enable ServiceLoader discovery across classloader layers

java.util.ServiceLoader uses findResources(), which was not
overriden in ReverseLookupClassLoader, causing resources available
in the descendant classloader not to be discovered when a service
loader instance was using the top classloader.

main/src/main/java/sbt/internal/ReverseLookupClassLoader.java

@@ -8,7 +8,9 @@
 package sbt.internal;
 
 import java.io.File;
+import java.io.IOException;
 import java.net.URL;
+import java.util.Enumeration;
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.concurrent.atomic.AtomicReference;
 import sbt.util.Logger;
@@ -64,6 +66,31 @@ final class ReverseLookupClassLoader extends ManagedClassLoader {
     return url != null ? url : directDescendant.get().findResource(name);
   }
 
+  @Override
+  public Enumeration<URL> findResources(String name) throws IOException {
+    final Enumeration<URL> parentResources = super.findResources(name);
+
+    if (directDescendant.get() == null) {
+      return parentResources;
+    }
+
+    final Enumeration<URL> directDescendantResources = directDescendant.get().findResources(name);
+    return new Enumeration<URL>() {
+      @Override
+      public boolean hasMoreElements() {
+        return parentResources.hasMoreElements() || directDescendantResources.hasMoreElements();
+      }
+
+      @Override
+      public URL nextElement() {
+        if (parentResources.hasMoreElements()) {
+          return parentResources.nextElement();
+        }
+        return directDescendantResources.nextElement();
+      }
+    };
+  }
+
   void setup(final File tmpDir) {
     setTempDir(tmpDir);
   }

sbt/src/sbt-test/classloader-cache/service-loader/build.sbt

@@ -0,0 +1,4 @@
+val dependency = project.settings(exportJars := true)
+val descendant = project.dependsOn(dependency).settings(
+  libraryDependencies += "org.scalatest" %% "scalatest" % "3.0.5" % "test"
+)

sbt/src/sbt-test/classloader-cache/service-loader/dependency/src/main/java/dependency/Runnable.java

@@ -0,0 +1,6 @@
+package dependency;
+
+public class Runnable implements java.lang.Runnable {
+    public void run() {}
+}
+

sbt/src/sbt-test/classloader-cache/service-loader/dependency/src/main/resources/META-INF/services/java.lang.Runnable

@@ -0,0 +1 @@
+dependency.Runnable

sbt/src/sbt-test/classloader-cache/service-loader/descendant/src/main/resources/META-INF/services/java.lang.Runnable

@@ -0,0 +1 @@
+descendant.Runnable

sbt/src/sbt-test/classloader-cache/service-loader/descendant/src/main/scala/test/Runnable.scala

@@ -0,0 +1,6 @@
+package descendant
+
+class Runnable extends java.lang.Runnable {
+  override def run(): Unit = ()
+}
+

sbt/src/sbt-test/classloader-cache/service-loader/descendant/src/test/scala/test/ServiceLoaderTest.scala

@@ -0,0 +1,20 @@
+package test
+
+import collection.JavaConverters._
+import org.scalatest._
+
+class ServiceLoaderTest extends FlatSpec {
+  val expected = Set(classOf[dependency.Runnable], classOf[descendant.Runnable])
+
+  val descendantClassLoader = classOf[descendant.Runnable].getClassLoader
+  val descendantRunnableLoader = java.util.ServiceLoader.load(classOf[java.lang.Runnable], descendantClassLoader)
+  val descendantLoadedClasses = descendantRunnableLoader.iterator().asScala.map(_.getClass).toSet
+  assert(descendantLoadedClasses == expected)
+
+  // this was the actual problem, when classLoaderLayeringStrategy := ClassLoaderLayeringStrategy.AllLibraryJars
+  val dependencyClassLoader = classOf[dependency.Runnable].getClassLoader
+  val dependencyRunnableLoader = java.util.ServiceLoader.load(classOf[java.lang.Runnable], dependencyClassLoader)
+  val dependencyLoadedClasses = dependencyRunnableLoader.iterator().asScala.map(_.getClass).toSet
+  assert(dependencyLoadedClasses == expected)
+}
+

sbt/src/sbt-test/classloader-cache/service-loader/test

@@ -0,0 +1,7 @@
+> set descendant / Test / classLoaderLayeringStrategy := ClassLoaderLayeringStrategy.AllLibraryJars
+
+> test
+
+> set descendant / Test / classLoaderLayeringStrategy := ClassLoaderLayeringStrategy.ScalaLibrary
+
+> test