Check that createLogHandler() isn't called more than once.

This would result in multiple log files encrypted with the same key.

briar-api/src/main/java/org/briarproject/briar/api/logging/PersistentLogManager.java

@@ -24,6 +24,8 @@ public interface PersistentLogManager {
 	/**
 	 * Creates and returns a persistent log handler that stores its logs in
 	 * the given directory.
+	 * <p>
+	 * This method should only be called once.
 	 */
 	Handler createLogHandler(File dir) throws IOException;
 

briar-core/src/main/java/org/briarproject/briar/logging/PersistentLogManagerImpl.java

@@ -28,6 +28,7 @@ import java.util.List;
 import java.util.Scanner;
 import java.util.concurrent.Executor;
 import java.util.concurrent.ScheduledExecutorService;
+import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.logging.Formatter;
 import java.util.logging.Handler;
 import java.util.logging.Logger;
@@ -63,6 +64,7 @@ class PersistentLogManagerImpl implements PersistentLogManager,
 	private final StreamWriterFactory streamWriterFactory;
 	private final Formatter formatter;
 	private final SecretKey logKey;
+	private final AtomicBoolean handlerCreated = new AtomicBoolean(false);
 
 	@Nullable
 	private volatile SecretKey oldLogKey = null;
@@ -103,6 +105,7 @@ class PersistentLogManagerImpl implements PersistentLogManager,
 
 	@Override
 	public Handler createLogHandler(File dir) throws IOException {
+		if (handlerCreated.getAndSet(true)) throw new IllegalStateException();
 		File logFile = new File(dir, LOG_FILE);
 		File oldLogFile = new File(dir, OLD_LOG_FILE);
 		if (oldLogFile.exists() && !oldLogFile.delete())