If no messages are added to a batch, don't call BatchWriter.finish() -

this allows the caller to avoid creating an empty packet by delaying
creation of the packet's header and trailer until something's written
to the packet's body. Changed the return semantics of
DatabaseComponent.generateBatch(ContactId, BatchWriter,
Collection<MessageId>) so that the IDs of messages considered for
inclusion in the batch but no longer sendable are also returned - this
allows the caller to remove them from the set of requested IDs.

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

@@ -79,8 +79,9 @@ public interface DatabaseComponent {
 
 	/**
 	 * Generates a batch of messages for the given contact from the given
-	 * collection of requested messages, and returns the IDs of the messages
+	 * collection of requested messages, and returns the IDs of any messages
-	 * added to the bacth.
+	 * that were either added to the batch, or were considered for inclusion
+	 * but are no longer sendable to the contact.
 	 */
 	Collection<MessageId> generateBatch(ContactId c, BatchWriter b,
 			Collection<MessageId> requested) throws DbException, IOException;

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

@@ -300,13 +300,11 @@ class ReadWriteLockDatabaseComponent<Txn> extends DatabaseComponentImpl<Txn> {
 					try {
 						Txn txn = db.startTransaction();
 						try {
+							sent = new ArrayList<MessageId>();
 							int capacity = b.getCapacity();
 							Collection<MessageId> sendable =
 								db.getSendableMessages(txn, c, capacity);
-							Iterator<MessageId> it = sendable.iterator();
+							for(MessageId m : sendable) {
-							sent = new ArrayList<MessageId>();
-							while(it.hasNext()) {
-								MessageId m = it.next();
 								byte[] raw = db.getMessage(txn, m);
 								if(!b.writeMessage(raw)) break;
 								bytesSent += raw.length;
@@ -326,9 +324,9 @@ class ReadWriteLockDatabaseComponent<Txn> extends DatabaseComponentImpl<Txn> {
 				} finally {
 					messageStatusLock.readLock().unlock();
 				}
-				BatchId id = b.finish();
 				// Record the contents of the batch, unless it's empty
 				if(sent.isEmpty()) return;
+				BatchId id = b.finish();
 				messageStatusLock.writeLock().lock();
 				try {
 					Txn txn = db.startTransaction();
@@ -357,7 +355,7 @@ class ReadWriteLockDatabaseComponent<Txn> extends DatabaseComponentImpl<Txn> {
 			if(!containsContact(c)) throw new NoSuchContactException();
 			messageLock.readLock().lock();
 			try {
-				Collection<MessageId> sent;
+				Collection<MessageId> sent, considered;
 				messageStatusLock.readLock().lock();
 				try{
 					subscriptionLock.readLock().lock();
@@ -365,13 +363,20 @@ class ReadWriteLockDatabaseComponent<Txn> extends DatabaseComponentImpl<Txn> {
 						Txn txn = db.startTransaction();
 						try {
 							sent = new ArrayList<MessageId>();
+							considered = new ArrayList<MessageId>();
 							int bytesSent = 0;
 							for(MessageId m : requested) {
 								byte[] raw = db.getMessageIfSendable(txn, c, m);
-								if(raw == null) continue;
+								// If the message is still sendable, try to add
-								if(!b.writeMessage(raw)) break;
+								// it to the batch. If the batch is full, don't
-								bytesSent += raw.length;
+								// treat the message as considered, and don't
-								sent.add(m);
+								// try to add any further messages.
+								if(raw != null) {
+									if(!b.writeMessage(raw)) break;
+									bytesSent += raw.length;
+									sent.add(m);
+								}
+								considered.add(m);
 							}
 							db.commitTransaction(txn);
 						} catch(DbException e) {
@@ -387,16 +392,16 @@ class ReadWriteLockDatabaseComponent<Txn> extends DatabaseComponentImpl<Txn> {
 				} finally {
 					messageStatusLock.readLock().unlock();
 				}
-				BatchId id = b.finish();
 				// Record the contents of the batch, unless it's empty
-				if(sent.isEmpty()) return sent;
+				if(sent.isEmpty()) return considered;
+				BatchId id = b.finish();
 				messageStatusLock.writeLock().lock();
 				try {
 					Txn txn = db.startTransaction();
 					try {
 						db.addOutstandingBatch(txn, c, id, sent);
 						db.commitTransaction(txn);
-						return sent;
+						return considered;
 					} catch(DbException e) {
 						db.abortTransaction(txn);
 						throw e;

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

@@ -223,23 +223,24 @@ class SynchronizedDatabaseComponent<Txn> extends DatabaseComponentImpl<Txn> {
 					synchronized(subscriptionLock) {
 						Txn txn = db.startTransaction();
 						try {
-							int capacity = b.getCapacity();
-							Collection<MessageId> sendable =
-								db.getSendableMessages(txn, c, capacity);
-							Iterator<MessageId> it = sendable.iterator();
 							Collection<MessageId> sent =
 								new ArrayList<MessageId>();
 							int bytesSent = 0;
-							while(it.hasNext()) {
+							int capacity = b.getCapacity();
-								MessageId m = it.next();
+							Collection<MessageId> sendable =
+								db.getSendableMessages(txn, c, capacity);
+							for(MessageId m : sendable) {
 								byte[] raw = db.getMessage(txn, m);
 								if(!b.writeMessage(raw)) break;
 								bytesSent += raw.length;
 								sent.add(m);
 							}
-							BatchId id = b.finish();
+							// If the batch is not empty, calculate its ID and
-							if(!sent.isEmpty())
+							// record it as outstanding
+							if(!sent.isEmpty()) {
+								BatchId id = b.finish();
 								db.addOutstandingBatch(txn, c, id, sent);
+							}
 							db.commitTransaction(txn);
 						} catch(DbException e) {
 							db.abortTransaction(txn);
@@ -265,19 +266,30 @@ class SynchronizedDatabaseComponent<Txn> extends DatabaseComponentImpl<Txn> {
 						try {
 							Collection<MessageId> sent =
 								new ArrayList<MessageId>();
+							Collection<MessageId> considered =
+								new ArrayList<MessageId>();
 							int bytesSent = 0;
 							for(MessageId m : requested) {
 								byte[] raw = db.getMessageIfSendable(txn, c, m);
-								if(raw == null) continue;
+								// If the message is still sendable, try to add
-								if(!b.writeMessage(raw)) break;
+								// it to the batch. If the batch is full, don't
-								bytesSent += raw.length;
+								// treat the message as considered, and don't
-								sent.add(m);
+								// try to add any further messages.
+								if(raw != null) {
+									if(!b.writeMessage(raw)) break;
+									bytesSent += raw.length;
+									sent.add(m);
+								}
+								considered.add(m);
 							}
-							BatchId id = b.finish();
+							// If the batch is not empty, calculate its ID and
-							if(!sent.isEmpty())
+							// record it as outstanding
+							if(!sent.isEmpty()) {
+								BatchId id = b.finish();
 								db.addOutstandingBatch(txn, c, id, sent);
+							}
 							db.commitTransaction(txn);
-							return sent;
+							return considered;
 						} catch(DbException e) {
 							db.abortTransaction(txn);
 							throw e;