1 files changed, 153 insertions, 0 deletions
diff --git a/packages/excalidraw/workers.ts b/packages/excalidraw/workers.ts
new file mode 100644
index 0000000..f5964d0
--- /dev/null
+++ b/packages/excalidraw/workers.ts
@@ -0,0 +1,153 @@
+import { WorkerInTheMainChunkError, WorkerUrlNotDefinedError } from "./errors";
+import { debounce } from "./utils";
+
+class IdleWorker {
+  public instance: Worker;
+
+  constructor(workerUrl: URL) {
+    this.instance = new Worker(workerUrl, { type: "module" });
+  }
+
+  /**
+   * Use to prolong the worker's life by `workerTTL` or terminate it with a flush immediately.
+   */
+  public debounceTerminate!: ReturnType<typeof debounce>;
+}
+
+/**
+ * Pool of idle short-lived workers.
+ *
+ * IMPORTANT: for simplicity it does not limit the number of newly created workers, leaving it up to the caller to manage the pool size.
+ */
+export class WorkerPool<T, R> {
+  private idleWorkers: Set<IdleWorker> = new Set();
+  private readonly workerUrl: URL;
+  private readonly workerTTL: number;
+
+  private constructor(
+    workerUrl: URL,
+    options: {
+      ttl?: number;
+    },
+  ) {
+    this.workerUrl = workerUrl;
+    // by default, active & idle workers will be terminated after 1s of inactivity
+    this.workerTTL = options.ttl || 1000;
+  }
+
+  /**
+   * Create a new worker pool.
+   *
+   * @param workerUrl - The URL of the worker file.
+   * @param options - The options for the worker pool.
+   * @throws If the worker is bundled into the main chunk.
+   * @returns A new worker pool instance.
+   */
+  public static create<T, R>(
+    workerUrl: URL | undefined,
+    options: {
+      ttl?: number;
+    } = {},
+  ): WorkerPool<T, R> {
+    if (!workerUrl) {
+      throw new WorkerUrlNotDefinedError();
+    }
+
+    if (!import.meta.url || workerUrl.toString() === import.meta.url) {
+      // in case the worker code is bundled into the main chunk
+      throw new WorkerInTheMainChunkError();
+    }
+
+    return new WorkerPool(workerUrl, options);
+  }
+
+  /**
+   * Take idle worker from the pool or create a new one and post a message to it.
+   */
+  public async postMessage(
+    data: T,
+    options: StructuredSerializeOptions,
+  ): Promise<R> {
+    let worker: IdleWorker;
+
+    const idleWorker = Array.from(this.idleWorkers).shift();
+    if (idleWorker) {
+      this.idleWorkers.delete(idleWorker);
+      worker = idleWorker;
+    } else {
+      worker = await this.createWorker();
+    }
+
+    return new Promise((resolve, reject) => {
+      worker.instance.onmessage = this.onMessageHandler(worker, resolve);
+      worker.instance.onerror = this.onErrorHandler(worker, reject);
+
+      worker.instance.postMessage(data, options);
+      worker.debounceTerminate(() =>
+        reject(
+          new Error(`Active worker did not respond for ${this.workerTTL}ms!`),
+        ),
+      );
+    });
+  }
+
+  /**
+   * Terminate the idle workers in the pool.
+   */
+  public async clear() {
+    for (const worker of this.idleWorkers) {
+      worker.debounceTerminate.cancel();
+      worker.instance.terminate();
+    }
+
+    this.idleWorkers.clear();
+  }
+
+  /**
+   * Used to get a worker from the pool or create a new one if there is no idle available.
+   */
+  private async createWorker(): Promise<IdleWorker> {
+    const worker = new IdleWorker(this.workerUrl);
+
+    worker.debounceTerminate = debounce((reject?: () => void) => {
+      worker.instance.terminate();
+
+      if (this.idleWorkers.has(worker)) {
+        this.idleWorkers.delete(worker);
+
+        // eslint-disable-next-line no-console
+        console.debug(
+          "Job finished! Idle worker has been released from the pool.",
+        );
+      } else if (reject) {
+        reject();
+      } else {
+        console.error("Worker has been terminated!");
+      }
+    }, this.workerTTL);
+
+    return worker;
+  }
+
+  private onMessageHandler(worker: IdleWorker, resolve: (value: R) => void) {
+    return (e: { data: R }) => {
+      worker.debounceTerminate();
+      this.idleWorkers.add(worker);
+      resolve(e.data);
+    };
+  }
+
+  private onErrorHandler(
+    worker: IdleWorker,
+    reject: (reason: ErrorEvent) => void,
+  ) {
+    return (e: ErrorEvent) => {
+      // terminate the worker immediately before rejection
+      worker.debounceTerminate(() => reject(e));
+      worker.debounceTerminate.flush();
+
+      // clear the worker pool in case there are some idle workers left
+      this.clear();
+    };
+  }
+}