Track Piper model bytes in the worker

tests/piper-download-progress.test.cjs

@@ -0,0 +1,33 @@
+const assert = require('node:assert/strict');
+const test = require('node:test');
+
+test('model response progress forwards each downloaded chunk and preserves the response', async () => {
+  const {trackModelDownload} = await import('../piper-download-progress.mjs');
+  const source = new ReadableStream({
+    start(controller) {
+      controller.enqueue(new Uint8Array([1, 2]));
+      controller.enqueue(new Uint8Array([3, 4, 5]));
+      controller.close();
+    },
+  });
+  const response = new Response(source, {headers: {'content-length': '5'}});
+  const messages = [];
+  const tracked = trackModelDownload(response, {id: 7, postMessage: message => messages.push(message)});
+
+  assert.deepEqual([...new Uint8Array(await tracked.arrayBuffer())], [1, 2, 3, 4, 5]);
+  assert.deepEqual(messages.map(message => message.progress), [
+    {loaded: 2, total: 5},
+    {loaded: 5, total: 5},
+  ]);
+  assert.ok(messages.every(message => message.id === 7 && message.type === 'progress'));
+});
+
+test('model response progress works when content length is absent', async () => {
+  const {trackModelDownload} = await import('../piper-download-progress.mjs');
+  const source = new Response(new Uint8Array([1, 2, 3]));
+  const messages = [];
+  const tracked = trackModelDownload(source, {id: 9, postMessage: message => messages.push(message)});
+
+  assert.equal((await tracked.arrayBuffer()).byteLength, 3);
+  assert.deepEqual(messages.map(message => message.progress), [{loaded: 3, total: 0}]);
+});