Slow memory leak when using GM_xmlhttpRequest

[rh314]

Expected Behavior

When using GM_xmlhttpRequest, there should not be any memory leaks. We expect Javascript's garbage collector to clean up requests when completed.

Actual Behavior

There is a small memory leak on every call to GM_xmlhttpRequest.
E.g. when GM_xmlhttpRequest is called once every millisecond (to more easily reproduce the bug) then memory consumption climbs by a few megabytes every second.

Initially I thought it might be the lack/presence of handling on* events, but the leak happens either way.
I've also checked if setting the fetch parameter changes the outcome - again, no effect, same outcome.

Specifications

• Firefox: 123.0.1 (64-bit)
• TM: 5.1.0
• OS: Linux Mint 21.3 Virginia

Script

First, run a small webserver locally on port 12300. (Anything goes, it just makes it easier to reproduce the leak locally)

Then, use the following script to reproduce the memory leak:

// ==UserScript==
// @name         Memory leak - reproduce issue
// @namespace    http://tampermonkey.net/
// @version      2024-03-21
// @author       You
// @match        https://github.com/Tampermonkey/tampermonkey/issues
// @icon         https://www.google.com/s2/favicons?sz=64&domain=github.com
// @grant        GM_xmlhttpRequest
// @connect      127.0.0.1
// ==/UserScript==

console.log('STARTING SCRIPT');
setInterval(() => {
    GM_xmlhttpRequest({
        method: 'GET',
        url: 'http://127.0.0.1:12300/',
    });
}, 1);

[rh314]

I've updated the ticket description to remove "optional/commented code".
I'll put the full optional/commented code in this comment.

// ==UserScript==
// @name         Memory leak - reproduce issue
// @namespace    http://tampermonkey.net/
// @version      2024-03-21
// @author       You
// @match        https://github.com/Tampermonkey/tampermonkey/issues
// @icon         https://www.google.com/s2/favicons?sz=64&domain=github.com
// @grant        GM_xmlhttpRequest
// @connect      127.0.0.1
// ==/UserScript==

console.log('STARTING SCRIPT');
// var xhr_cnts = {
//     onload: 0,
//     onloadstart: 0,
//     onerror: 0,
//     onabort: 0,
//     ontimeout: 0,
//     onprogress: 0,
//     onreadystatechange: 0,
// };
setInterval(() => {
    GM_xmlhttpRequest({
        method: 'GET',
        url: 'http://127.0.0.1:12300/',

        // fetch: true,  // NOTE: Memory leak happens with any value here

        // onload: response => {
        //     xhr_cnts.onload += 1;
        //     if (xhr_cnts.onload % 1000 == 0) {
        //         console.log(xhr_cnts);
        //     }
        // },
        // onloadstart: response => { xhr_cnts.onloadstart += 1; },
        // onerror: response => { xhr_cnts.onerror += 1; },
        // onabort: response => { xhr_cnts.onabort += 1; },
        // ontimeout: response => { xhr_cnts.ontimeout += 1; },
        // onprogress: response => { xhr_cnts.onprogress += 1; },
        // onreadystatechange: response => { xhr_cnts.onreadystatechange += 1; },
    });
}, 1);