fix(sdk): use lockfiles to prevent multiple simulators on the same state directory

docs/api/04-standard-library/aws/api-reference.md

@@ -1747,6 +1747,8 @@ Add a Lambda layer to the function.
 
 - *Type:* str
 
+The ARN of the layer.
+
 ---
 
 #### Properties <a name="Properties" id="Properties"></a>

libs/wingsdk/src/simulator/simulator.ts

@@ -1,5 +1,10 @@
 import { existsSync } from "fs";
-import { mkdir, rm } from "fs/promises";
+import {
+  mkdir,
+  rm,
+  open as openFileHandle,
+  type FileHandle,
+} from "fs/promises";
 import type { Server, IncomingMessage, ServerResponse } from "http";
 import { join, resolve } from "path";
 import { makeSimulatorClient } from "./client";
@@ -225,9 +230,13 @@ export class Simulator {
   // keeps the running state of all resources.
   private runningState: Record<string, ResourceRunningState> = {};
 
+  private lockfileFilename: string;
+  private lockfile: FileHandle | undefined;
+
   constructor(props: SimulatorProps) {
     const simdir = resolve(props.simfile);
     this.statedir = props.stateDir ?? join(simdir, ".state");
+    this.lockfileFilename = join(this.statedir, "lock");
 
     this._running = "stopped";
     this._handles = new HandleManager();
@@ -303,9 +312,9 @@ export class Simulator {
     }
     this._running = "starting";
 
-    await this.startServer();
-
     try {
+      await this.acquireStateLock();
+      await this.startServer();
       await this.startResources();
     } catch (err: any) {
       this.stopServer();
@@ -314,6 +323,36 @@ export class Simulator {
     }
   }
 
+  private async acquireStateLock() {
+    if (this.lockfile) {
+      return; // already locked
+    }
+
+    await mkdir(this.statedir, { recursive: true });
+    try {
+      this.lockfile = await openFileHandle(this.lockfileFilename, "wx");
+    } catch (error) {
+      if (
+        error instanceof Error // && error.code === "EEXIST"
+      ) {
+        throw new Error(
+          "Another instance of the simulator is already running. Please stop the current simulation before starting a new one."
+        );
+      }
+      throw error;
+    }
+  }
+
+  private async releaseStateLock() {
+    if (!this.lockfile) {
+      return; // not locked
+    }
+
+    await this.lockfile.close();
+    await rm(this.lockfileFilename);
+    this.lockfile = undefined;
+  }
+
   private async startResources() {
     const retries: Record<string, number> = {};
     const queue = this._model.graph.nodes.map((n) => n.path);
@@ -413,6 +452,8 @@ export class Simulator {
 
     this.stopServer();
 
+    await this.releaseStateLock();
+
     this._handles.reset();
     this._running = "stopped";
   }