Add LocalDB Locking

FIxes #32

client/src/main/java/envoy/client/data/LocalDB.java

@@ -1,11 +1,14 @@
 package envoy.client.data;
 
 import java.io.*;
+import java.nio.channels.*;
+import java.nio.file.StandardOpenOption;
 import java.time.Instant;
 import java.util.*;
 
 import envoy.data.*;
 import envoy.event.*;
+import envoy.exception.EnvoyException;
 import envoy.util.SerializationUtils;
 
 import dev.kske.eventbus.Event;
@@ -35,6 +38,7 @@ public final class LocalDB implements EventListener {
 	private Instant				lastSync	= Instant.EPOCH;
 	private String				authToken;
 	private File				dbDir, userFile, idGeneratorFile, lastLoginFile, usersFile;
+	private FileLock			instanceLock;
 
 	/**
 	 * Constructs an empty local database. To serialize any user-specific data to
@@ -74,6 +78,28 @@ public final class LocalDB implements EventListener {
 		userFile = new File(dbDir, user.getID() + ".db");
 	}
 
+	FileChannel fc;
+
+	/**
+	 * Ensured that only one Envoy instance is using this local database by creating
+	 * a lock file.
+	 * The lock file is deleted on application exit.
+	 *
+	 * @throws EnvoyException if the lock cannot by acquired
+	 * @since Envoy Client v0.2-beta
+	 */
+	public void lock() throws EnvoyException {
+		File file = new File(dbDir, "instance.lock");
+		file.deleteOnExit();
+		try {
+			FileChannel fc = FileChannel.open(file.toPath(), StandardOpenOption.CREATE, StandardOpenOption.WRITE);
+			instanceLock = fc.tryLock();
+			if (instanceLock == null) throw new EnvoyException("Another Envoy instance is using this local database!");
+		} catch (IOException e) {
+			throw new EnvoyException("Could not create lock file!", e);
+		}
+	}
+
 	/**
 	 * Stores all users. If the client user is specified, their chats will be stored
 	 * as well. The message id generator will also be saved if present.

client/src/main/java/envoy/client/ui/Startup.java

@@ -67,10 +67,13 @@ public final class Startup extends Application {
 
 		// Initialize the local database
 		try {
-			localDB = new LocalDB(new File(config.getHomeDirectory(), config.getLocalDB().getPath()));
-		} catch (final IOException e3) {
-			logger.log(Level.SEVERE, "Could not initialize local database: ", e3);
-			new Alert(AlertType.ERROR, "Could not initialize local database!\n" + e3).showAndWait();
+			File localDBDir = new File(config.getHomeDirectory(), config.getLocalDB().getPath());
+			logger.info("Initializing LocalDB at " + localDBDir);
+			localDB = new LocalDB(localDBDir);
+			localDB.lock();
+		} catch (IOException | EnvoyException e) {
+			logger.log(Level.SEVERE, "Could not initialize local database: ", e);
+			new Alert(AlertType.ERROR, "Could not initialize local database!\n" + e).showAndWait();
 			System.exit(1);
 			return;
 		}