Add backup comparator test harness

ts/services/backups/util/FileStream.ts

@@ -0,0 +1,61 @@
+// Copyright 2024 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
+import { type FileHandle, open } from 'node:fs/promises';
+import { Buffer } from 'node:buffer';
+import { InputStream } from '@signalapp/libsignal-client/dist/io';
+
+export class FileStream extends InputStream {
+  private file: FileHandle | undefined;
+  private position = 0;
+  private buffer = Buffer.alloc(16 * 1024);
+  private initPromise: Promise<unknown> | undefined;
+
+  constructor(private readonly filePath: string) {
+    super();
+  }
+
+  public async close(): Promise<void> {
+    await this.initPromise;
+    await this.file?.close();
+  }
+
+  // Only for comparator tests
+  public async size(): Promise<number> {
+    const file = await this.lazyOpen();
+    const { size } = await file.stat();
+    return size;
+  }
+
+  async read(amount: number): Promise<Buffer> {
+    const file = await this.lazyOpen();
+    if (this.buffer.length < amount) {
+      this.buffer = Buffer.alloc(amount);
+    }
+    const { bytesRead } = await file.read(
+      this.buffer,
+      0,
+      amount,
+      this.position
+    );
+    this.position += bytesRead;
+    return this.buffer.slice(0, bytesRead);
+  }
+
+  async skip(amount: number): Promise<void> {
+    this.position += amount;
+  }
+
+  private async lazyOpen(): Promise<FileHandle> {
+    await this.initPromise;
+
+    if (this.file) {
+      return this.file;
+    }
+
+    const filePromise = open(this.filePath);
+    this.initPromise = filePromise;
+    this.file = await filePromise;
+    return this.file;
+  }
+}