Module.runMain causes ESM loaders in the chain to be duplicated

[cspotcode]

Version

v19.0.0-nightly202207093d575a4f1b

Platform

Linux DESKTOP-5K7R6JP 5.10.102.1-microsoft-standard-WSL2 #1 SMP Wed Mar 2 00:30:59 UTC 2022 x86_64 x86_64 x86_64 GNU/Linux

Subsystem

No response

What steps will reproduce the bug?

Detailed reproduction here:
https://github.com/cspotcode/repros/tree/node-loader-chaining-duplication

On node nightly, use node --loader ./loader.mjs ./index.mjs

Within index.mjs, call import {runMain} from 'module'; runMain() and then do something that uses loader hooks, such as importing or calling import.meta.resolve(). The hooks in loader.mjs have been duplicated in the chain and thus will be called more than they should.

The linked reproduction demonstrates this by implementing a loader hook that logs a message and delegates to the next hook in the chain.

How often does it reproduce? Is there a required condition?

Always

What is the expected behavior?

Module.runMain understands if the loader hooks have already been initialized and does not duplicate them in the chain.

What do you see instead?

Module.runMain duplicates all loaders in the chain.

Additional information

No response

[aduh95]

/cc @nodejs/loaders

[daeyeon]

I guess that this could be fixed if we ensure initializeLoader() is called only once.

Question: runMain() seems not to be documented yet. (#32643)
If so, how should we deal with this issue?

[JakobJingleheimer]

I guess that this could be fixed if we ensure initializeLoader() is called only once.

That sounds good to me. Happy to tackle that part.