[2.x] fix: Fixes stale resources in test (#9469)

**Problem**
sbt 2.x currently constructs the test classloader using symlinks
to the JARs in CAS (content-addressable storage).
This causes issues because JVM apparently caches opened JAR files by path,
and since the symlink itself doesn't change it could end up
serving stale resource files.

**Solution**
We can workaround this issue by resolving the symlinks to the real path.

main/src/main/scala/sbt/internal/ClassLoaders.scala

@@ -9,7 +9,7 @@
 package sbt
 package internal
 
-import java.io.File
+import java.io.{ IOException, File }
 import java.net.URL
 import java.nio.file.Path
 import sbt.ClassLoaderLayeringStrategy.*
@@ -29,8 +29,22 @@ import xsbti.ArtifactInfo
 import xsbti.HashedVirtualFileRef
 
 private[sbt] object ClassLoaders {
+
+  /**
+   * https://github.com/sbt/sbt/issues/9468
+   * Resolves symlinks before converting to a URL. Classpath entries backed by a
+   * CAS cache would otherwise all share one URL across builds;
+   * the JVM's jar/URL connection caching is keyed by path, so once a jar: URL for that
+   * stable path has been opened, later reads can keep returning the first content seen\
+   * even after the symlink target (and the underlying file content) changes.
+   */
+  private[sbt] def toRealURL(f: File): URL =
+    val real = try f.toPath.toRealPath().toFile
+    catch case _: IOException => f
+    real.toURI.toURL
+
   extension (files: Seq[File]) {
-    def urls: Array[URL] = files.toArray.map(_.toURI.toURL)
+    def urls: Array[URL] = files.toArray.map(toRealURL)
   }
   private val interfaceLoader = classOf[sbt.testing.Framework].getClassLoader
   /*

main/src/main/scala/sbt/internal/LayeredClassLoaders.scala

@@ -45,10 +45,10 @@ private[internal] final class ReverseLookupClassLoaderHolder(
     val closeThis: Boolean,
     val allowZombies: Boolean,
     val logger: Logger
-) extends URLClassLoader(Array.empty, null) {
+) extends URLClassLoader(Array.empty, null):
   private val cached: AtomicReference[ReverseLookupClassLoader] = new AtomicReference
   private val closed = new AtomicBoolean(false)
-  private val urls = classpath.map(_.toURI.toURL).toArray
+  private val urls = classpath.map(ClassLoaders.toRealURL).toArray
 
   /**
    * Get a classloader. If there is a loader available in the cache, it will use that loader,
@@ -69,7 +69,7 @@ private[internal] final class ReverseLookupClassLoaderHolder(
     reverseLookupClassLoader.setup(tempDir)
     new BottomClassLoader(
       ReverseLookupClassLoaderHolder.this,
-      fullClasspath.map(_.toURI.toURL).toArray,
+      fullClasspath.map(ClassLoaders.toRealURL).toArray,
       reverseLookupClassLoader,
       tempDir,
       closeThis,
@@ -97,7 +97,7 @@ private[internal] final class ReverseLookupClassLoaderHolder(
       case c    => c.close()
     }
   }
-}
+end ReverseLookupClassLoaderHolder
 
 /**
  * This is more or less copied from the NativeCopyLoader in zinc. It differs from the zinc

sbt-app/src/sbt-test/compiler-project/run-test/build.sbt

@@ -6,3 +6,5 @@ libraryDependencies ++= Seq(
   "commons-io" % "commons-io" % "2.5" % Runtime,
 )
 libraryDependencies += scalaVersion("org.scala-lang" % "scala-compiler" % _ ).value
+
+Test / fork := true

sbt-app/src/sbt-test/tests/test-resource/build.sbt

@@ -0,0 +1,7 @@
+val munit = "org.scalameta" %% "munit" % "1.0.4"
+scalaVersion := "3.8.4"
+
+lazy val root = rootProject
+  .settings(
+    libraryDependencies += munit % Test
+  )

sbt-app/src/sbt-test/tests/test-resource/changes/bad.txt

@@ -0,0 +1 @@
+bad

sbt-app/src/sbt-test/tests/test-resource/src/test/resources/hello.txt

@@ -0,0 +1 @@
+hello

sbt-app/src/sbt-test/tests/test-resource/src/test/scala/ATest.scala

@@ -0,0 +1,13 @@
+package testpkg
+
+import munit.*
+import scala.io.Source
+import scala.util.Using
+
+class ATest extends FunSuite:
+  test("hello.txt should contain hello") {
+    Using.resource(Source.fromResource("hello.txt")): r =>
+      val actual = r.mkString.trim
+      assert(actual == "hello", s"actual: $actual")
+  }
+end ATest

sbt-app/src/sbt-test/tests/test-resource/test

@@ -0,0 +1,5 @@
+> test
+
+$ copy-file changes/bad.txt src/test/resources/hello.txt
+
+-> test