Memory leak with Node.js 20+

[leaveswoods]

Prerequisites

• I confirm my issue is not in the opened issues
• I confirm the Frequently Asked Questions didn't contain the answer to my issue

Environment check

• I'm using the latest msw version
• I'm using Node.js version 20 or higher

Node.js version

20.13.0

Reproduction repository

https://github.com/leaveswoods/msw-mem-leak

Reproduction steps

1. Clone the repo: https://github.com/leaveswoods/msw-mem-leak
2. Install Node.js 20
3. Under test project root
4. Run ./runMswMinimalTest.sh
5. Check the logs for heap usage before and after the tests.

Current behavior

The heap usage is increasing significantly after test with msw even though

• we force gc after test completed.
• we close msw server in the end.

Expected behavior

The heap usage should not increase overtime with msw enabled.

[jordems]

We are also running into this with similar symptoms as #2405

[leaveswoods]

We are also running into this with similar symptoms as #2405

I also checked this thread and try to use 2.8.5 as suggested, but no luck. Latest version also does not work.

[jordems]

@kettanaito We’d love your input here, as this issue seems closely related to #2405.

We’re currently using MSW in a long-lived server context to mock a few external services in a load-testing environment. Unfortunately, the memory leak is causing our servers to exhaust memory over time, which skews our test results.

If you have any thoughts, guidance, or suggested workarounds, they’d be greatly appreciated. Thanks for all your work on this!

[kettanaito]

Hi, @leaveswoods. Thanks for reporting this (and for the reproduction repository). I will take a look at this once I have a moment. I encourage others to dive into this, too, following the reproduction steps and analyzing the heap map/other artifacts to see what's accumulating memory. Share your findings here so we could discuss the ways to fix the issue. Thanks.

[kettanaito]

I'm trying to put the reproduction scenario in an automated test (see #2542) but the memory usage delta I'm getting is always high, even when not using MSW. Does anybody see any obvious errors with this test? Would appreciate any feedback here. Making an automated test out of this is the first step to fixing the issue.

[leaveswoods]

Hi @kettanaito thanks for replying us. I think your test is fine. I copy your test code, and create two js files, with and without msw

import http from 'node:http'
import { setupServer } from 'msw/node'

async function profile() {
  const startMemoryUsage = process.memoryUsage().heapUsed

  for (let i = 0; i < 1000; i++) {
    const server = setupServer()
    server.listen({ onUnhandledRequest: 'bypass' })

    await new Promise((resolve) => {
      http
        .get('http://localhost/non-existent', () => resolve())
        .on('error', () => resolve())
    })

    server.close()
  }

  const endMemoryUsage = process.memoryUsage().heapUsed
  const memoryUsed = endMemoryUsage - startMemoryUsage

  global.gc?.()
  console.log(memoryUsed / 1024 / 1024, 'MB')
}

profile()

import http from 'node:http'

async function profile() {
  const startMemoryUsage = process.memoryUsage().heapUsed

  for (let i = 0; i < 1000; i++) {

    await new Promise((resolve) => {
      http
        .get('http://localhost/non-existent', () => resolve())
        .on('error', () => resolve())
    })
  }

  const endMemoryUsage = process.memoryUsage().heapUsed
  const memoryUsed = endMemoryUsage - startMemoryUsage

  global.gc?.()
  console.log(memoryUsed / 1024 / 1024)
}

profile()

And I run two js files separately multiple times, you can see from screenshot, the memory delta is much higher when msw is in used.

➜  msw git:(main) ✗ node withMsw.js 
6.177215576171875 MB
➜  msw git:(main) ✗ node withoutMsw.js 
1.280303955078125
➜  msw git:(main) ✗ node withMsw.js
9.632209777832031 MB
➜  msw git:(main) ✗  node withoutMsw.js
0.668487548828125
➜  msw git:(main) ✗ node withMsw.js 
7.8651275634765625 MB
➜  msw git:(main) ✗ node withoutMsw.js
0.6793899536132812
➜  msw git:(main) ✗ 

[leaveswoods]

And I found an interesting thing, different node version can have different behavior here.

My apology, the above test was run on node 18. But I have run the test again in node 20, still have memory leak, but is better than node 18.

[leaveswoods]

My current solution is switching to use another library "nock": "^13.5.6", which does not have memory leak issue.

However, nock >= 14 also has memory leak issue, and I think it is likely because nock >= 14 starts to use @mswjs/interceptors, while nock 13 does not.

Hope this can help you for debugging @kettanaito .

[kettanaito]

Thanks, @leaveswoods. The root cause it likely in Interceptors and how we handle the socket connection.

It's a bit hard to have a reproducible test if both scenarios consume some memory, the faulty one just happens to consume more... I will try to wrap my head about this, but your insight helped, thank you.

As to why different Node.js versions reflect differently on this issue is likely because we are tapping into the native net and its sockets, and Node.js might have different behaviors there across versions.

[rajyan]

We encountered a crash when upgrading msw. Through a binary search, I identified that the issue is caused by #2508 (which includes @mswjs/interceptors update). I also confirmed a memory leak using --logHeapUsage option.

We are using node 22, the error message is as follows.

node: ../deps/uv/src/unix/stream.c:456: uv__stream_destroy: Assertion `!uv__io_active(&stream->io_watcher, POLLIN | POLLOUT)' failed.

Hope this helps for further investigation.