feat: lock file

This introduces a lock file for xud that is created in the xud data
directory any time xud starts up. Anytime a lock file already exists,
an error message is logged and xud exits. This prevents multiple
xud processes from trying to use the same node key or directory at the
same time.

The lock files are unique to each network, so running multiple processes
with different networks (e.g. simnet and mainnet) is allowed. They are
deleted when xud shuts down.

Closes #1989.

lib/Xud.ts

@@ -35,9 +35,9 @@ class Xud extends EventEmitter {
   public service!: Service;
   private logger!: Logger;
   private config: Config;
-  private db!: DB;
-  private pool!: Pool;
-  private orderBook!: OrderBook;
+  private db?: DB;
+  private pool?: Pool;
+  private orderBook?: OrderBook;
   private rpcServer?: GrpcServer;
   private httpServer?: HttpServer;
   private grpcAPIProxy?: GrpcWebProxyServer;
@@ -46,6 +46,7 @@ class Xud extends EventEmitter {
   private swapClientManager?: SwapClientManager;
   private unitConverter?: UnitConverter;
   private simnetChannels$?: Subscription;
+  private lockFilePath?: string;
 
   /**
    * Create an Exchange Union daemon.
@@ -88,6 +89,21 @@ class Xud extends EventEmitter {
     }
 
     try {
+      // create a lock file to prevent multiple xud instances from trying to run
+      // with the same node key and/or database
+      const lockFilePath = path.join(this.config.xudir, `${this.config.network}.lock`);
+      try {
+        await (await fs.open(lockFilePath, 'wx')).close();
+        this.lockFilePath = lockFilePath;
+      } catch (err) {
+        if (err.code === 'EEXIST') {
+          this.logger.error(`A lock file exists at ${lockFilePath}. Another xud process is running or a previous process exited ungracefully.`);
+          return;
+        } else {
+          throw err;
+        }
+      }
+
       if (!this.config.rpc.disable) {
         // start rpc server first, it will respond with UNAVAILABLE error
         // indicating xud is starting until xud finishes initializing
@@ -296,6 +312,10 @@ class Xud extends EventEmitter {
     // TODO: ensure we are not in the middle of executing any trades
     const closePromises: Promise<void>[] = [];
 
+    if (this.lockFilePath) {
+      closePromises.push(fs.unlink(this.lockFilePath).catch(this.logger.error));
+    }
+
     this.simnetChannels$?.unsubscribe();
 
     if (this.swapClientManager) {
@@ -318,7 +338,7 @@ class Xud extends EventEmitter {
     }
     await Promise.all(closePromises);
 
-    await this.db.close();
+    await this.db?.close();
     this.logger.info('XUD shutdown gracefully');
 
     this.emit('shutdown');