feat: support async customDiff implementations

The customDiff option now accepts functions returning Promise<Buffer>
(e.g. node-hdiffpatch's callback-based async diff), so servers can run
diffs off the event loop. Sync implementations keep working unchanged.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: sunnylqm

src/diff.ts

@@ -19,7 +19,10 @@ import {
   zipOptionsForPayloadEntry,
 } from './utils/zip-options';
 
-type Diff = (oldSource?: Buffer, newSource?: Buffer) => Buffer;
+type Diff = (
+  oldSource?: Buffer,
+  newSource?: Buffer,
+) => Buffer | Promise<Buffer>;
 type HdiffModule = {
   diff?: Diff;
 };
@@ -83,7 +86,7 @@ async function addBundlePatch(
 ) {
   const newSource = await readEntry(entry, nextZipfile);
   zipfile.addBuffer(
-    diffFn(originSource, newSource),
+    await diffFn(originSource, newSource),
     `${entry.fileName}.patch`,
     zipOptionsForPatchEntry(),
   );

tests/diff.test.ts

@@ -22,7 +22,10 @@ type ZipContent = {
 
 type DiffContextOptions = {
   output?: unknown;
-  customDiff?: (oldSource?: Buffer, newSource?: Buffer) => Buffer;
+  customDiff?: (
+    oldSource?: Buffer,
+    newSource?: Buffer,
+  ) => Buffer | Promise<Buffer>;
   customHdiffModule?: {
     diff?: (oldSource?: Buffer, newSource?: Buffer) => Buffer;
   };
@@ -185,6 +188,37 @@ describe('diff commands', () => {
     expect(diffMeta.deletes['old-only.txt']).toBe(1);
   });
 
+  test('hdiff supports an async customDiff implementation', async () => {
+    const originPath = path.join(tempRoot, 'origin.ppk');
+    const nextPath = path.join(tempRoot, 'next.ppk');
+    const outputPath = path.join(tempRoot, 'out', 'diff-async.ppk');
+
+    await createZip(originPath, {
+      'index.bundlejs': 'old-bundle',
+    });
+
+    await createZip(nextPath, {
+      'index.bundlejs': 'new-bundle',
+    });
+
+    await diffCommands.hdiff(
+      createContext([originPath, nextPath], {
+        output: outputPath,
+        customDiff: async (oldSource, newSource) => {
+          await new Promise((resolve) => setTimeout(resolve, 1));
+          return Buffer.from(
+            `async-patch:${oldSource?.toString('utf-8')}:${newSource?.toString('utf-8')}`,
+          );
+        },
+      }),
+    );
+
+    const result = await readZipContent(outputPath);
+    expect(result.files['index.bundlejs.patch']?.toString('utf-8')).toBe(
+      'async-patch:old-bundle:new-bundle',
+    );
+  });
+
   test('hdiff compresses large manifest entries', async () => {
     const originPath = path.join(tempRoot, 'origin-large-manifest.ppk');
     const nextPath = path.join(tempRoot, 'next-large-manifest.ppk');