staging: fix classpath discovery with sbt >= 1.3

This fixes the sbt-dotty/quoted-example-project scripted test after the
previous commit.

Getting a classpath from a classloader is impossible in general, so we
really shouldn't be relying on it, but I fixed the thing we currently
use to at least work with sbt >= 1.3, sbt now uses multiple layers of
classloaders so we have to recurse on the parent of the classloader to
find all the URLs.

I also made it more correct by not appending
System.getProperty("java.class.path") to the classpath, this is
incorrect in general and can lead to classpath pollution (e.g., when
launched from sbt where java.class.path will contain the jars used by
sbt itself, which might include a different version of scala-library
than the one we use).

Author: smarter

staging/src/scala/quoted/staging/QuoteDriver.scala

@@ -11,6 +11,9 @@ import dotty.tools.dotc.reporting._
 import scala.quoted._
 import scala.quoted.staging.Toolbox
 import java.net.URLClassLoader
+import java.nio.file.Paths
+import java.io.File
+import scala.annotation.tailrec
 
 /** Driver to compile quoted code
  *
@@ -53,7 +56,7 @@ private class QuoteDriver(appClassloader: ClassLoader) extends Driver {
 
   override def initCtx: Context = {
     val ictx = contextBase.initialCtx
-    ictx.settings.classpath.update(getCurrentClasspath(appClassloader))(ictx)
+    ictx.settings.classpath.update(classpathFromClassloader(appClassloader))(ictx)
     ictx
   }
 
@@ -64,17 +67,29 @@ private class QuoteDriver(appClassloader: ClassLoader) extends Driver {
     ctx.setReporter(new ThrowingReporter(ctx.reporter))
   }
 
-  private def getCurrentClasspath(cl: ClassLoader): String = {
-    val classpath0 = System.getProperty("java.class.path")
-    cl match {
-      case cl: URLClassLoader =>
-        // Loads the classes loaded by this class loader
-        // When executing `run` or `test` in sbt the classpath is not in the property java.class.path
-        import java.nio.file.Paths
-        val newClasspath = cl.getURLs.map(url => Paths.get(url.toURI).toString)
-        newClasspath.mkString("", java.io.File.pathSeparator, if (classpath0 == "") "" else java.io.File.pathSeparator + classpath0)
-      case _ => classpath0
-    }
+  /** Attempt to recreate a classpath from a classloader.
+   *
+   *  BEWARE: with exotic enough classloaders, this may not work at all or do
+   *  the wrong thing.
+   */
+  private def classpathFromClassloader(cl: ClassLoader): String = {
+    @tailrec
+    def loop(cl: ClassLoader, suffixClasspath: String): String =
+      cl match {
+        case cl: URLClassLoader =>
+          val updatedClasspath = cl.getURLs
+            .map(url => Paths.get(url.toURI).toAbsolutePath.toString)
+            .mkString(
+              "",
+              File.pathSeparator,
+              if (suffixClasspath.isEmpty) "" else File.pathSeparator + suffixClasspath
+            )
+          loop(cl.getParent, updatedClasspath)
+        case _ =>
+          suffixClasspath
+      }
+
+    loop(cl, "")
   }
 }