Scala Native crash when wrong cached ClassLoader is used

[lolgab]

Given a Cross ScalaNativeModule, multiple classloaders are used,
Since we cache the output nativeConfig, it can happen that a nativeConfig generated
with a Classloader is used when linking with another classloader.
This can lead to ClassCastException since objects from different classloader can't be shared.

Repro

import mill._
import mill.scalalib._
import mill.scalanativelib._

object app extends Cross[App]("a", "b", "c", "d")
trait App extends ScalaNativeModule with Cross.Module[String] {
  def scalaVersion = "3.5.2"
  def scalaNativeVersion = "0.5.6"
}

mill __.nativeLink

Output

app[aarch64-macos-none].nativeLink java.lang.ClassCastException: class scala.scalanative.build.Config$Impl cannot be cast to class scala.scalanative.build.Config (scala.scalanative.build.Config$Impl is in unnamed module of loader mill.api.ClassLoader$$anon$1 @278c6ad9; scala.scalanative.build.Config is in unnamed module of loader mill.api.ClassLoader$$anon$1 @3d4e7cb1)
    mill.scalanativelib.worker.ScalaNativeWorkerImpl.nativeLink(ScalaNativeWorkerImpl.scala:105)
    mill.scalanativelib.ScalaNativeModule.$anonfun$nativeLink$4(ScalaNativeModule.scala:286)
    mill.scalanativelib.ScalaNativeModule$ScalaNativeBridge.$anonfun$apply$1(ScalaNativeModule.scala:159)
    mill.api.CachedFactory.withValue(CachedFactory.scala:39)
    mill.scalanativelib.ScalaNativeModule$ScalaNativeBridge.apply(ScalaNativeModule.scala:158)
    mill.scalanativelib.ScalaNativeModule.$anonfun$nativeLink$3(ScalaNativeModule.scala:287)
app[aarch64-windows-gnu].nativeLink scala.scalanative.build.BuildException: Failed to compile /Users/lorenzo/scala/scala-native-cross/out/app/aarch64-windows-gnu/nativeWorkdir.dest/native/dependencies/nativelib_native0.5_3-0.5.6-0/scala-native/nativeThreadTLS.c
app[x86_64-linux-gnu].nativeLink java.lang.ClassCastException: class scala.scalanative.build.Config$Impl cannot be cast to class scala.scalanative.build.Config (scala.scalanative.build.Config$Impl is in unnamed module of loader mill.api.ClassLoader$$anon$1 @2bd047bc; scala.scalanative.build.Config is in unnamed module of loader mill.api.ClassLoader$$anon$1 @709c8fd6)
    mill.scalanativelib.worker.ScalaNativeWorkerImpl.nativeLink(ScalaNativeWorkerImpl.scala:105)
    mill.scalanativelib.ScalaNativeModule.$anonfun$nativeLink$4(ScalaNativeModule.scala:286)
    mill.scalanativelib.ScalaNativeModule$ScalaNativeBridge.$anonfun$apply$1(ScalaNativeModule.scala:159)
    mill.api.CachedFactory.withValue(CachedFactory.scala:39)
    mill.scalanativelib.ScalaNativeModule$ScalaNativeBridge.apply(ScalaNativeModule.scala:158)
    mill.scalanativelib.ScalaNativeModule.$anonfun$nativeLink$3(ScalaNativeModule.scala:287)

Possible solutions

• Add the module as the cache key, to make a module always use the same ClassLoader.

Workaround

Pass -j 1 so a single Classloader is used.