[fetch] Fix double free in fetch_free() with streaming fetch requests

src/Fetch.js

@@ -576,7 +576,7 @@ function fetchXHR(fetch, onsuccess, onerror, onprogress, onreadystatechange) {
 #endif
       // The data pointer malloc()ed here has the same lifetime as the emscripten_fetch_t structure itself has, and is
       // freed when emscripten_fetch_close() is called.
-      ptr = _malloc(ptrLen);
+      ptr = _realloc({{{ makeGetValue('fetch', C_STRUCTS.emscripten_fetch_t.data, '*') }}}, ptrLen);
       HEAPU8.set(new Uint8Array(/** @type{Array<number>} */(xhr.response)), ptr);
     }
     {{{ makeSetValue('fetch', C_STRUCTS.emscripten_fetch_t.data, 'ptr', '*') }}}
@@ -653,8 +653,9 @@ function fetchXHR(fetch, onsuccess, onerror, onprogress, onreadystatechange) {
 #if ASSERTIONS
       assert(onprogress, 'When doing a streaming fetch, you should have an onprogress handler registered to receive the chunks!');
 #endif
-      // Allocate byte data in Emscripten heap for the streamed memory block (freed immediately after onprogress call)
-      ptr = _malloc(ptrLen);
+      // The data pointer malloc()ed here has the same lifetime as the emscripten_fetch_t structure itself has, and is
+      // freed when emscripten_fetch_close() is called.
+      ptr = _realloc({{{ makeGetValue('fetch', C_STRUCTS.emscripten_fetch_t.data, '*') }}}, ptrLen);
       HEAPU8.set(new Uint8Array(/** @type{Array<number>} */(xhr.response)), ptr);
     }
     {{{ makeSetValue('fetch', C_STRUCTS.emscripten_fetch_t.data, 'ptr', '*') }}}
@@ -668,7 +669,6 @@ function fetchXHR(fetch, onsuccess, onerror, onprogress, onreadystatechange) {
     {{{ makeSetValue('fetch', C_STRUCTS.emscripten_fetch_t.status, 'status', 'i16') }}}
     if (xhr.statusText) stringToUTF8(xhr.statusText, fetch + {{{ C_STRUCTS.emscripten_fetch_t.statusText }}}, 64);
     onprogress(fetch, e);
-    _free(ptr);
   };
   xhr.onreadystatechange = (e) => {
     // check if xhr was aborted by user and don't try to call back

src/lib/libfetch.js

@@ -29,7 +29,7 @@ var LibraryFetch = {
   emscripten_start_fetch: startFetch,
   emscripten_start_fetch__deps: [
     'malloc',
-    'free',
+    'realloc',
     '$Fetch',
     '$fetchXHR',
     '$callUserCallback',

test/fetch/test_fetch_stream_abort.cpp

@@ -0,0 +1,43 @@
+// Copyright 2025 The Emscripten Authors.  All rights reserved.
+// Emscripten is available under two separate licenses, the MIT license and the
+// University of Illinois/NCSA Open Source License.  Both these licenses can be
+// found in the LICENSE file.
+
+#include <string.h>
+#include <stdlib.h>
+#include <stdio.h>
+#include <assert.h>
+#include <emscripten.h>
+#include <emscripten/fetch.h>
+#include <emscripten/eventloop.h>
+
+int main() {
+  emscripten_fetch_attr_t attr;
+  emscripten_fetch_attr_init(&attr);
+  strcpy(attr.requestMethod, "GET");
+  attr.onsuccess = [](emscripten_fetch_t *fetch) {
+    printf("Finished downloading %llu bytes\n", fetch->totalBytes);
+    emscripten_fetch_close(fetch);
+  };
+  attr.onerror = [](emscripten_fetch_t *fetch) {
+    printf("Downloading failed with status code: %d.\n", fetch->status);
+    if (fetch->status != (uint16_t)-1) { // if not aborted with emscripten_fetch_close()
+      emscripten_fetch_close(fetch);
+    }
+  };
+  attr.onprogress = [](emscripten_fetch_t *fetch) {
+    printf("Downloading.. %.2f%s complete. Received chunk [%llu, %llu[\n", 
+      (fetch->totalBytes > 0) ? ((fetch->dataOffset + fetch->numBytes) * 100.0 / fetch->totalBytes) : (double)(fetch->dataOffset + fetch->numBytes),
+      (fetch->totalBytes > 0) ? "%" : " bytes",
+      fetch->dataOffset,
+      fetch->dataOffset + fetch->numBytes);
+
+    emscripten_set_immediate([](void *arg) {
+      emscripten_fetch_t *fetch = (emscripten_fetch_t *)arg;
+      printf("Abort fetch when downloading\n");
+      emscripten_fetch_close(fetch);
+    }, fetch);
+  };
+  attr.attributes = EMSCRIPTEN_FETCH_LOAD_TO_MEMORY | EMSCRIPTEN_FETCH_STREAM_DATA;
+  emscripten_fetch(&attr, "largefile.txt");
+}

test/test_browser.py

@@ -4460,18 +4460,21 @@ def test_fetch_response_headers(self, args):
     shutil.copy(test_file('gears.png'), '.')
     self.btest_exit('fetch/test_fetch_response_headers.cpp', cflags=['-sFETCH_DEBUG', '-sFETCH', '-pthread', '-sPROXY_TO_PTHREAD'] + args)
 
-  # Test emscripten_fetch() usage to stream a fetch in to memory without storing the full file in memory
-  # Streaming only works the fetch backend.
-  @also_with_wasm2js
-  def test_fetch_stream_file(self):
-    # Strategy: create a large 128MB file, and compile with a small 16MB Emscripten heap, so that the tested file
-    # won't fully fit in the heap. This verifies that streaming works properly.
+  def make_largefile(self):
     s = '12345678'
     for _ in range(14):
       s = s[::-1] + s # length of str will be 2^17=128KB
     with open('largefile.txt', 'w') as f:
       for _ in range(1024):
         f.write(s)
+
+  # Test emscripten_fetch() usage to stream a fetch in to memory without storing the full file in memory
+  # Streaming only works the fetch backend.
+  @also_with_wasm2js
+  def test_fetch_stream_file(self):
+    # Strategy: create a large 128MB file, and compile with a small 16MB Emscripten heap, so that the tested file
+    # won't fully fit in the heap. This verifies that streaming works properly.
+    self.make_largefile()
     self.btest_exit('fetch/test_fetch_stream_file.cpp', cflags=['-sFETCH_DEBUG', '-sFETCH', '-sFETCH_STREAMING'])
 
   @also_with_fetch_streaming
@@ -4538,6 +4541,11 @@ def test_fetch_stream_async(self):
     create_file('myfile.dat', 'hello world\n' * 1000)
     self.btest_exit('fetch/test_fetch_stream_async.c', cflags=['-sFETCH', '-sFETCH_STREAMING'])
 
+  @also_with_asan
+  def test_fetch_stream_abort(self):
+    self.make_largefile()
+    self.btest_exit('fetch/test_fetch_stream_abort.cpp', cflags=['-sFETCH', '-sFETCH_STREAMING', '-sALLOW_MEMORY_GROWTH'])
+
   @also_with_fetch_streaming
   def test_fetch_persist(self):
     create_file('myfile.dat', 'hello world\n')