageShared(T txn, MessageId m) throws DbException;
 
 	/**
 	 * Sets the validation and delivery state of the given message.

briar-core/src/org/briarproject/db/DatabaseComponentImpl.java

@@ -94,6 +94,7 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		this.shutdown = shutdown;
 	}
 
+	@Override
 	public boolean open() throws DbException {
 		Runnable shutdownHook = new Runnable() {
 			public void run() {
@@ -110,12 +111,14 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		return reopened;
 	}
 
+	@Override
 	public void close() throws DbException {
 		if (closed.getAndSet(true)) return;
 		shutdown.removeShutdownHook(shutdownHandle);
 		db.close();
 	}
 
+	@Override
 	public Transaction startTransaction(boolean readOnly) throws DbException {
 		// Don't allow reentrant locking
 		if (lock.getReadHoldCount() > 0) throw new IllegalStateException();
@@ -135,6 +138,7 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		}
 	}
 
+	@Override
 	public void endTransaction(Transaction transaction) throws DbException {
 		try {
 			T txn = txnClass.cast(transaction.unbox());
@@ -153,6 +157,7 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		return txnClass.cast(transaction.unbox());
 	}
 
+	@Override
 	public ContactId addContact(Transaction transaction, Author remote,
 			AuthorId local, boolean verified, boolean active)
 			throws DbException {
@@ -170,6 +175,7 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		return c;
 	}
 
+	@Override
 	public void addGroup(Transaction transaction, Group g) throws DbException {
 		if (transaction.isReadOnly()) throw new IllegalArgumentException();
 		T txn = unbox(transaction);
@@ -179,6 +185,7 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		}
 	}
 
+	@Override
 	public void addLocalAuthor(Transaction transaction, LocalAuthor a)
 			throws DbException {
 		if (transaction.isReadOnly()) throw new IllegalArgumentException();
@@ -189,6 +196,7 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		}
 	}
 
+	@Override
 	public void addLocalMessage(Transaction transaction, Message m,
 			Metadata meta, boolean shared) throws DbException {
 		if (transaction.isReadOnly()) throw new IllegalArgumentException();
@@ -215,6 +223,7 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		}
 	}
 
+	@Override
 	public void addTransport(Transaction transaction, TransportId t,
 			int maxLatency) throws DbException {
 		if (transaction.isReadOnly()) throw new IllegalArgumentException();
@@ -223,6 +232,7 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 			db.addTransport(txn, t, maxLatency);
 	}
 
+	@Override
 	public void addTransportKeys(Transaction transaction, ContactId c,
 			TransportKeys k) throws DbException {
 		if (transaction.isReadOnly()) throw new IllegalArgumentException();
@@ -234,6 +244,7 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		db.addTransportKeys(txn, c, k);
 	}
 
+	@Override
 	public boolean containsContact(Transaction transaction, AuthorId remote,
 			AuthorId local) throws DbException {
 		T txn = unbox(transaction);
@@ -242,6 +253,7 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		return db.containsContact(txn, remote, local);
 	}
 
+	@Override
 	public boolean containsGroup(Transaction transaction, GroupId g)
 			throws DbException {
 		T txn = unbox(transaction);
@@ -255,6 +267,7 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		return db.containsLocalAuthor(txn, local);
 	}
 
+	@Override
 	public void deleteMessage(Transaction transaction, MessageId m)
 			throws DbException {
 		if (transaction.isReadOnly()) throw new IllegalArgumentException();
@@ -264,6 +277,7 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		db.deleteMessage(txn, m);
 	}
 
+	@Override
 	public void deleteMessageMetadata(Transaction transaction, MessageId m)
 			throws DbException {
 		if (transaction.isReadOnly()) throw new IllegalArgumentException();
@@ -274,6 +288,7 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 	}
 
 	@Nullable
+	@Override
 	public Ack generateAck(Transaction transaction, ContactId c,
 			int maxMessages) throws DbException {
 		if (transaction.isReadOnly()) throw new IllegalArgumentException();
@@ -287,6 +302,7 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 	}
 
 	@Nullable
+	@Override
 	public Collection<byte[]> generateBatch(Transaction transaction,
 			ContactId c, int maxLength, int maxLatency) throws DbException {
 		if (transaction.isReadOnly()) throw new IllegalArgumentException();
@@ -306,6 +322,7 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 	}
 
 	@Nullable
+	@Override
 	public Offer generateOffer(Transaction transaction, ContactId c,
 			int maxMessages, int maxLatency) throws DbException {
 		if (transaction.isReadOnly()) throw new IllegalArgumentException();
@@ -319,6 +336,7 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 	}
 
 	@Nullable
+	@Override
 	public Request generateRequest(Transaction transaction, ContactId c,
 			int maxMessages) throws DbException {
 		if (transaction.isReadOnly()) throw new IllegalArgumentException();
@@ -333,6 +351,7 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 	}
 
 	@Nullable
+	@Override
 	public Collection<byte[]> generateRequestedBatch(Transaction transaction,
 			ContactId c, int maxLength, int maxLatency) throws DbException {
 		if (transaction.isReadOnly()) throw new IllegalArgumentException();
@@ -352,6 +371,7 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		return Collections.unmodifiableList(messages);
 	}
 
+	@Override
 	public Contact getContact(Transaction transaction, ContactId c)
 			throws DbException {
 		T txn = unbox(transaction);
@@ -360,18 +380,21 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		return db.getContact(txn, c);
 	}
 
+	@Override
 	public Collection<Contact> getContacts(Transaction transaction)
 			throws DbException {
 		T txn = unbox(transaction);
 		return db.getContacts(txn);
 	}
 
+	@Override
 	public Collection<Contact> getContactsByAuthorId(Transaction transaction,
 			AuthorId remote) throws DbException {
 		T txn = unbox(transaction);
 		return db.getContactsByAuthorId(txn, remote);
 	}
 
+	@Override
 	public Collection<ContactId> getContacts(Transaction transaction,
 			AuthorId a) throws DbException {
 		T txn = unbox(transaction);
@@ -380,11 +403,13 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		return db.getContacts(txn, a);
 	}
 
+	@Override
 	public DeviceId getDeviceId(Transaction transaction) throws DbException {
 		T txn = unbox(transaction);
 		return db.getDeviceId(txn);
 	}
 
+	@Override
 	public Group getGroup(Transaction transaction, GroupId g)
 			throws DbException {
 		T txn = unbox(transaction);
@@ -393,6 +418,7 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		return db.getGroup(txn, g);
 	}
 
