From f9909fc5a3b24ac7d51acd50cf15e3675bdd26a8 Mon Sep 17 00:00:00 2001
From: Devin Ivy <devinivy@gmail.com>
Date: Wed, 8 Apr 2026 16:16:08 -0400
Subject: [PATCH] feat: auto-transfer return values from workers

Worker return values that are transferable (ArrayBuffer, TypedArray,
streams, etc.) are automatically added to the postMessage transfer
list. No explicit transfer() wrapping needed on the return path since
the worker is done with the value.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
---
 src/transfer.ts           |  2 +-
 src/worker-entry.ts       | 10 ++++++----
 test/fixtures/transfer.ts |  9 +++++++++
 test/transfer.test.ts     | 14 +++++++++++++-
 4 files changed, 29 insertions(+), 6 deletions(-)

diff --git a/src/transfer.ts b/src/transfer.ts
index 7e499fe..a6073de 100644
--- a/src/transfer.ts
+++ b/src/transfer.ts
@@ -25,7 +25,7 @@ export function extractTransferables(args: unknown[]): { args: unknown[]; transf
   return { args: processedArgs, transfer: transferList };
 }
 
-function collectTransferables(value: unknown, list: Transferable[]): void {
+export function collectTransferables(value: unknown, list: Transferable[]): void {
   if (!(typeof value === 'object' && value !== null)) return;
 
   // Directly transferable
diff --git a/src/worker-entry.ts b/src/worker-entry.ts
index 6bdab8b..4aafa70 100644
--- a/src/worker-entry.ts
+++ b/src/worker-entry.ts
@@ -1,7 +1,8 @@
 import { parentPort } from 'node:worker_threads';
 import { registry } from './registry.ts';
 import { deserializeArg, serializeArg } from './sync/reconstruct.ts';
-import { extractTransferables } from './transfer.ts';
+import { collectTransferables } from './transfer.ts';
+import type { Transferable } from 'node:worker_threads';
 
 const imported = new Set<string>();
 
@@ -19,9 +20,10 @@ parentPort!.on('message', async (msg: { callId: number; id: string; args: unknow
 
     const deserializedArgs = args.map(deserializeArg);
     const value = await fn(...deserializedArgs);
-    const extracted = extractTransferables([value]);
-    const returnValue = serializeArg(extracted.args[0]);
-    parentPort!.postMessage({ callId, value: returnValue }, extracted.transfer);
+    const returnValue = serializeArg(value);
+    const transferList: Transferable[] = [];
+    collectTransferables(value, transferList);
+    parentPort!.postMessage({ callId, value: returnValue }, transferList);
   } catch (err) {
     const message = err instanceof Error ? err.message : String(err);
     parentPort!.postMessage({ callId, error: message });
diff --git a/test/fixtures/transfer.ts b/test/fixtures/transfer.ts
index 7ea975e..9f535a4 100644
--- a/test/fixtures/transfer.ts
+++ b/test/fixtures/transfer.ts
@@ -16,3 +16,12 @@ export const sumUint8 = mo(import.meta, (arr: Uint8Array): number => {
   }
   return sum;
 });
+
+export const makeBuffer = mo(import.meta, (size: number): ArrayBuffer => {
+  const buf = new ArrayBuffer(size);
+  const view = new Uint8Array(buf);
+  for (let i = 0; i < size; i++) {
+    view[i] = i + 1;
+  }
+  return buf;
+});
diff --git a/test/transfer.test.ts b/test/transfer.test.ts
index e42e6f5..fc4d489 100644
--- a/test/transfer.test.ts
+++ b/test/transfer.test.ts
@@ -1,7 +1,7 @@
 import { describe, it } from 'node:test';
 import assert from 'node:assert/strict';
 import { workerPool, transfer } from 'moroutine';
-import { sumBuffer, sumUint8 } from './fixtures/transfer.ts';
+import { sumBuffer, sumUint8, makeBuffer } from './fixtures/transfer.ts';
 
 describe('transfer', () => {
   it('transfers an ArrayBuffer to a worker (zero-copy)', async () => {
@@ -35,6 +35,18 @@ describe('transfer', () => {
     }
   });
 
+  it('auto-transfers return values from worker (zero-copy)', async () => {
+    const pool = workerPool(1);
+    try {
+      const buf = await pool(makeBuffer(4));
+      const view = new Uint8Array(buf);
+      assert.equal(buf.byteLength, 4);
+      assert.deepEqual([...view], [1, 2, 3, 4]);
+    } finally {
+      pool[Symbol.dispose]();
+    }
+  });
+
   it('without transfer, buffer is copied (not detached)', async () => {
     const buf = new ArrayBuffer(4);
     const view = new Uint8Array(buf);
-- 
2.51.2