ESM custom loader loading code twice

[mgagliardo91]

With the recent release of Node v18.19.0, the dd-trace-js custom ESM loader is loading our application twice, causing it to fail with a port already bound error (this is an express application).

It appears the the v18.19.0 release added some breaking changes that would alter the way the dd-trace-js loader hook works. They indicate that v20 moves loaders to their own thread, which sounds like the culprit, but I'm unsure why that would affect v18. See

• ESM --loader cause duplicate app load nodejs/node#49082
• Breaking changes in 18.19.0 (including The --loader flag was deprecated in Node v20.6.0) nodejs/node#51048
• Node 20 ESM loaders cannot hook into the CommonJS loader nodejs/node#47880

I have tried to switch to use the --import syntax, as outlined here, however, this still results in duplicate loading.

I am unsure whether this is a bug with Node v18.19.0, or if the breaking change is in fact correct and dd-trace-js loader hook needs to be updated to reflect the changes.

Expected behaviour
Loading dd-trace-js using the custom ESM loader node --loader=dd-trace/loader-hook.mjs properly starts our application, once.

Actual behaviour
Custom loader loads our application twice, which triggers a port conflict given that its trying to start two threads of the same application.

Steps to reproduce

1. Create a simple express app using ESM
2. Instrument with dd-trace-js custom loader
3. Run with Node v18.19.0

Environment

• Operation system: node:18-alpine
• Node.js version: v18.19.0
• Tracer version: 4.20.0
• Agent version:
• Relevant library versions:

[mgagliardo91]

For now, we will pin our docker node version to v18.18.0; however, we hope to resolve this without leaving the pin in place.

[Rowanto]

Same issue. Pinning to 18.18 (node:18.18-alpine) solves the issue temporarily as outlined by @mgagliardo91

[khanayan123]

Hi is this still an issue?

the fix was applied in this PR: DataDog/import-in-the-middle#44

& was released with this release: https://github.com/DataDog/import-in-the-middle/releases/tag/v1.6.0