+	@Override
 	public Metadata getGroupMetadata(Transaction transaction, GroupId g)
 			throws DbException {
 		T txn = unbox(transaction);
@@ -401,12 +427,14 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		return db.getGroupMetadata(txn, g);
 	}
 
+	@Override
 	public Collection<Group> getGroups(Transaction transaction, ClientId c)
 			throws DbException {
 		T txn = unbox(transaction);
 		return db.getGroups(txn, c);
 	}
 
+	@Override
 	public LocalAuthor getLocalAuthor(Transaction transaction, AuthorId a)
 			throws DbException {
 		T txn = unbox(transaction);
@@ -415,25 +443,36 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		return db.getLocalAuthor(txn, a);
 	}
 
+	@Override
 	public Collection<LocalAuthor> getLocalAuthors(Transaction transaction)
 			throws DbException {
 		T txn = unbox(transaction);
 		return db.getLocalAuthors(txn);
 	}
 
+	@Override
 	public Collection<MessageId> getMessagesToValidate(Transaction transaction,
 			ClientId c) throws DbException {
 		T txn = unbox(transaction);
 		return db.getMessagesToValidate(txn, c);
 	}
 
+	@Override
 	public Collection<MessageId> getPendingMessages(Transaction transaction,
 			ClientId c) throws DbException {
 		T txn = unbox(transaction);
 		return db.getPendingMessages(txn, c);
 	}
 
+	@Override
+	public Collection<MessageId> getMessagesToShare(
+			Transaction transaction, ClientId c) throws DbException {
+		T txn = unbox(transaction);
+		return db.getMessagesToShare(txn, c);
+	}
+
 	@Nullable
+	@Override
 	public byte[] getRawMessage(Transaction transaction, MessageId m)
 			throws DbException {
 		T txn = unbox(transaction);
@@ -442,6 +481,7 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		return db.getRawMessage(txn, m);
 	}
 
+	@Override
 	public Map<MessageId, Metadata> getMessageMetadata(Transaction transaction,
 			GroupId g) throws DbException {
 		T txn = unbox(transaction);
@@ -450,6 +490,7 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		return db.getMessageMetadata(txn, g);
 	}
 
+	@Override
 	public Map<MessageId, Metadata> getMessageMetadata(Transaction transaction,
 			GroupId g, Metadata query) throws DbException {
 		T txn = unbox(transaction);
@@ -458,6 +499,7 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		return db.getMessageMetadata(txn, g, query);
 	}
 
+	@Override
 	public Metadata getMessageMetadata(Transaction transaction, MessageId m)
 			throws DbException {
 		T txn = unbox(transaction);
@@ -466,6 +508,7 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		return db.getMessageMetadata(txn, m);
 	}
 
+	@Override
 	public Metadata getMessageMetadataForValidator(Transaction transaction,
 			MessageId m)
 			throws DbException {
@@ -475,6 +518,7 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		return db.getMessageMetadataForValidator(txn, m);
 	}
 
+	@Override
 	public State getMessageState(Transaction transaction, MessageId m)
 			throws DbException {
 		T txn = unbox(transaction);
@@ -483,6 +527,7 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		return db.getMessageState(txn, m);
 	}
 
+	@Override
 	public Collection<MessageStatus> getMessageStatus(Transaction transaction,
 			ContactId c, GroupId g) throws DbException {
 		T txn = unbox(transaction);
@@ -493,6 +538,7 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		return db.getMessageStatus(txn, c, g);
 	}
 
+	@Override
 	public MessageStatus getMessageStatus(Transaction transaction, ContactId c,
 			MessageId m) throws DbException {
 		T txn = unbox(transaction);
@@ -503,6 +549,7 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		return db.getMessageStatus(txn, c, m);
 	}
 
+	@Override
 	public Map<MessageId, State> getMessageDependencies(Transaction transaction,
 			MessageId m) throws DbException {
 		T txn = unbox(transaction);
@@ -511,6 +558,7 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		return db.getMessageDependencies(txn, m);
 	}
 
+	@Override
 	public Map<MessageId, State> getMessageDependents(Transaction transaction,
 			MessageId m) throws DbException {
 		T txn = unbox(transaction);
@@ -519,12 +567,14 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		return db.getMessageDependents(txn, m);
 	}
 
+	@Override
 	public Settings getSettings(Transaction transaction, String namespace)
 			throws DbException {
 		T txn = unbox(transaction);
 		return db.getSettings(txn, namespace);
 	}
 
+	@Override
 	public Map<ContactId, TransportKeys> getTransportKeys(
 			Transaction transaction, TransportId t) throws DbException {
 		T txn = unbox(transaction);
@@ -533,6 +583,7 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		return db.getTransportKeys(txn, t);
 	}
 
+	@Override
 	public void incrementStreamCounter(Transaction transaction, ContactId c,
 			TransportId t, long rotationPeriod) throws DbException {
 		if (transaction.isReadOnly()) throw new IllegalArgumentException();
@@ -544,6 +595,7 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		db.incrementStreamCounter(txn, c, t, rotationPeriod);
 	}
 
+	@Override
 	public boolean isVisibleToContact(Transaction transaction, ContactId c,
 			GroupId g) throws DbException {
 		T txn = unbox(transaction);
@@ -554,6 +606,7 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		return db.containsVisibleGroup(txn, c, g);
 	}
 
+	@Override
 	public void mergeGroupMetadata(Transaction transaction, GroupId g,
 			Metadata meta) throws DbException {
 		if (transaction.isReadOnly()) throw new IllegalArgumentException();
@@ -563,6 +616,7 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		db.mergeGroupMetadata(txn, g, meta);
 	}
 
+	@Override
 	public void mergeMessageMetadata(Transaction transaction, MessageId m,
 			Metadata meta) throws DbException {
 		if (transaction.isReadOnly()) throw new IllegalArgumentException();
@@ -572,6 +626,7 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		db.mergeMessageMetadata(txn, m, meta);
 	}
 
+	@Override
 	public void mergeSettings(Transaction transaction, Settings s,
 			String namespace) throws DbException {
 		if (transaction.isReadOnly()) throw new IllegalArgumentException();
@@ -586,6 +641,7 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		}
 	}
 
+	@Override
 	public void receiveAck(Transaction transaction, ContactId c, Ack a)
 			throws DbException {
 		if (transaction.isReadOnly()) throw new IllegalArgumentException();
@@ -602,6 +658,7 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		transaction.attach(new MessagesAckedEvent(c, acked));
 	}
 
