fix: new instance of FlatESLint should load latest config file version

[mdjermanovic]

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Aims to fix #16576.

What changes did you make? (Give an overview)

This change appends query string with config file's mtime to its URL in order to force reload of the config file when it has been changed since the last load.

Is there anything you'd like reviewers to focus on?

Marked as draft as this approach doesn't seem to work with CJS config files.

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[fasttime]

I managed to get the CJS test passing by removing the config from the CommonJS module cache before the import. Basically:

index f041ac6a3..40ef95d39 100644
--- a/lib/eslint/flat-eslint.js
+++ b/lib/eslint/flat-eslint.js
@@ -284,6 +284,7 @@ async function loadFlatConfigFile(filePath) {

     debug(`Config file URL is ${fileURL}`);

+    delete require.cache[filePath]; // Uncache CJS module if exists.
     return (await import(fileURL)).default;
 }

This keeps the ESM cache unchanged, so the module should only be reloaded when the URL changes because of the timestamp. Obviously, this is not a very clean solution. I'd wish there was a standard way to reload CJS or ESM modules without starting a new process.

[mdjermanovic]

Seems to work with delete require.cache[filePath], @fasttime thanks for the suggestion!

[mdjermanovic]

This keeps the ESM cache unchanged, so the module should only be reloaded when the URL changes because of the timestamp.

Oh, I overlooked this. Hmm, I think you're right, URL would already have a cache hit in the import cache, so deleting from the underlying require cache wouldn't cause reloading. Still, might be better to minimize side effects by deleting the entry only when it's necessary?

[fasttime]

Yes, I can think of a few edge cases, although they don't seem very likely. For example, if a config file is required by some plugin after being manually deleted from the CJS module cache, that would trigger a reload. I don't know why a plugin or tool that uses ESLint would require a config file manually, but for such cases it would be safest to keep the config in the CJS module cache if it is not going to be reloaded.

It should be even possible to delete the config module from require.cache unconditionally before import, but restore it afterwards (probably in a finally block) if no new entry with the same key (filePath) was created. That would make a map of filePath to mtime superfluous.

[nzakas]

I wonder if all of this is strictly necessary. Can't we just append ?time=${new Date().getTime()} to the URL and forgo needing to check the actual modification time?

[mdjermanovic]

That would work, but every time we call loadFlatConfigFile a new instance would be created and cached forever. I left a note on why this solution might be problematic here:

eslint/lib/eslint/flat-eslint.js

Lines 297 to 299 in b82bd56

	* Note that we should not overuse queries (e.g., by appending the current time
	* to always reload the config file module) as that could cause memory leaks
	* because entries are never removed from the import cache.

[nzakas]

Ah, totally missed that. Makes sense. 👍

[nzakas]

LGTM.

[nzakas]

Ah, totally missed that. Makes sense. 👍

[nzakas]

@mdjermanovic because it's been a couple of weeks, didn't want to merge without being sure this was considered done. If so, please feel free to merge.