buffer: improve blob read performance

Fix: #42108

PR-URL: #42117
Fixes: #42108
Reviewed-By: Benjamin Gruenbaum <[email protected]>
Reviewed-By: Mestery <[email protected]>

Author: meixg

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;
         }
       }
     });