+	@Override
 	public void receiveMessage(Transaction transaction, ContactId c, Message m)
 			throws DbException {
 		if (transaction.isReadOnly()) throw new IllegalArgumentException();
@@ -620,6 +677,7 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		}
 	}
 
+	@Override
 	public void receiveOffer(Transaction transaction, ContactId c, Offer o)
 			throws DbException {
 		if (transaction.isReadOnly()) throw new IllegalArgumentException();
@@ -643,6 +701,7 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		if (request) transaction.attach(new MessageToRequestEvent(c));
 	}
 
+	@Override
 	public void receiveRequest(Transaction transaction, ContactId c, Request r)
 			throws DbException {
 		if (transaction.isReadOnly()) throw new IllegalArgumentException();
@@ -660,6 +719,7 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		if (requested) transaction.attach(new MessageRequestedEvent(c));
 	}
 
+	@Override
 	public void removeContact(Transaction transaction, ContactId c)
 			throws DbException {
 		if (transaction.isReadOnly()) throw new IllegalArgumentException();
@@ -670,6 +730,7 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		transaction.attach(new ContactRemovedEvent(c));
 	}
 
+	@Override
 	public void removeGroup(Transaction transaction, Group g)
 			throws DbException {
 		if (transaction.isReadOnly()) throw new IllegalArgumentException();
@@ -683,6 +744,7 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		transaction.attach(new GroupVisibilityUpdatedEvent(affected));
 	}
 
+	@Override
 	public void removeLocalAuthor(Transaction transaction, AuthorId a)
 			throws DbException {
 		if (transaction.isReadOnly()) throw new IllegalArgumentException();
@@ -693,6 +755,7 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		transaction.attach(new LocalAuthorRemovedEvent(a));
 	}
 
+	@Override
 	public void removeTransport(Transaction transaction, TransportId t)
 			throws DbException {
 		if (transaction.isReadOnly()) throw new IllegalArgumentException();
@@ -702,6 +765,7 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		db.removeTransport(txn, t);
 	}
 
+	@Override
 	public void setContactVerified(Transaction transaction, ContactId c)
 			throws DbException {
 		if (transaction.isReadOnly()) throw new IllegalArgumentException();
@@ -712,6 +776,7 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		transaction.attach(new ContactVerifiedEvent(c));
 	}
 
+	@Override
 	public void setContactActive(Transaction transaction, ContactId c,
 			boolean active) throws DbException {
 		if (transaction.isReadOnly()) throw new IllegalArgumentException();
@@ -722,16 +787,20 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		transaction.attach(new ContactStatusChangedEvent(c, active));
 	}
 
-	public void setMessageShared(Transaction transaction, MessageId m,
-			boolean shared) throws DbException {
+	@Override
+	public void setMessageShared(Transaction transaction, MessageId m)
+			throws DbException {
 		if (transaction.isReadOnly()) throw new IllegalArgumentException();
 		T txn = unbox(transaction);
 		if (!db.containsMessage(txn, m))
 			throw new NoSuchMessageException();
-		db.setMessageShared(txn, m, shared);
-		if (shared) transaction.attach(new MessageSharedEvent(m));
+		if (db.getMessageState(txn, m) != DELIVERED)
+			throw new IllegalArgumentException("Shared undelivered message");
+		db.setMessageShared(txn, m);
+		transaction.attach(new MessageSharedEvent(m));
 	}
 
+	@Override
 	public void setMessageState(Transaction transaction, MessageId m,
 			State state) throws DbException {
 		if (transaction.isReadOnly()) throw new IllegalArgumentException();
@@ -742,6 +811,7 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		transaction.attach(new MessageStateChangedEvent(m, false, state));
 	}
 
+	@Override
 	public void addMessageDependencies(Transaction transaction,
 			Message dependent, Collection<MessageId> dependencies)
 			throws DbException {
@@ -755,6 +825,7 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		}
 	}
 
+	@Override
 	public void setReorderingWindow(Transaction transaction, ContactId c,
 			TransportId t, long rotationPeriod, long base, byte[] bitmap)
 			throws DbException {
@@ -767,6 +838,7 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		db.setReorderingWindow(txn, c, t, rotationPeriod, base, bitmap);
 	}
 
+	@Override
 	public void setVisibleToContact(Transaction transaction, ContactId c,
 			GroupId g, boolean visible) throws DbException {
 		if (transaction.isReadOnly()) throw new IllegalArgumentException();
@@ -793,6 +865,7 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		}
 	}
 
+	@Override
 	public void updateTransportKeys(Transaction transaction,
 			Map<ContactId, TransportKeys> keys) throws DbException {
 		if (transaction.isReadOnly()) throw new IllegalArgumentException();

briar-core/src/org/briarproject/db/JdbcDatabase.java

@@ -1692,6 +1692,35 @@ abstract class JdbcDatabase implements Database<Connection> {
 		}
 	}
 
+	public Collection<MessageId> getMessagesToShare(
+			Connection txn, ClientId c) throws DbException {
+		PreparedStatement ps = null;
+		ResultSet rs = null;
+		try {
+			String sql = "SELECT m.messageId FROM messages AS m"
+					+ " JOIN messageDependencies AS d"
+					+ " ON m.messageId = d.dependencyId"
+					+ " JOIN messages AS m1"
+					+ " ON d.messageId = m1.messageId"
+					+ " JOIN groups AS g"
+					+ " ON m.groupId = g.groupId"
+					+ " WHERE m.shared = FALSE AND m1.shared = TRUE"
+					+ " AND g.clientId = ?";
+			ps = txn.prepareStatement(sql);
+			ps.setBytes(1, c.getBytes());
+			rs = ps.executeQuery();
+			List<MessageId> ids = new ArrayList<MessageId>();
+			while (rs.next()) ids.add(new MessageId(rs.getBytes(1)));
+			rs.close();
+			ps.close();
+			return Collections.unmodifiableList(ids);
+		} catch (SQLException e) {
+			tryToClose(rs);
+			tryToClose(ps);
+			throw new DbException(e);
+		}
+	}
+
 	@Nullable
 	public byte[] getRawMessage(Connection txn, MessageId m)
 			throws DbException {
@@ -2321,14 +2350,13 @@ abstract class JdbcDatabase implements Database<Connection> {
 		}
 	}
 
