From d40f5a177a45c649c8bd3c85ac4fab44d43ee228 Mon Sep 17 00:00:00 2001
From: Xuguang Mei <meixg@foxmail.com>
Date: Mon, 28 Feb 2022 02:48:47 +0800
Subject: [PATCH] buffer: improve blob read performance

Fix: https://github.com/nodejs/node/issues/42108

PR-URL: https://github.com/nodejs/node/pull/42117
Fixes: https://github.com/nodejs/node/issues/42108
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Mestery <mestery@protonmail.com>
---
 lib/internal/blob.js | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/lib/internal/blob.js b/lib/internal/blob.js
index 6c1388744b17d4..a620552d846051 100644
--- a/lib/internal/blob.js
+++ b/lib/internal/blob.js
@@ -65,6 +65,7 @@ const {
 
 const kHandle = Symbol('kHandle');
 const kState = Symbol('kState');
+const kIndex = Symbol('kIndex');
 const kType = Symbol('kType');
 const kLength = Symbol('kLength');
 const kArrayBufferPromise = Symbol('kArrayBufferPromise');
@@ -325,17 +326,17 @@ class Blob {
     return new lazyReadableStream({
       async start() {
         this[kState] = await self.arrayBuffer();
+        this[kIndex] = 0;
       },
 
       pull(controller) {
-        if (this[kState].byteLength <= kMaxChunkSize) {
-          controller.enqueue(new Uint8Array(this[kState]));
+        if (this[kState].byteLength - this[kIndex] <= kMaxChunkSize) {
+          controller.enqueue(new Uint8Array(this[kState], this[kIndex]));
           controller.close();
           this[kState] = undefined;
         } else {
-          const slice = this[kState].slice(0, kMaxChunkSize);
-          this[kState] = this[kState].slice(kMaxChunkSize);
-          controller.enqueue(new Uint8Array(slice));
+          controller.enqueue(new Uint8Array(this[kState], this[kIndex], kMaxChunkSize));
+          this[kIndex] += kMaxChunkSize;
         }
       }
     });