From a68b949f120ad11329c66d4d58575cee1a304e75 Mon Sep 17 00:00:00 2001
From: Brendan Dahl <brendan.dahl@gmail.com>
Date: Tue, 6 Jan 2026 22:23:30 +0000
Subject: [PATCH] [fetch] Do not collect all data when streaming requests.

Previously, when `EMSCRIPTEN_FETCH_STREAM_DATA` was used we would collect
all the streamed data chunks and create the full response when finished.
This caused excessive memory usage. Now we leave it up to the user to
process the data and collect it as needed.
---
 src/Fetch.js | 39 ++++++++++++++++++++++++++++-----------
 1 file changed, 28 insertions(+), 11 deletions(-)

diff --git a/src/Fetch.js b/src/Fetch.js
index 52619b454e936..41a8e30e1d32c 100644
--- a/src/Fetch.js
+++ b/src/Fetch.js
@@ -41,6 +41,10 @@ var FetchXHR = class {
   _aborted = false;
   _responseHeaders = null;
 
+  // When enabled the the data will be streamed using progress events. If the full result is needed
+  // the data must be collected during the progress events.
+  _streamData = false;
+
   // --- Private state management ---
   _changeReadyState(state) {
     this.readyState = state;
@@ -174,7 +178,9 @@ var FetchXHR = class {
       const contentLength = +response.headers.get('Content-Length');
 
       let receivedLength = 0;
-      const chunks = [];
+      // When streaming data don't collect all of the chunks into one large chunk. It's up to the
+      // user to collect the data as it comes in.
+      const chunks = this._streamData ? null : [];
 
       while (true) {
         const { done, value } = await reader.read();
@@ -182,7 +188,9 @@ var FetchXHR = class {
           break;
         }
 
-        chunks.push(value);
+        if (!this._streamData) {
+          chunks.push(value);
+        }
         receivedLength += value.length;
 
         if (this.onprogress) {
@@ -197,16 +205,19 @@ var FetchXHR = class {
         }
       }
 
-      // Combine chunks into a single Uint8Array.
-      const allChunks = new Uint8Array(receivedLength);
-      let position = 0;
-      for (const chunk of chunks) {
-        allChunks.set(chunk, position);
-        position += chunk.length;
+      if (this._streamData) {
+        this.response = null;
+      } else {
+        // Combine chunks into a single Uint8Array.
+        const allChunks = new Uint8Array(receivedLength);
+        let position = 0;
+        for (const chunk of chunks) {
+          allChunks.set(chunk, position);
+          position += chunk.length;
+        }
+        // Convert to ArrayBuffer as requested by responseType
+        this.response = allChunks.buffer;
       }
-
-      // Convert to ArrayBuffer as requested by responseType
-      this.response = allChunks.buffer;
     } catch (error) {
       this.statusText = error.message;
 
@@ -517,6 +528,7 @@ function fetchXHR(fetch, onsuccess, onerror, onprogress, onreadystatechange) {
   var xhr = new XMLHttpRequest();
 #endif
   xhr.withCredentials = !!{{{ makeGetValue('fetch_attr', C_STRUCTS.emscripten_fetch_attr_t.withCredentials, 'u8') }}};;
+  xhr._streamData = fetchAttrStreamData;
 #if FETCH_DEBUG
   dbg(`fetch: xhr.timeout: ${xhr.timeout}, xhr.withCredentials: ${xhr.withCredentials}`);
   dbg(`fetch: xhr.open(requestMethod="${requestMethod}", url: "${url}", userName: ${userNameStr}, password: ${passwordStr}`);
@@ -609,6 +621,11 @@ function fetchXHR(fetch, onsuccess, onerror, onprogress, onreadystatechange) {
     if (xhr.status >= 200 && xhr.status < 300) {
 #if FETCH_DEBUG
       dbg(`fetch: xhr of URL "${xhr.url_}" / responseURL "${xhr.responseURL}" succeeded with status ${xhr.status}`);
+#endif
+#if ASSERTIONS
+      if (fetchAttrStreamData) {
+        assert(xhr.response === null);
+      }
 #endif
       onsuccess(fetch, xhr, e);
     } else {