-	public void setMessageShared(Connection txn, MessageId m, boolean shared)
-			throws DbException {
+	public void setMessageShared(Connection txn, MessageId m) throws DbException {
 		PreparedStatement ps = null;
 		try {
-			String sql = "UPDATE messages SET shared = ? WHERE messageId = ?";
+			String sql = "UPDATE messages SET shared = TRUE"
+					+ " WHERE messageId = ?";
 			ps = txn.prepareStatement(sql);
-			ps.setBoolean(1, shared);
-			ps.setBytes(2, m.getBytes());
+			ps.setBytes(1, m.getBytes());
 			int affected = ps.executeUpdate();
 			if (affected < 0 || affected > 1) throw new DbStateException();
 			ps.close();

briar-core/src/org/briarproject/forum/ForumManagerImpl.java

@@ -73,15 +73,16 @@ class ForumManagerImpl extends BdfIncomingMessageHook implements ForumManager {
 	}
 
 	@Override
-	protected void incomingMessage(Transaction txn, Message m, BdfList body,
+	protected boolean incomingMessage(Transaction txn, Message m, BdfList body,
 			BdfDictionary meta) throws DbException, FormatException {
 
-		clientHelper.setMessageShared(txn, m.getId(), true);
-
 		ForumPostHeader post = getForumPostHeader(txn, m.getId(), meta);
 		ForumPostReceivedEvent event =
 				new ForumPostReceivedEvent(post, m.getGroupId());
 		txn.attach(event);
+
+		// share message
+		return true;
 	}
 
 	@Override

briar-core/src/org/briarproject/introduction/IntroductionManagerImpl.java

@@ -207,7 +207,7 @@ class IntroductionManagerImpl extends BdfIncomingMessageHook
 	 * in the introduction protocol and which engine we need to start.
 	 */
 	@Override
-	protected void incomingMessage(Transaction txn, Message m, BdfList body,
+	protected boolean incomingMessage(Transaction txn, Message m, BdfList body,
 			BdfDictionary message)	throws DbException {
 
 		// Get message data and type
@@ -233,7 +233,7 @@ class IntroductionManagerImpl extends BdfIncomingMessageHook
 					LOG.log(WARNING, e.toString(), e);
 				}
 				deleteMessage(txn, m.getId());
-				return;
+				return false;
 			}
 			try {
 				introduceeManager.incomingMessage(txn, state, message);
@@ -254,7 +254,7 @@ class IntroductionManagerImpl extends BdfIncomingMessageHook
 			} catch (FormatException e) {
 				LOG.warning("Could not find state for message, deleting...");
 				deleteMessage(txn, m.getId());
-				return;
+				return false;
 			}
 
 			long role = state.getLong(ROLE, -1L);
@@ -285,6 +285,7 @@ class IntroductionManagerImpl extends BdfIncomingMessageHook
 				LOG.warning("Unknown message type '" + type + "', deleting...");
 			}
 		}
+		return false;
 	}
 
 	@Override

briar-core/src/org/briarproject/messaging/MessagingManagerImpl.java

@@ -93,7 +93,7 @@ class MessagingManagerImpl extends BdfIncomingMessageHook
 	}
 
 	@Override
-	protected void incomingMessage(Transaction txn, Message m, BdfList body,
+	protected boolean incomingMessage(Transaction txn, Message m, BdfList body,
 			BdfDictionary meta) throws DbException, FormatException {
 
 		GroupId groupId = m.getGroupId();
@@ -106,6 +106,9 @@ class MessagingManagerImpl extends BdfIncomingMessageHook
 		PrivateMessageReceivedEvent event = new PrivateMessageReceivedEvent(
 				header, groupId);
 		txn.attach(event);
+
+		// don't share message
+		return false;
 	}
 
 	@Override

briar-core/src/org/briarproject/sharing/SharingManagerImpl.java

@@ -192,7 +192,7 @@ abstract class SharingManagerImpl<S extends Shareable, I extends Invitation, IS
 	}
 
 	@Override
-	protected void incomingMessage(Transaction txn, Message m, BdfList body,
+	protected boolean incomingMessage(Transaction txn, Message m, BdfList body,
 			BdfDictionary d) throws DbException, FormatException {
 
 		BaseMessage msg = BaseMessage.from(getIFactory(), m.getGroupId(), d);
@@ -263,6 +263,8 @@ abstract class SharingManagerImpl<S extends Shareable, I extends Invitation, IS
 			// message has passed validator, so that should never happen
 			throw new RuntimeException("Illegal Sharing Message");
 		}
+		// don't share message as other party already has it
+		return false;
 	}
 
 	@Override

briar-core/src/org/briarproject/sync/ValidationManagerImpl.java

@@ -72,6 +72,7 @@ class ValidationManagerImpl implements ValidationManager, Service,
 		for (ClientId c : validators.keySet()) {
 			validateOutstandingMessagesAsync(c);
 			deliverOutstandingMessagesAsync(c);
+			shareOutstandingMessagesAsync(c);
 		}
 	}
 
@@ -191,6 +192,7 @@ class ValidationManagerImpl implements ValidationManager, Service,
 	private void deliverNextPendingMessage(Queue<MessageId> pending) {
 		try {
 			boolean anyInvalid = false, allDelivered = true;
+			Queue<MessageId> toShare = null;
 			Queue<MessageId> invalidate = null;
 			Transaction txn = db.startTransaction(false);
 			try {
@@ -213,8 +215,14 @@ class ValidationManagerImpl implements ValidationManager, Service,
 						ClientId c = g.getClientId();
 						Metadata meta = db.getMessageMetadataForValidator(txn,
 								id);
-						if (deliverMessage(txn, m, c, meta)) {
+						DeliveryResult result = deliverMessage(txn, m, c, meta);
+						if (result.valid) {
 							pending.addAll(getPendingDependents(txn, id));
+							if (result.share) {
+								db.setMessageShared(txn, id);
+								toShare = new LinkedList<MessageId>(
+										states.keySet());
+							}
 						} else {
 							invalidate = getDependentsToInvalidate(txn, id);
 						}
@@ -226,6 +234,7 @@ class ValidationManagerImpl implements ValidationManager, Service,
 			}
 			if (invalidate != null) invalidateNextMessageAsync(invalidate);
 			deliverNextPendingMessageAsync(pending);
+			if (toShare != null) shareNextMessageAsync(toShare);
 		} catch (NoSuchMessageException e) {
 			LOG.info("Message removed before delivery");
 			deliverNextPendingMessageAsync(pending);
@@ -284,16 +293,17 @@ class ValidationManagerImpl implements ValidationManager, Service,
 	}
 
