No way to subscribe to files unmounting in development

Verify canary release

• I verified that the issue exists in the latest Next.js canary release

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 21.6.0: Mon Aug 22 20:20:05 PDT 2022; root:xnu-8020.140.49~2/RELEASE_ARM64_T8101
Binaries:
  Node: 16.15.0
  npm: 8.5.5
  Yarn: N/A
  pnpm: 7.25.1
Relevant packages:
  next: 12.3.0
  eslint-config-next: N/A
  react: 18.1.0
  react-dom: 18.1.0

Which area(s) of Next.js are affected? (leave empty if unsure)

No response

Link to the code that reproduces this issue

https://stackblitz.com/edit/vercel-next-js-jqfycd?file=server%2FBackendMock.ts

To Reproduce

1. Create a singleton file which tracks its own initialization, then logs every 10s:

class Test {
    static hasInitialized = false;
    static init() {
        if (!this.hasInitialized) {
            this.hasInitialized = true;
            setInterval(() => console.log("Polling something"), 10000);
        }
    }
}

2. call this function from a getServersideProps() or API endpoint
3. Make a change to the API or one of its dependencies

Describe the Bug

Related to https://github.com/vercel/next.js/issues/45204, unchanged files will be reloaded by nextjs if a different dependency of an API route (e.g.) is changed. While this alone seems to be a bug, the lack of a solution for "shutting down" files that get reloaded also seems to be an issue.

In particular, database connections or singleton handler classes, or anything polling, will not shut down when files are reloaded. This leads to redundant work being done, excessive memory consumption (eventually leading to OOMs), and pain in debugging things (multiple instances of singletons all living around at the same time).

One workaround is storing the instance in a global variable, and either shutting down any existing one if detected or just reusing the existing one. However, this is rather hacky and relies on abusing the global variable space to dump every singleton. Additionally the need to shutdown and restart database connections in these situations is rather painful, so I opt to reuse existing connections, but that prevents reloads of changes to initialization logic (would be nice if I could detect a reload due to change vs. reload due to other file changing)

Expected Behavior

There should be a way (similar to hot module reloading's module.hot.*) to "subscribe" to a module being unloaded. When this happens, it could clear any intervals, and close any connections.

Which browser are you using? (if relevant)

No response

How are you deploying your application? (if relevant)

No response

Start with the linked StackBlitz reproduction, especially server/BackendMock.ts, and compare its reload behavior with the related issue #45204. Define how a development-server module-unload subscription should work, then verify that intervals, database connections, and singleton handlers can be cleaned up when dependencies reload.