buildSrc/test loads LibrariesForLibs twice from 2 different classloaders.

[yogurtearl]

When using to the work-around posted here:
#15383 (comment)

i.e.

./buildSrc/build.gradle.kts
plugins {
    `kotlin-dsl`
}

repositories {
    mavenCentral()
}

println("from build-src build script: ${libs.versions.bb.get()}")

dependencies {
    implementation(files(libs.javaClass.superclass.protectionDomain.codeSource.location))
}

./buildSrc/settings.gradle.kts
dependencyResolutionManagement {
    versionCatalogs {
        create("libs") {
            from(files("../gradle/libs.versions.toml"))
        }
    }
}

./buildSrc/src/main/kotlin/foo.gradle.kts
import org.gradle.accessors.dm.LibrariesForLibs

val libs = the<LibrariesForLibs>()
println("from pre compiled script plugin: ${libs.versions.bb.get()}")

./build.gradle.kts
plugins {
    id("foo")
}

println("from main build script: ${libs.versions.bb.get()}")

./gradle/libs.versions.toml
[versions]
bb = "3.2.1"

./settings.gradle.kts
rootProject.name = "showcase"

When that workaround is used with tests in buildSrc/test, then org.gradle.accessors.dm.LibrariesForLibs is loaded from 2 different class loaders:

VisitableURLClassLoader(ClassLoaderScopeIdentifier.Id{coreAndPlugins:injected-plugin(local)})

and

VisitableURLClassLoader(ClassLoaderScopeIdentifier.Id{coreAndPlugins:settings[:]:buildSrc[:](export)})

Expected Behavior

Should work with buildSrc/test with GradleRunner and withPluginClasspath

Current Behavior

but gives this error:

> Failed to apply plugin 'foo.bar'.
   > class org.gradle.accessors.dm.LibrariesForLibs_Decorated cannot be cast to class org.gradle.accessors.dm.LibrariesForLibs (org.gradle.accessors.dm.LibrariesForLibs_Decorated is in unnamed module of loader org.gradle.internal.classloader.VisitableURLClassLoader @30747123; org.gradle.accessors.dm.LibrariesForLibs is in unnamed module of loader org.gradle.internal.classloader.VisitableURLClassLoader @66bdd123)

Context

if I print the classloaders like this:

import org.gradle.accessors.dm.LibrariesForLibs


internal val Project.libs: LibrariesForLibs
    get() {
        printClassloader(libExtension().javaClass)
        printClassloader(libExtension().javaClass.superclass)
        printClassloader(LibrariesForLibs::class.java)
        return libExtension() as LibrariesForLibs
    }

private fun Project.libExtension(): Any = extensions.getByName("libs")

fun printClassloader(clazz: Class<*>) {
    println("class ${clazz.canonicalName} has CL ${clazz.classLoader}")
    println("location ${clazz.protectionDomain.codeSource.location}")

    var parent = clazz.classLoader.parent
    while(parent != null) {
        println("with parent $parent")
        parent = parent.parent
    }
}

I will print something like this:

> Configure project :foo:bar:whevs
class org.gradle.accessors.dm.LibrariesForLibs_Decorated has CL VisitableURLClassLoader(ClassLoaderScopeIdentifier.Id{coreAndPlugins:settings[:]:buildSrc[:](export)})
location file:/private/var/folders/zg/tjjb_fkx1355xx67t7pyl43r123/T/junit15680257868112259141/.gradle/7.5.1/dependencies-accessors/f5ba69d2465650fe8c727e76995fffc6234da247/classes/
with parent CachingClassLoader(FilteringClassLoader(VisitableURLClassLoader(legacy-mixin-loader)))
with parent FilteringClassLoader(VisitableURLClassLoader(legacy-mixin-loader))
with parent VisitableURLClassLoader(legacy-mixin-loader)
with parent VisitableURLClassLoader(ant-and-gradle-loader)
with parent VisitableURLClassLoader(ant-loader)
with parent jdk.internal.loader.ClassLoaders$PlatformClassLoader@4c180213
class org.gradle.accessors.dm.LibrariesForLibs has CL VisitableURLClassLoader(ClassLoaderScopeIdentifier.Id{coreAndPlugins:settings[:]:buildSrc[:](export)})
location file:/private/var/folders/zg/tjjb_fkx1355xx67t7pyl43r123/T/junit15680257868112259141/.gradle/7.5.1/dependencies-accessors/f5ba69d2465650fe8c727e76995fffc6234da247/classes/
with parent CachingClassLoader(FilteringClassLoader(VisitableURLClassLoader(legacy-mixin-loader)))
with parent FilteringClassLoader(VisitableURLClassLoader(legacy-mixin-loader))
with parent VisitableURLClassLoader(legacy-mixin-loader)
with parent VisitableURLClassLoader(ant-and-gradle-loader)
with parent VisitableURLClassLoader(ant-loader)
with parent jdk.internal.loader.ClassLoaders$PlatformClassLoader@4c180213
class org.gradle.accessors.dm.LibrariesForLibs has CL VisitableURLClassLoader(ClassLoaderScopeIdentifier.Id{coreAndPlugins:injected-plugin(local)})
location file:/Users/me/git/myproject/buildSrc/build/tmp/test/work/.gradle-test-kit/caches/jars-9/749352387bd7e916adb995fc3d840123/classes.jar
with parent CachingClassLoader(FilteringClassLoader(VisitableURLClassLoader(legacy-mixin-loader)))
with parent FilteringClassLoader(VisitableURLClassLoader(legacy-mixin-loader))
with parent VisitableURLClassLoader(legacy-mixin-loader)
with parent VisitableURLClassLoader(ant-and-gradle-loader)
with parent VisitableURLClassLoader(ant-loader)
with parent jdk.internal.loader.ClassLoaders$PlatformClassLoader@4c180213

Your Environment

Gradle 7.5.1