 	private void storeMessageContext(Message m, ClientId c,
-			MessageContext result) {
+			MessageContext context) {
 		try {
 			MessageId id = m.getId();
 			boolean anyInvalid = false, allDelivered = true;
 			Queue<MessageId> invalidate = null;
 			Queue<MessageId> pending = null;
+			Queue<MessageId> toShare = null;
 			Transaction txn = db.startTransaction(false);
 			try {
 				// Check if message has any dependencies
-				Collection<MessageId> dependencies = result.getDependencies();
+				Collection<MessageId> dependencies = context.getDependencies();
 				if (!dependencies.isEmpty()) {
 					db.addMessageDependencies(txn, m, dependencies);
 					// Check if dependencies are valid and delivered
@@ -310,11 +320,17 @@ class ValidationManagerImpl implements ValidationManager, Service,
 						invalidate = getDependentsToInvalidate(txn, id);
 					}
 				} else {
-					Metadata meta = result.getMetadata();
+					Metadata meta = context.getMetadata();
 					db.mergeMessageMetadata(txn, id, meta);
 					if (allDelivered) {
-						if (deliverMessage(txn, m, c, meta)) {
+						DeliveryResult result = deliverMessage(txn, m, c, meta);
+						if (result.valid) {
 							pending = getPendingDependents(txn, id);
+							if (result.share) {
+								db.setMessageShared(txn, id);
+								toShare =
+										new LinkedList<MessageId>(dependencies);
+							}
 						} else {
 							invalidate = getDependentsToInvalidate(txn, id);
 						}
@@ -328,6 +344,7 @@ class ValidationManagerImpl implements ValidationManager, Service,
 			}
 			if (invalidate != null) invalidateNextMessageAsync(invalidate);
 			if (pending != null) deliverNextPendingMessageAsync(pending);
+			if (toShare != null) shareNextMessageAsync(toShare);
 		} catch (NoSuchMessageException e) {
 			LOG.info("Message removed during validation");
 		} catch (NoSuchGroupException e) {
@@ -337,18 +354,21 @@ class ValidationManagerImpl implements ValidationManager, Service,
 		}
 	}
 
-	private boolean deliverMessage(Transaction txn, Message m, ClientId c,
-			Metadata meta) throws DbException {
+	private DeliveryResult deliverMessage(Transaction txn, Message m,
+			ClientId c, Metadata meta) throws DbException {
 		// Deliver the message to the client if it's registered a hook
+		boolean shareMsg = false;
 		IncomingMessageHook hook = hooks.get(c);
-		if (hook != null) hook.incomingMessage(txn, m, meta);
+		if (hook != null) {
+			shareMsg = hook.incomingMessage(txn, m, meta);
+		}
 		// TODO: Find a better way for clients to signal validity, #643
 		if (db.getRawMessage(txn, m.getId()) == null) {
 			db.setMessageState(txn, m.getId(), INVALID);
-			return false;
+			return new DeliveryResult(false, false);
 		} else {
 			db.setMessageState(txn, m.getId(), DELIVERED);
-			return true;
+			return new DeliveryResult(true, shareMsg);
 		}
 	}
 
@@ -362,6 +382,70 @@ class ValidationManagerImpl implements ValidationManager, Service,
 		return pending;
 	}
 
+	private void shareOutstandingMessagesAsync(final ClientId c) {
+		dbExecutor.execute(new Runnable() {
+			@Override
+			public void run() {
+				shareOutstandingMessages(c);
+			}
+		});
+	}
+
+	private void shareOutstandingMessages(ClientId c) {
+		try {
+			Queue<MessageId> toShare = new LinkedList<MessageId>();
+			Transaction txn = db.startTransaction(true);
+			try {
+				toShare.addAll(db.getMessagesToShare(txn, c));
+				txn.setComplete();
+			} finally {
+				db.endTransaction(txn);
+			}
+			shareNextMessageAsync(toShare);
+		} catch (DbException e) {
+			if (LOG.isLoggable(WARNING)) LOG.log(WARNING, e.toString(), e);
+		}
+	}
+
+	/**
+	 * Shares the next message from the toShare queue asynchronously.
+	 *
+	 * This method should only be called for messages that have all their
+	 * dependencies delivered and have been delivered themselves.
+	 */
+	private void shareNextMessageAsync(final Queue<MessageId> toShare) {
+		if (toShare.isEmpty()) return;
+		dbExecutor.execute(new Runnable() {
+			@Override
+			public void run() {
+				shareNextMessage(toShare);
+			}
+		});
+	}
+
+	private void shareNextMessage(Queue<MessageId> toShare) {
+		try {
+			Transaction txn = db.startTransaction(false);
+			try {
+				MessageId id = toShare.poll();
+				db.setMessageShared(txn, id);
+				toShare.addAll(db.getMessageDependencies(txn, id).keySet());
+				txn.setComplete();
+			} finally {
+				db.endTransaction(txn);
+			}
+			shareNextMessageAsync(toShare);
+		} catch (NoSuchMessageException e) {
+			LOG.info("Message removed before sharing");
+			shareNextMessageAsync(toShare);
+		} catch (NoSuchGroupException e) {
+			LOG.info("Group removed before sharing");
+			shareNextMessageAsync(toShare);
+		} catch (DbException e) {
+			if (LOG.isLoggable(WARNING)) LOG.log(WARNING, e.toString(), e);
+		}
+	}
+
 	private void invalidateNextMessageAsync(final Queue<MessageId> invalidate) {
 		if (invalidate.isEmpty()) return;
 		dbExecutor.execute(new Runnable() {
@@ -447,4 +531,13 @@ class ValidationManagerImpl implements ValidationManager, Service,
 			if (LOG.isLoggable(WARNING)) LOG.log(WARNING, e.toString(), e);
 		}
 	}
+
+	private static class DeliveryResult {
+		private final boolean valid, share;
+
+		private DeliveryResult(boolean valid, boolean share) {
+			this.valid = valid;
+			this.share = share;
+		}
+	}
 }

briar-tests/src/org/briarproject/blogs/BlogManagerImplTest.java

@@ -199,7 +199,6 @@ public class BlogManagerImplTest extends BriarTestCase {
 		);
 
 		context.checking(new Expectations() {{
-			oneOf(clientHelper).setMessageShared(txn, messageId, true);
 			oneOf(identityManager)
 					.getAuthorStatus(txn, blog1.getAuthor().getId());
 			will(returnValue(VERIFIED));

briar-tests/src/org/briarproject/db/DatabaseComponentImplTest.java

@@ -760,7 +760,7 @@ public class DatabaseComponentImplTest extends BriarTestCase {
 
 		transaction = db.startTransaction(false);
 		try {
-			db.setMessageShared(transaction, message.getId(), true);
+			db.setMessageShared(transaction, message.getId());
 			fail();
 		} catch (NoSuchMessageException expected) {
 			// Expected

briar-tests/src/org/briarproject/db/H2DatabaseTest.java

@@ -272,19 +272,12 @@ public class H2DatabaseTest extends BriarTestCase {
 		assertTrue(ids.isEmpty());
 
