From 70fa4cee50e7900f322b4c077b5fa0da0875b1a4 Mon Sep 17 00:00:00 2001
From: "Kamat, Trivikram" <16024985+trivikr@users.noreply.github.com>
Date: Wed, 8 Jul 2026 20:57:10 -0700
Subject: [PATCH] stream: copy SAB-backed chunks in iter consumers

Ensure bytes() copies single chunks backed by SharedArrayBuffer so
arrayBuffer() and arrayBufferSync() return ArrayBuffer instances as
specified.

Signed-off-by: Kamat, Trivikram <16024985+trivikr@users.noreply.github.com>
Assisted-by: openai:gpt-5.5
---
 lib/internal/streams/iter/consumers.js        | 14 ++++++--------
 lib/internal/streams/iter/utils.js            | 19 ++++++++++++-------
 .../test-stream-iter-sharedarraybuffer.js     |  6 ++++--
 3 files changed, 22 insertions(+), 17 deletions(-)

diff --git a/lib/internal/streams/iter/consumers.js b/lib/internal/streams/iter/consumers.js
index 7a2d5719b9b97b..8e18c1b5b930ea 100644
--- a/lib/internal/streams/iter/consumers.js
+++ b/lib/internal/streams/iter/consumers.js
@@ -22,6 +22,8 @@ const {
   TypedArrayPrototypeGetBuffer,
   TypedArrayPrototypeGetByteLength,
   TypedArrayPrototypeGetByteOffset,
+  TypedArrayPrototypeSet,
+  Uint8Array,
 } = primordials;
 
 const {
@@ -164,21 +166,17 @@ async function collectAsync(source, signal, limit) {
 
 /**
  * Convert a Uint8Array to its backing ArrayBuffer, slicing if necessary.
- * Handles both ArrayBuffer and SharedArrayBuffer backing stores.
  * @param {Uint8Array} data
- * @returns {ArrayBuffer|SharedArrayBuffer}
+ * @returns {ArrayBuffer}
  */
 function toArrayBuffer(data) {
   const byteOffset = TypedArrayPrototypeGetByteOffset(data);
   const byteLength = TypedArrayPrototypeGetByteLength(data);
   const buffer = TypedArrayPrototypeGetBuffer(data);
-  // SharedArrayBuffer is not available in primordials, so use
-  // direct property access for its byteLength and slice.
   if (isSharedArrayBuffer(buffer)) {
-    if (byteOffset === 0 && byteLength === buffer.byteLength) {
-      return buffer;
-    }
-    return buffer.slice(byteOffset, byteOffset + byteLength);
+    const copy = new Uint8Array(byteLength);
+    TypedArrayPrototypeSet(copy, data);
+    return TypedArrayPrototypeGetBuffer(copy);
   }
   if (byteOffset === 0 &&
       byteLength === ArrayBufferPrototypeGetByteLength(buffer)) {
diff --git a/lib/internal/streams/iter/utils.js b/lib/internal/streams/iter/utils.js
index ca3d2531d6f9e0..c52393493bc0a8 100644
--- a/lib/internal/streams/iter/utils.js
+++ b/lib/internal/streams/iter/utils.js
@@ -203,6 +203,12 @@ function allUint8Array(chunks) {
   return true;
 }
 
+function copyBytes(chunk) {
+  const copy = new Uint8Array(TypedArrayPrototypeGetByteLength(chunk));
+  TypedArrayPrototypeSet(copy, chunk);
+  return copy;
+}
+
 /**
  * Concatenate multiple Uint8Arrays into a single Uint8Array.
  * @param {Uint8Array[]} chunks
@@ -220,16 +226,15 @@ function concatBytes(chunks) {
     // If non-zero offset, skip the remaining buffer checks.
     if (TypedArrayPrototypeGetByteOffset(chunk) === 0) {
       const buf = TypedArrayPrototypeGetBuffer(chunk);
-      // SharedArrayBuffer is not available in primordials, so use
-      // direct property access for its byteLength.
-      const bufByteLength = isSharedArrayBuffer(buf) ?
-        buf.byteLength :
-        ArrayBufferPrototypeGetByteLength(buf);
-      if (TypedArrayPrototypeGetByteLength(chunk) === bufByteLength) {
+      if (
+        !isSharedArrayBuffer(buf) &&
+        TypedArrayPrototypeGetByteLength(chunk) ===
+          ArrayBufferPrototypeGetByteLength(buf)
+      ) {
         return chunk;
       }
     }
-    return new Uint8Array(chunk);
+    return copyBytes(chunk);
   }
   // Multiple chunks: concatenate
   let totalByteLength = 0;
diff --git a/test/parallel/test-stream-iter-sharedarraybuffer.js b/test/parallel/test-stream-iter-sharedarraybuffer.js
index 7aff205bf7664a..de0f8d6f3fc636 100644
--- a/test/parallel/test-stream-iter-sharedarraybuffer.js
+++ b/test/parallel/test-stream-iter-sharedarraybuffer.js
@@ -52,6 +52,7 @@ function testBytesSyncSAB() {
   const sab = new SharedArrayBuffer(5);
   new Uint8Array(sab).set([104, 101, 108, 108, 111]); // 'hello'
   const data = bytesSync(fromSync(sab));
+  assert.ok(data.buffer instanceof ArrayBuffer);
   assert.deepStrictEqual(data, new Uint8Array([104, 101, 108, 108, 111]));
 }
 
@@ -59,6 +60,7 @@ async function testBytesAsyncSAB() {
   const sab = new SharedArrayBuffer(5);
   new Uint8Array(sab).set([104, 101, 108, 108, 111]); // 'hello'
   const data = await bytes(from(sab));
+  assert.ok(data.buffer instanceof ArrayBuffer);
   assert.deepStrictEqual(data, new Uint8Array([104, 101, 108, 108, 111]));
 }
 
@@ -80,7 +82,7 @@ function testArrayBufferSyncSAB() {
   const sab = new SharedArrayBuffer(4);
   new Uint8Array(sab).set([1, 2, 3, 4]);
   const result = arrayBufferSync(fromSync(sab));
-  assert.ok(result instanceof SharedArrayBuffer || result instanceof ArrayBuffer);
+  assert.ok(result instanceof ArrayBuffer);
   assert.deepStrictEqual(new Uint8Array(result), new Uint8Array([1, 2, 3, 4]));
 }
 
@@ -88,7 +90,7 @@ async function testArrayBufferAsyncSAB() {
   const sab = new SharedArrayBuffer(4);
   new Uint8Array(sab).set([1, 2, 3, 4]);
   const result = await arrayBuffer(from(sab));
-  assert.ok(result instanceof SharedArrayBuffer || result instanceof ArrayBuffer);
+  assert.ok(result instanceof ArrayBuffer);
   assert.deepStrictEqual(new Uint8Array(result), new Uint8Array([1, 2, 3, 4]));
 }