Add placeholder BlockSource implementation to DB.

bramble-api/src/main/java/org/briarproject/bramble/api/db/DatabaseComponent.java

@@ -208,6 +208,24 @@ public interface DatabaseComponent {
 	Collection<Message> generateRequestedBatch(Transaction txn, ContactId c,
 			int maxLength, int maxLatency) throws DbException;
 
+	/**
+	 * Returns the number of blocks in the given message.
+	 * <p>
+	 * Read-only.
+	 */
+	int getBlockCount(Transaction txn, MessageId m) throws DbException;
+
+	/**
+	 * Returns the given block of the given message.
+	 * <p>
+	 * Read-only.
+	 *
+	 * @throws NoSuchBlockException if 'blockNumber' is greater than or equal
+	 * to the number of blocks in the message
+	 */
+	byte[] getBlock(Transaction txn, MessageId m, int blockNumber)
+			throws DbException;
+
 	/**
 	 * Returns the contact with the given ID.
 	 * <p/>

bramble-api/src/main/java/org/briarproject/bramble/api/db/NoSuchBlockException.java

@@ -0,0 +1,9 @@
+package org.briarproject.bramble.api.db;
+
+/**
+ * Thrown when a database operation is attempted for a block that is not in
+ * the database. This exception may occur due to concurrent updates and does
+ * not indicate a database error.
+ */
+public class NoSuchBlockException extends DbException {
+}

bramble-core/src/main/java/org/briarproject/bramble/db/BlockSourceImpl.java

@@ -0,0 +1,29 @@
+package org.briarproject.bramble.db;
+
+import org.briarproject.bramble.api.db.DatabaseComponent;
+import org.briarproject.bramble.api.db.DbException;
+import org.briarproject.bramble.api.io.BlockSource;
+import org.briarproject.bramble.api.sync.MessageId;
+
+import javax.inject.Inject;
+
+class BlockSourceImpl implements BlockSource {
+
+	private final DatabaseComponent db;
+
+	@Inject
+	BlockSourceImpl(DatabaseComponent db) {
+		this.db = db;
+	}
+
+	@Override
+	public int getBlockCount(MessageId m) throws DbException {
+		return db.transactionWithResult(true, txn -> db.getBlockCount(txn, m));
+	}
+
+	@Override
+	public byte[] getBlock(MessageId m, int blockNumber) throws DbException {
+		return db.transactionWithResult(true, txn ->
+				db.getBlock(txn, m, blockNumber));
+	}
+}

bramble-core/src/main/java/org/briarproject/bramble/db/DatabaseComponentImpl.java

@@ -14,6 +14,7 @@ import org.briarproject.bramble.api.db.DbException;
 import org.briarproject.bramble.api.db.DbRunnable;
 import org.briarproject.bramble.api.db.Metadata;
 import org.briarproject.bramble.api.db.MigrationListener;
+import org.briarproject.bramble.api.db.NoSuchBlockException;
 import org.briarproject.bramble.api.db.NoSuchContactException;
 import org.briarproject.bramble.api.db.NoSuchGroupException;
 import org.briarproject.bramble.api.db.NoSuchLocalAuthorException;
@@ -420,6 +421,26 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		return messages;
 	}
 
+	@Override
+	public int getBlockCount(Transaction transaction, MessageId m)
+			throws DbException {
+		T txn = unbox(transaction);
+		if (!db.containsMessage(txn, m))
+			throw new NoSuchMessageException();
+		return 1;
+	}
+
+	@Override
+	public byte[] getBlock(Transaction transaction, MessageId m,
+			int blockNumber) throws DbException {
+		T txn = unbox(transaction);
+		if (!db.containsMessage(txn, m))
+			throw new NoSuchMessageException();
+		if (blockNumber != 0)
+			throw new NoSuchBlockException();
+		return db.getMessage(txn, m).getBody();
+	}
+
 	@Override
 	public Contact getContact(Transaction transaction, ContactId c)
 			throws DbException {

bramble-core/src/main/java/org/briarproject/bramble/db/DatabaseModule.java

@@ -3,6 +3,7 @@ package org.briarproject.bramble.db;
 import org.briarproject.bramble.api.db.DatabaseComponent;
 import org.briarproject.bramble.api.db.DatabaseConfig;
 import org.briarproject.bramble.api.event.EventBus;
+import org.briarproject.bramble.api.io.BlockSource;
 import org.briarproject.bramble.api.lifecycle.ShutdownManager;
 import org.briarproject.bramble.api.sync.MessageFactory;
 import org.briarproject.bramble.api.system.Clock;
@@ -31,4 +32,9 @@ public class DatabaseModule {
 		return new DatabaseComponentImpl<>(db, Connection.class, eventBus,
 				shutdown);
 	}
+
+	@Provides
+	BlockSource provideBlockSource(BlockSourceImpl blockSource) {
+		return blockSource;
+	}
 }