 		// Sharing the message should make it sendable
-		db.setMessageShared(txn, messageId, true);
+		db.setMessageShared(txn, messageId);
 		ids = db.getMessagesToSend(txn, contactId, ONE_MEGABYTE);
 		assertEquals(Collections.singletonList(messageId), ids);
 		ids = db.getMessagesToOffer(txn, contactId, 100);
 		assertEquals(Collections.singletonList(messageId), ids);
 
-		// Unsharing the message should make it unsendable
-		db.setMessageShared(txn, messageId, false);
-		ids = db.getMessagesToSend(txn, contactId, ONE_MEGABYTE);
-		assertTrue(ids.isEmpty());
-		ids = db.getMessagesToOffer(txn, contactId, 100);
-		assertTrue(ids.isEmpty());
-
 		db.commitTransaction(txn);
 		db.close();
 	}
@@ -1363,6 +1356,43 @@ public class H2DatabaseTest extends BriarTestCase {
 		db.close();
 	}
 
+	@Test
+	public void testGetMessagesToShare() throws Exception {
+		MessageId mId1 = new MessageId(TestUtils.getRandomId());
+		MessageId mId2 = new MessageId(TestUtils.getRandomId());
+		MessageId mId3 = new MessageId(TestUtils.getRandomId());
+		MessageId mId4 = new MessageId(TestUtils.getRandomId());
+		Message m1 = new Message(mId1, groupId, timestamp, raw);
+		Message m2 = new Message(mId2, groupId, timestamp, raw);
+		Message m3 = new Message(mId3, groupId, timestamp, raw);
+		Message m4 = new Message(mId4, groupId, timestamp, raw);
+
+		Database<Connection> db = open(false);
+		Connection txn = db.startTransaction();
+
+		// Add a group and some messages
+		db.addGroup(txn, group);
+		db.addMessage(txn, m1, DELIVERED, true);
+		db.addMessage(txn, m2, DELIVERED, false);
+		db.addMessage(txn, m3, DELIVERED, false);
+		db.addMessage(txn, m4, DELIVERED, true);
+
+		// Introduce dependencies between the messages
+		db.addMessageDependency(txn, groupId, mId1, mId2);
+		db.addMessageDependency(txn, groupId, mId3, mId1);
+		db.addMessageDependency(txn, groupId, mId4, mId3);
+
+		// Retrieve messages to be shared
+		Collection<MessageId> result =
+				db.getMessagesToShare(txn, clientId);
+		assertEquals(2, result.size());
+		assertTrue(result.contains(mId2));
+		assertTrue(result.contains(mId3));
+
+		db.commitTransaction(txn);
+		db.close();
+	}
+
 	@Test
 	public void testGetMessageStatus() throws Exception {
 		Database<Connection> db = open(false);

briar-tests/src/org/briarproject/sync/ValidationManagerImplTest.java

@@ -84,6 +84,7 @@ public class ValidationManagerImplTest extends BriarTestCase {
 		final Transaction txn3 = new Transaction(null, true);
 		final Transaction txn4 = new Transaction(null, false);
 		final Transaction txn5 = new Transaction(null, true);
+		final Transaction txn6 = new Transaction(null, true);
 		context.checking(new Expectations() {{
 			// Get messages to validate
 			oneOf(db).startTransaction(true);
@@ -108,6 +109,7 @@ public class ValidationManagerImplTest extends BriarTestCase {
 			oneOf(db).mergeMessageMetadata(txn2, messageId, metadata);
 			// Deliver the first message
 			oneOf(hook).incomingMessage(txn2, message, metadata);
+			will(returnValue(false));
 			oneOf(db).getRawMessage(txn2, messageId);
 			will(returnValue(raw));
 			oneOf(db).setMessageState(txn2, messageId, DELIVERED);
@@ -144,6 +146,12 @@ public class ValidationManagerImplTest extends BriarTestCase {
 			oneOf(db).getPendingMessages(txn5, clientId);
 			will(returnValue(Collections.emptyList()));
 			oneOf(db).endTransaction(txn5);
+			// Get messages to share
+			oneOf(db).startTransaction(true);
+			will(returnValue(txn6));
+			oneOf(db).getMessagesToShare(txn6, clientId);
+			will(returnValue(Collections.emptyList()));
+			oneOf(db).endTransaction(txn6);
 		}});
 
 		ValidationManagerImpl vm = new ValidationManagerImpl(db, dbExecutor,
@@ -160,6 +168,7 @@ public class ValidationManagerImplTest extends BriarTestCase {
 		assertTrue(txn3.isComplete());
 		assertTrue(txn4.isComplete());
 		assertTrue(txn5.isComplete());
+		assertTrue(txn6.isComplete());
 	}
 
 	@Test
@@ -175,6 +184,7 @@ public class ValidationManagerImplTest extends BriarTestCase {
 		final Transaction txn1 = new Transaction(null, true);
 		final Transaction txn2 = new Transaction(null, false);
 		final Transaction txn3 = new Transaction(null, false);
+		final Transaction txn4 = new Transaction(null, true);
 
 		context.checking(new Expectations() {{
 			// Get messages to validate
@@ -205,6 +215,7 @@ public class ValidationManagerImplTest extends BriarTestCase {
 			will(returnValue(new Metadata()));
 			// Deliver the message
 			oneOf(hook).incomingMessage(txn2, message, metadata);
+			will(returnValue(false));
 			oneOf(db).getRawMessage(txn2, messageId);
 			will(returnValue(raw));
 			oneOf(db).setMessageState(txn2, messageId, DELIVERED);
@@ -228,6 +239,7 @@ public class ValidationManagerImplTest extends BriarTestCase {
 			will(returnValue(metadata));
 			// Deliver the dependent
 			oneOf(hook).incomingMessage(txn3, message2, metadata);
+			will(returnValue(false));
 			oneOf(db).getRawMessage(txn3, messageId2);
 			will(returnValue(raw));
 			oneOf(db).setMessageState(txn3, messageId2, DELIVERED);
@@ -235,6 +247,13 @@ public class ValidationManagerImplTest extends BriarTestCase {
 			oneOf(db).getMessageDependents(txn3, messageId2);
 			will(returnValue(Collections.emptyMap()));
 			oneOf(db).endTransaction(txn3);
+
+			// Get messages to share
+			oneOf(db).startTransaction(true);
+			will(returnValue(txn4));
+			oneOf(db).getMessagesToShare(txn4, clientId);
+			will(returnValue(Collections.emptyList()));
+			oneOf(db).endTransaction(txn4);
 		}});
 
