Turbopack Error: internal error: entered unreachable code: there must be a path to a root

[Maverick283]

Turbopack version: > v15.4.1
Next.js version: > 15.4.1

Error message:

Turbopack Error: internal error: entered unreachable code: there must be a path to a root

Details:

Starting with Next.js 15.4.1, we get an error when trying to run next dev --turbopack.

yarn run v1.22.22
$ next dev --turbopack
   ▲ Next.js 15.4.1 (Turbopack)
   - Local:        http://localhost:3000
   - Network:      http://192.168.1.126:3000
   - Environments: .env.local

 ✓ Starting...

thread 'tokio-runtime-worker' panicked at turbopack/crates/turbopack-core/src/module_graph/mod.rs:934:25:
internal error: entered unreachable code: there must be a path to a root
stack backtrace:
 ○ Compiling instrumentation Node.js ...
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

thread 'tokio-runtime-worker' panicked at turbopack/crates/turbopack-core/src/module_graph/mod.rs:934:25:
internal error: entered unreachable code: there must be a path to a root
stack backtrace:
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

thread 'tokio-runtime-worker' panicked at turbopack/crates/turbopack-core/src/module_graph/mod.rs:934:25:
internal error: entered unreachable code: there must be a path to a root
stack backtrace:
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

[... This repeats a couple of times]

thread 'tokio-runtime-worker' panicked at turbopack/crates/turbopack-core/src/module_graph/mod.rs:934:25:
internal error: entered unreachable code: there must be a path to a root
stack backtrace:
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

thread 'tokio-runtime-worker' panicked at turbopack/crates/turbopack-core/src/module_graph/mod.rs:934:25:
internal error: entered unreachable code: there must be a path to a root
stack backtrace:
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
[Error [TurbopackInternalError]: internal error: entered unreachable code: there must be a path to a root

Debug info:
- Execution of TaskId { id: 2147483651 } transient failed
- Execution of get_written_endpoint_with_issues_operation failed
- Execution of PlainIssue::from_issue failed
- Execution of <ModuleGraphImportTracer as ImportTracer>::get_traces failed
- internal error: entered unreachable code: there must be a path to a root]

✨  Done in 6.24s.

When setting `RUST_BACKTRACE=full`, there is some more information:

thread 'tokio-runtime-worker' panicked at turbopack/crates/turbopack-core/src/module_graph/mod.rs:934:25:
internal error: entered unreachable code: there must be a path to a root
stack backtrace:
   0:        0x58f10992c - _napi_register_module_v1
   1:        0x58f12d968 - _napi_register_module_v1
   2:        0x58f1048c8 - _napi_register_module_v1
   3:        0x58f1097e0 - _napi_register_module_v1
   4:        0x58f10be38 - _napi_register_module_v1
   5:        0x58f10bc88 - _napi_register_module_v1
   6:        0x58f10c938 - _napi_register_module_v1
   7:        0x58f10c500 - _napi_register_module_v1
   8:        0x58f109df0 - _napi_register_module_v1
   9:        0x58f10c1d0 - _napi_register_module_v1
  10:        0x58f27e8ac - _napi_register_module_v1
  11:        0x58d409838 - _BrotliDecoderVersion
  12:        0x58d60fea4 - _BrotliDecoderVersion
  13:        0x58d3f464c - _BrotliDecoderVersion
  14:        0x58a243aac - <unknown>
  15:        0x58a29f35c - <unknown>
  16:        0x58a26b730 - <unknown>
  17:        0x58a26a680 - <unknown>
  18:        0x58a29eea0 - <unknown>
  19:        0x58a0392c8 - <unknown>
  20:        0x589f43d2c - <unknown>
  21:        0x589f50db0 - <unknown>
  22:        0x589f51164 - <unknown>
  23:        0x58f0c92ac - _napi_register_module_v1
  24:        0x58f0c8a00 - _napi_register_module_v1
  25:        0x58f0b0400 - _napi_register_module_v1
  26:        0x58f0aff00 - _napi_register_module_v1
  27:        0x58f0c8028 - _napi_register_module_v1
  28:        0x58f0b7d68 - _napi_register_module_v1
  29:        0x58f0c2d90 - _napi_register_module_v1
  30:        0x58f0990ac - _napi_register_module_v1
  31:        0x58f093168 - _napi_register_module_v1
  32:        0x58f0b69a0 - _napi_register_module_v1
  33:        0x58f0a34e0 - _napi_register_module_v1
  34:        0x58f09bec4 - _napi_register_module_v1
  35:        0x58f09c278 - _napi_register_module_v1
  36:        0x58f10ed50 - _napi_register_module_v1
  37:        0x198d95c0c - __pthread_cond_wait

Running without --turbopack still works (obviously).

I cannot find anything anywhere about this issue related to Next.js.
While I am aware of this being one of the annoying "crying for help" posts, I don't know what else to do. Therefore:
Any help is much appreciated!!

[Maverick283]

I am absolutely confused, which is why I'll leave some of the things I've found out here. My lack of deeper understanding when turbopack(?) decides to execute and import things leave me quite riddled.

From what I understand, there must have been some change in how the turbopack resolves imports with Next.js v15.4.1

TL;DR

A circular dependency of some sort causes the turbopack ModuleGraphImportTracer to fail

---

Not too long, I am curious

For the sake of keeping this a minimal example, let's just say we want to call another function on startup that gives us a value we can log. I now realize that the placement of some code is not ideal, but let's ignore this for now - I am just trying to provide insight for anyone struggling with this in the future.

To achieve that, we added the following to the instrumentation.ts

import { getEnv } from '@/getEnv'

let logger
const setupSpecialLogger = () => {
  logger = console
}

export const specialLogger = (text: string) => {
  setupSpecialLogger()
  logger!.log(text)
}

export async function register() {
    specialLogger(await getEnv())
}

As the name suggest, we want to setup our logger here. Let's keep it simple and just use console here for the example, while this would obviously be more complex with a proper custom logger.
We also want to log our env variable at startup.

For now, this should still work. However, if we implement the getEnv.ts like so, it will crash while starting:

import { specialLogger } from '@/instrumentation'

export const getEnv = async () => {
  specialLogger(`We're returning the env value here`)
  return process.env.THE_VALUE
}

This is, to my understanding, because of the circular imports. Why it didn't crash before but starts crashing with 15.4.1 => I don't know.