KeyRotatorImpl looks a *lot* like DatabaseCleanerImpl.

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

@@ -19,9 +19,9 @@ interface DatabaseCleaner {
 		/**
 		 * Checks how much free storage space is available to the database, and
 		 * if necessary expires old messages until the free space is at least
-		 * DatabaseConstants.MIN_FREE_SPACE. While the free space is less than
-		 * DatabaseConstants.CRITICAL_FREE_SPACE, operations that attempt to
-		 * store messages in the database will block.
+		 * DatabaseConstants.MIN_FREE_SPACE. If the free space is less than
+		 * DatabaseConstants.CRITICAL_FREE_SPACE and there are no more messages
+		 * to expire, an Error will be thrown.
 		 */
 		void checkFreeSpaceAndClean() throws DbException;
 

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

@@ -33,12 +33,10 @@ class DatabaseCleanerImpl extends TimerTask implements DatabaseCleaner {
 				callback.checkFreeSpaceAndClean();
 			}
 		} catch(DbException e) {
-			if(LOG.isLoggable(Level.WARNING))
-				LOG.warning(e.toString());
+			if(LOG.isLoggable(Level.WARNING)) LOG.warning(e.toString());
 			throw new Error(e); // Kill the application
 		} catch(RuntimeException e) {
-			if(LOG.isLoggable(Level.WARNING))
-				LOG.warning(e.toString());
+			if(LOG.isLoggable(Level.WARNING)) LOG.warning(e.toString());
 			throw new Error(e); // Kill the application
 		}
 	}

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

@@ -1548,8 +1548,7 @@ DatabaseCleaner.Callback {
 			if(freeSpace < CRITICAL_FREE_SPACE && !expired) {
 				// FIXME: Work out what to do here - the amount of free space
 				// is critically low and there are no messages left to expire
-				System.err.println("Disk space is critical - shutting down");
-				System.exit(1);
+				throw new Error("Disk space is critical");
 			}
 			Thread.yield();
 			freeSpace = db.getFreeSpace();

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

@@ -14,7 +14,7 @@ interface DatabaseConstants {
 	/**
 	 * The minimum amount of space in bytes that must be kept free on the device
 	 * where the database is stored. If less than this much space is free and
-	 * there are no more messages to expire, the program will shut down.
+	 * there are no more messages to expire, an Error will be thrown.
 	 */
 	static final long CRITICAL_FREE_SPACE = 100 * 1024 * 1024; // 100 MiB
 

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

@@ -0,0 +1,24 @@
+package net.sf.briar.db;
+
+import net.sf.briar.api.db.DbException;
+
+interface KeyRotator {
+
+	/**
+	 * Starts a new thread to rotate keys periodically. The rotator will pause
+	 * for the given number of milliseconds between rotations.
+	 */
+	void startRotating(Callback callback, long msBetweenRotations);
+
+	/** Tells the rotator thread to exit. */
+	void stopRotating();
+
+	interface Callback {
+
+		/**
+		 * Rotates keys, replacing and destroying any keys that have passed the
+		 * ends of their respective retention periods.
+		 */
+		void rotateKeys() throws DbException;
+	}
+}

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

@@ -0,0 +1,41 @@
+package net.sf.briar.db;
+
+import java.util.Timer;
+import java.util.TimerTask;
+import java.util.logging.Level;
+import java.util.logging.Logger;
+
+import net.sf.briar.api.db.DbException;
+
+class KeyRotatorImpl extends TimerTask implements KeyRotator {
+
+	private static final Logger LOG =
+			Logger.getLogger(KeyRotatorImpl.class.getName());
+
+	private volatile Callback callback = null;
+	private volatile Timer timer = null;
+
+	public void startRotating(Callback callback, long msBetweenRotations) {
+		this.callback = callback;
+		timer = new Timer();
+		timer.scheduleAtFixedRate(this, 0L, msBetweenRotations);
+	}
+
+	public void stopRotating() {
+		if(timer == null) throw new IllegalStateException();
+		timer.cancel();
+	}
+
+	public void run() {
+		if(callback == null) throw new IllegalStateException();
+		try {
+			callback.rotateKeys();
+		} catch(DbException e) {
+			if(LOG.isLoggable(Level.WARNING)) LOG.warning(e.toString());
+			throw new Error(e); // Kill the application
+		} catch(RuntimeException e) {
+			if(LOG.isLoggable(Level.WARNING)) LOG.warning(e.toString());
+			throw new Error(e); // Kill the application
+		}
+	}
+}