 		ValidationManagerImpl vm = new ValidationManagerImpl(db, dbExecutor,
@@ -249,6 +268,137 @@ public class ValidationManagerImplTest extends BriarTestCase {
 		assertTrue(txn1.isComplete());
 		assertTrue(txn2.isComplete());
 		assertTrue(txn3.isComplete());
+		assertTrue(txn4.isComplete());
+	}
+
+	@Test
+	public void testMessagesAreSharedAtStartup() throws Exception {
+		Mockery context = new Mockery();
+		final DatabaseComponent db = context.mock(DatabaseComponent.class);
+		final Executor dbExecutor = new ImmediateExecutor();
+		final Executor cryptoExecutor = new ImmediateExecutor();
+		final MessageValidator validator = context.mock(MessageValidator.class);
+		final IncomingMessageHook hook =
+				context.mock(IncomingMessageHook.class);
+		final Transaction txn = new Transaction(null, true);
+		final Transaction txn1 = new Transaction(null, true);
+		final Transaction txn2 = new Transaction(null, true);
+		final Transaction txn3 = new Transaction(null, false);
+		final Transaction txn4 = new Transaction(null, false);
+
+		context.checking(new Expectations() {{
+			// No messages to validate
+			oneOf(db).startTransaction(true);
+			will(returnValue(txn));
+			oneOf(db).getMessagesToValidate(txn, clientId);
+			will(returnValue(Collections.emptyList()));
+			oneOf(db).endTransaction(txn);
+			// No pending messages to deliver
+			oneOf(db).startTransaction(true);
+			will(returnValue(txn1));
+			oneOf(db).getPendingMessages(txn1, clientId);
+			will(returnValue(Collections.emptyList()));
+			oneOf(db).endTransaction(txn1);
+
+			// Get messages to share
+			oneOf(db).startTransaction(true);
+			will(returnValue(txn2));
+			oneOf(db).getMessagesToShare(txn2, clientId);
+			will(returnValue(Collections.singletonList(messageId)));
+			oneOf(db).endTransaction(txn2);
+			// Share message and get dependencies
+			oneOf(db).startTransaction(false);
+			will(returnValue(txn3));
+			oneOf(db).setMessageShared(txn3, messageId);
+			oneOf(db).getMessageDependencies(txn3, messageId);
+			will(returnValue(Collections.singletonMap(messageId2, DELIVERED)));
+			oneOf(db).endTransaction(txn3);
+			// Share dependency
+			oneOf(db).startTransaction(false);
+			will(returnValue(txn4));
+			oneOf(db).setMessageShared(txn4, messageId2);
+			oneOf(db).getMessageDependencies(txn4, messageId2);
+			will(returnValue(Collections.emptyMap()));
+			oneOf(db).endTransaction(txn4);
+		}});
+
+		ValidationManagerImpl vm = new ValidationManagerImpl(db, dbExecutor,
+				cryptoExecutor);
+		vm.registerMessageValidator(clientId, validator);
+		vm.registerIncomingMessageHook(clientId, hook);
+		vm.startService();
+
+		context.assertIsSatisfied();
+
+		assertTrue(txn.isComplete());
+		assertTrue(txn1.isComplete());
+		assertTrue(txn2.isComplete());
+		assertTrue(txn3.isComplete());
+		assertTrue(txn4.isComplete());
+	}
+
+	@Test
+	public void testIncomingMessagesAreShared() throws Exception {
+		Mockery context = new Mockery();
+		final DatabaseComponent db = context.mock(DatabaseComponent.class);
+		final Executor dbExecutor = new ImmediateExecutor();
+		final Executor cryptoExecutor = new ImmediateExecutor();
+		final MessageValidator validator = context.mock(MessageValidator.class);
+		final IncomingMessageHook hook =
+				context.mock(IncomingMessageHook.class);
+		final Transaction txn = new Transaction(null, true);
+		final Transaction txn1 = new Transaction(null, false);
+		final Transaction txn2 = new Transaction(null, false);
+		context.checking(new Expectations() {{
+			// Load the group
+			oneOf(db).startTransaction(true);
+			will(returnValue(txn));
+			oneOf(db).getGroup(txn, groupId);
+			will(returnValue(group));
+			oneOf(db).endTransaction(txn);
+			// Validate the message: valid
+			oneOf(validator).validateMessage(message, group);
+			will(returnValue(validResultWithDependencies));
+			// Store the validation result
+			oneOf(db).startTransaction(false);
+			will(returnValue(txn1));
+			oneOf(db).addMessageDependencies(txn1, message,
+					validResultWithDependencies.getDependencies());
+			oneOf(db).getMessageDependencies(txn1, messageId);
+			will(returnValue(Collections.singletonMap(messageId1, DELIVERED)));
+			oneOf(db).mergeMessageMetadata(txn1, messageId, metadata);
+			// Deliver the message
+			oneOf(hook).incomingMessage(txn1, message, metadata);
+			will(returnValue(true));
+			oneOf(db).getRawMessage(txn1, messageId);
+			will(returnValue(raw));
+			oneOf(db).setMessageState(txn1, messageId, DELIVERED);
+			// Get any pending dependents
+			oneOf(db).getMessageDependents(txn1, messageId);
+			will(returnValue(Collections.emptyMap()));
+			// Share message
+			oneOf(db).setMessageShared(txn1, messageId);
+			oneOf(db).endTransaction(txn1);
+			// Share dependencies
+			oneOf(db).startTransaction(false);
+			will(returnValue(txn2));
+			oneOf(db).setMessageShared(txn2, messageId1);
+			oneOf(db).getMessageDependencies(txn2, messageId1);
+			will(returnValue(Collections.emptyMap()));
+			oneOf(db).endTransaction(txn2);
+		}});
+
+		ValidationManagerImpl vm = new ValidationManagerImpl(db, dbExecutor,
+				cryptoExecutor);
+		vm.registerMessageValidator(clientId, validator);
+		vm.registerIncomingMessageHook(clientId, hook);
+		vm.eventOccurred(new MessageAddedEvent(message, contactId));
+
+		context.assertIsSatisfied();
+
+		assertTrue(txn.isComplete());
+		assertTrue(txn1.isComplete());
+		assertTrue(txn2.isComplete());
 	}
 
