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[fetch] Fix memory leak inside saveResponseAndStatus in streaming mode #26273

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[fetch] Fix memory leak inside saveResponseAndStatus in streaming mode #26273

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44 changes: 26 additions & 18 deletions src/Fetch.js
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Original file line number Diff line number Diff line change
Expand Up @@ -576,21 +576,23 @@ function fetchXHR(fetch, onsuccess, onerror, onprogress, onreadystatechange) {
// This receives a condition, which determines whether to save the xhr's
// response, or just 0.
function saveResponseAndStatus() {
var ptr = 0;
var ptrLen = 0;
if (xhr.response && fetchAttrLoadToMemory && {{{ makeGetValue('fetch', C_STRUCTS.emscripten_fetch_t.data, '*') }}} === 0) {
let ptrLen = 0;
if (fetchAttrLoadToMemory && xhr.response) {
let ptr = {{{ makeGetValue('fetch', C_STRUCTS.emscripten_fetch_t.data, '*') }}};
ptrLen = xhr.response.byteLength;
}
if (ptrLen > 0) {
#if FETCH_DEBUG
dbg(`fetch: allocating ${ptrLen} bytes in Emscripten heap for xhr data`);
#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 = _realloc({{{ makeGetValue('fetch', C_STRUCTS.emscripten_fetch_t.data, '*') }}}, ptrLen);
if (ptrLen > readI53FromI64(fetch + {{{ C_STRUCTS.emscripten_fetch_t.numBytes }}})) {
#if FETCH_DEBUG
dbg(`fetch: allocating ${ptrLen} bytes in Emscripten heap for xhr data`);
#endif
ptr = _realloc(ptr, ptrLen);
{{{ makeSetValue('fetch', C_STRUCTS.emscripten_fetch_t.data, 'ptr', '*') }}}
}

HEAPU8.set(new Uint8Array(/** @type{Array<number>} */(xhr.response)), ptr);
}
{{{ makeSetValue('fetch', C_STRUCTS.emscripten_fetch_t.data, 'ptr', '*') }}}
writeI53ToI64(fetch + {{{ C_STRUCTS.emscripten_fetch_t.numBytes }}}, ptrLen);
writeI53ToI64(fetch + {{{ C_STRUCTS.emscripten_fetch_t.dataOffset }}}, 0);
var len = xhr.response ? xhr.response.byteLength : 0;
Expand Down Expand Up @@ -660,21 +662,27 @@ function fetchXHR(fetch, onsuccess, onerror, onprogress, onreadystatechange) {
if (!Fetch.xhrs.has(id)) {
return;
}
var ptrLen = (fetchAttrLoadToMemory && fetchAttrStreamData && xhr.response) ? xhr.response.byteLength : 0;
var ptr = 0;
if (ptrLen > 0 && fetchAttrLoadToMemory && fetchAttrStreamData) {
#if FETCH_DEBUG
dbg(`fetch: allocating ${ptrLen} bytes in Emscripten heap for xhr data`);
#endif
let ptrLen = 0;
if (fetchAttrLoadToMemory && fetchAttrStreamData && xhr.response) {
let ptr = {{{ makeGetValue('fetch', C_STRUCTS.emscripten_fetch_t.data, '*') }}};
ptrLen = xhr.response.byteLength;

#if ASSERTIONS
assert(onprogress, 'When doing a streaming fetch, you should have an onprogress handler registered to receive the chunks!');
#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 = _realloc({{{ makeGetValue('fetch', C_STRUCTS.emscripten_fetch_t.data, '*') }}}, ptrLen);
if (ptrLen > readI53FromI64(fetch + {{{ C_STRUCTS.emscripten_fetch_t.numBytes }}})) {
#if FETCH_DEBUG
dbg(`fetch: allocating ${ptrLen} bytes in Emscripten heap for xhr data`);
#endif
ptr = _realloc(ptr, ptrLen);
{{{ makeSetValue('fetch', C_STRUCTS.emscripten_fetch_t.data, 'ptr', '*') }}}
}

HEAPU8.set(new Uint8Array(/** @type{Array<number>} */(xhr.response)), ptr);
}
{{{ makeSetValue('fetch', C_STRUCTS.emscripten_fetch_t.data, 'ptr', '*') }}}
writeI53ToI64(fetch + {{{ C_STRUCTS.emscripten_fetch_t.numBytes }}}, ptrLen);
writeI53ToI64(fetch + {{{ C_STRUCTS.emscripten_fetch_t.dataOffset }}}, e.loaded - ptrLen);
writeI53ToI64(fetch + {{{ C_STRUCTS.emscripten_fetch_t.totalBytes }}}, e.total);
Expand Down
1 change: 1 addition & 0 deletions src/lib/libfetch.js
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Expand Up @@ -33,6 +33,7 @@ var LibraryFetch = {
'$Fetch',
'$fetchXHR',
'$callUserCallback',
'$readI53FromI64',
'$writeI53ToI64',
'$stringToUTF8',
'$stringToNewUTF8',
Expand Down
1 change: 0 additions & 1 deletion test/fetch/test_fetch_stream_file.cpp
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Expand Up @@ -20,7 +20,6 @@ int main() {
attr.onsuccess = [](emscripten_fetch_t *fetch) {
printf("Finished downloading %llu bytes\n", fetch->totalBytes);
printf("Data checksum: %08X\n", checksum);
assert(fetch->data == 0); // The data was streamed via onprogress, no bytes available here.
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@sbc100 sbc100 Feb 15, 2026

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It seems like the idea is that with streaming you get the bytes only in the progress events and not in the onsuccess event.

At least that has been the way the API has been since this test was added 10 years ago: 786f2fe

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@inolen inolen Feb 16, 2026

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Yep, but the same structure (with the ->data pointer) is passed to both events. To keep the test as is, ->data would have to be free'd and NULL'd before calling onsuccess.

As I said in a previous reply, I think of ->data as the persistent buffer between events and ->numBytes as the amount of data in that buffer that's valid for the current event. ->numBytes being 0 is important, but ->data being required to be NULL in onload seems odd.

assert(fetch->numBytes == 0);
assert(fetch->totalBytes == 134217728);
assert(checksum == 0xA7F8E858U);
Expand Down