 	@Test
@@ -266,6 +416,7 @@ public class ValidationManagerImplTest extends BriarTestCase {
 		final Transaction txn2 = new Transaction(null, true);
 		final Transaction txn3 = new Transaction(null, false);
 		final Transaction txn4 = new Transaction(null, true);
+		final Transaction txn5 = new Transaction(null, true);
 		context.checking(new Expectations() {{
 			// Get messages to validate
 			oneOf(db).startTransaction(true);
@@ -308,6 +459,12 @@ public class ValidationManagerImplTest extends BriarTestCase {
 			oneOf(db).getPendingMessages(txn4, clientId);
 			will(returnValue(Collections.emptyList()));
 			oneOf(db).endTransaction(txn4);
+			// Get messages to share
+			oneOf(db).startTransaction(true);
+			will(returnValue(txn5));
+			oneOf(db).getMessagesToShare(txn5, clientId);
+			will(returnValue(Collections.emptyList()));
+			oneOf(db).endTransaction(txn5);
 		}});
 
 		ValidationManagerImpl vm = new ValidationManagerImpl(db, dbExecutor,
@@ -323,6 +480,7 @@ public class ValidationManagerImplTest extends BriarTestCase {
 		assertTrue(txn2.isComplete());
 		assertTrue(txn3.isComplete());
 		assertTrue(txn4.isComplete());
+		assertTrue(txn5.isComplete());
 	}
 
 	@Test
@@ -340,6 +498,7 @@ public class ValidationManagerImplTest extends BriarTestCase {
 		final Transaction txn2 = new Transaction(null, true);
 		final Transaction txn3 = new Transaction(null, false);
 		final Transaction txn4 = new Transaction(null, true);
+		final Transaction txn5 = new Transaction(null, true);
 		context.checking(new Expectations() {{
 			// Get messages to validate
 			oneOf(db).startTransaction(true);
@@ -385,6 +544,12 @@ public class ValidationManagerImplTest extends BriarTestCase {
 			oneOf(db).getPendingMessages(txn4, clientId);
 			will(returnValue(Collections.emptyList()));
 			oneOf(db).endTransaction(txn4);
+			// Get messages to share
+			oneOf(db).startTransaction(true);
+			will(returnValue(txn5));
+			oneOf(db).getMessagesToShare(txn5, clientId);
+			will(returnValue(Collections.emptyList()));
+			oneOf(db).endTransaction(txn5);
 		}});
 
 		ValidationManagerImpl vm = new ValidationManagerImpl(db, dbExecutor,
@@ -400,6 +565,7 @@ public class ValidationManagerImplTest extends BriarTestCase {
 		assertTrue(txn2.isComplete());
 		assertTrue(txn3.isComplete());
 		assertTrue(txn4.isComplete());
+		assertTrue(txn5.isComplete());
 	}
 
 	@Test
@@ -429,6 +595,7 @@ public class ValidationManagerImplTest extends BriarTestCase {
 			oneOf(db).mergeMessageMetadata(txn1, messageId, metadata);
 			// Deliver the message
 			oneOf(hook).incomingMessage(txn1, message, metadata);
+			will(returnValue(false));
 			oneOf(db).getRawMessage(txn1, messageId);
 			will(returnValue(raw));
 			oneOf(db).setMessageState(txn1, messageId, DELIVERED);
@@ -548,6 +715,7 @@ public class ValidationManagerImplTest extends BriarTestCase {
 			oneOf(db).mergeMessageMetadata(txn1, messageId, metadata);
 			// Deliver the message
 			oneOf(hook).incomingMessage(txn1, message, metadata);
+			will(returnValue(false));
 			oneOf(db).getRawMessage(txn1, messageId);
 			will(returnValue(raw));
 			oneOf(db).setMessageState(txn1, messageId, DELIVERED);
@@ -792,6 +960,7 @@ public class ValidationManagerImplTest extends BriarTestCase {
 			oneOf(db).mergeMessageMetadata(txn1, messageId, metadata);
 			// Deliver the message
 			oneOf(hook).incomingMessage(txn1, message, metadata);
+			will(returnValue(false));
 			oneOf(db).getRawMessage(txn1, messageId);
 			will(returnValue(raw));
 			oneOf(db).setMessageState(txn1, messageId, DELIVERED);
@@ -815,6 +984,7 @@ public class ValidationManagerImplTest extends BriarTestCase {
 			will(returnValue(metadata));
 			// Deliver message 1
 			oneOf(hook).incomingMessage(txn2, message1, metadata);
+			will(returnValue(false));
 			oneOf(db).getRawMessage(txn2, messageId1);
 			will(returnValue(raw));
 			oneOf(db).setMessageState(txn2, messageId1, DELIVERED);
@@ -838,6 +1008,7 @@ public class ValidationManagerImplTest extends BriarTestCase {
 			will(returnValue(metadata));
 			// Deliver message 2
 			oneOf(hook).incomingMessage(txn3, message2, metadata);
+			will(returnValue(false));
 			oneOf(db).getRawMessage(txn3, messageId2);
 			will(returnValue(raw));
 			oneOf(db).setMessageState(txn3, messageId2, DELIVERED);
@@ -890,6 +1061,7 @@ public class ValidationManagerImplTest extends BriarTestCase {
 			will(returnValue(metadata));
 			// Deliver message 4
 			oneOf(hook).incomingMessage(txn6, message4, metadata);
+			will(returnValue(false));
 			oneOf(db).getRawMessage(txn6, messageId4);
 			will(returnValue(raw));
 			oneOf(db).setMessageState(txn6, messageId4, DELIVERED);
@@ -947,6 +1119,7 @@ public class ValidationManagerImplTest extends BriarTestCase {
 			oneOf(db).mergeMessageMetadata(txn1, messageId, metadata);
 			// Deliver the message
 			oneOf(hook).incomingMessage(txn1, message, metadata);
+			will(returnValue(false));
 			oneOf(db).getRawMessage(txn1, messageId);
 			will(returnValue(raw));
 			oneOf(db).setMessageState(txn1, messageId, DELIVERED);