Support Java 9 modular runtime images

http://openjdk.java.net/jeps/220 changes the layout of the
JDK to encapsulate the provided libraries with the new module
system.

This commit modifies the compiler's classpath implementation
to scan the new location of these, the `jrt://` virtual filesystem.

This might need to be adjusted once we provide a means for
users to specify the subset of modules that they want to
depend on, but for now reclaims the ground we lost.

```
⚡ (java_use 9-ea; qscala)
Welcome to Scala 2.12.0-20160908-223617-7e4ebda (Java HotSpot(TM) 64-Bit Server VM, Java 9-ea).
Type in expressions for evaluation. Or try :help.

scala> import StackWalker._, java.util.stream._, scala.collection.JavaConverters._
import StackWalker._
import java.util.stream._
import scala.collection.JavaConverters._

scala> (() => StackWalker.getInstance(java.util.EnumSet.of(Option.RETAIN_CLASS_REFERENCE)).walk[Seq[String]]((s: java.util.stream.Stream[StackFrame]) => s.iterator.asScala.take(3).map(_.toString).toList)).apply().mkString("\n")
res0: String =
.$anonfun$res0$1(<console>:21)
.<init>(<console>:21)
.<clinit>(<console>)

scala>
```

I've marked the new class, `NioFile` as `private[scala]` to justify
the forward compatibility whitelist entry.

In principle we could use NioFile more widely rather than `PlainFile`
I tried this out in https://github.com/retronym/scala/commit/b2d0a17a
which passed CI. But to be conservative, I'm not submitting that change
at this point.

4 files changed, 57 insertions, 3 deletions

diff --git a/src/compiler/scala/tools/nsc/classpath/ClassPathFactory.scala b/src/compiler/scala/tools/nsc/classpath/ClassPathFactory.scala
index 3a29f1ba11..80c5ec8828 100644
--- a/src/compiler/scala/tools/nsc/classpath/ClassPathFactory.scala
+++ b/src/compiler/scala/tools/nsc/classpath/ClassPathFactory.scala
@@ -4,6 +4,7 @@
 package scala.tools.nsc.classpath
 
 import scala.reflect.io.{AbstractFile, VirtualDirectory}
+import scala.reflect.io.Path.string2path
 import scala.tools.nsc.Settings
 import FileUtils.AbstractFileOps
 import scala.tools.nsc.util.ClassPath
@@ -52,7 +53,10 @@ class ClassPathFactory(settings: Settings) {
   protected def classesInPathImpl(path: String, expand: Boolean) =
     for {
       file <- expandPath(path, expand)
-      dir <- Option(AbstractFile.getDirectory(file))
+      dir <- {
+        def asImage = if (file.endsWith(".jimage")) Some(AbstractFile.getFile(file)) else None
+        Option(AbstractFile.getDirectory(file)).orElse(asImage)
+      }
     } yield newClassPath(dir)
 
   private def createSourcePath(file: AbstractFile): ClassPath =
diff --git a/src/compiler/scala/tools/nsc/classpath/DirectoryClassPath.scala b/src/compiler/scala/tools/nsc/classpath/DirectoryClassPath.scala
index c4be59d7eb..133a656206 100644
--- a/src/compiler/scala/tools/nsc/classpath/DirectoryClassPath.scala
+++ b/src/compiler/scala/tools/nsc/classpath/DirectoryClassPath.scala
@@ -4,7 +4,9 @@
 package scala.tools.nsc.classpath
 
 import java.io.File
-import java.net.URL
+import java.net.{URI, URL}
+import java.nio.file.{FileSystems, Files, SimpleFileVisitor}
+import java.util.function.IntFunction
 import java.util
 import java.util.Comparator
 
@@ -119,6 +121,53 @@ trait JFileDirectoryLookup[FileEntryType <: ClassRepresentation] extends Directo
   def asClassPathStrings: Seq[String] = Seq(dir.getPath)
 }
 
+object JImageDirectoryLookup {
+  import java.nio.file._, java.net.URI, scala.collection.JavaConverters._
+  def apply(): List[ClassPath] = {
+    try {
+      val fs = FileSystems.getFileSystem(URI.create("jrt:/"))
+      val dir: Path = fs.getPath("/modules")
+      val modules = Files.list(dir).iterator().asScala.toList
+      modules.map(m => new JImageDirectoryLookup(fs, m.getFileName.toString))
+    } catch {
+      case _: ProviderNotFoundException | _: FileSystemNotFoundException =>
+        Nil
+    }
+  }
+}
+class JImageDirectoryLookup(fs: java.nio.file.FileSystem, module: String) extends DirectoryLookup[ClassFileEntryImpl] with NoSourcePaths {
+  import java.nio.file.Path, java.nio.file._
+  type F = Path
+  val dir: Path = fs.getPath("/modules/" + module)
+
+  protected def emptyFiles: Array[Path] = Array.empty
+  protected def getSubDir(packageDirName: String): Option[Path] = {
+    val packageDir = dir.resolve(packageDirName)
+    if (Files.exists(packageDir) && Files.isDirectory(packageDir)) Some(packageDir)
+    else None
+  }
+  protected def listChildren(dir: Path, filter: Option[Path => Boolean]): Array[Path] = {
+    import scala.collection.JavaConverters._
+    val f = filter.getOrElse((p: Path) => true)
+    Files.list(dir).iterator().asScala.filter(f).toArray[Path]
+  }
+  protected def getName(f: Path): String = f.getFileName.toString
+  protected def toAbstractFile(f: Path): AbstractFile = new scala.reflect.io.PlainNioFile(f)
+  protected def isPackage(f: Path): Boolean = Files.isDirectory(f) && mayBeValidPackage(f.getFileName.toString)
+
+  def asURLs: Seq[URL] = Seq(dir.toUri.toURL)
+  def asClassPathStrings: Seq[String] = asURLs.map(_.toString)
+
+  def findClassFile(className: String): Option[AbstractFile] = {
+    val relativePath = FileUtils.dirPath(className) + ".class"
+    val classFile = dir.resolve(relativePath)
+    if (Files.exists(classFile)) Some(new scala.reflect.io.PlainNioFile(classFile)) else None
+  }
+  override protected def createFileEntry(file: AbstractFile): ClassFileEntryImpl = ClassFileEntryImpl(file)
+  override protected def isMatchingFile(f: Path): Boolean = Files.isRegularFile(f) && f.getFileName.toString.endsWith(".class")
+  override private[nsc] def classes(inPackage: String): Seq[ClassFileEntry] = files(inPackage)
+}
+
 case class DirectoryClassPath(dir: File) extends JFileDirectoryLookup[ClassFileEntryImpl] with NoSourcePaths {
   override def findClass(className: String): Option[ClassRepresentation] = findClassFile(className) map ClassFileEntryImpl
 
diff --git a/src/compiler/scala/tools/nsc/classpath/FileUtils.scala b/src/compiler/scala/tools/nsc/classpath/FileUtils.scala
index bbcfcb24ca..2ade83c6f9 100644
--- a/src/compiler/scala/tools/nsc/classpath/FileUtils.scala
+++ b/src/compiler/scala/tools/nsc/classpath/FileUtils.scala
@@ -63,7 +63,7 @@ object FileUtils {
 
   // probably it should match a pattern like [a-z_]{1}[a-z0-9_]* but it cannot be changed
   // because then some tests in partest don't pass
-  private def mayBeValidPackage(dirName: String): Boolean =
+  def mayBeValidPackage(dirName: String): Boolean =
     (dirName != "META-INF") && (dirName != "") && (dirName.charAt(0) != '.')
 
   def mkFileFilter(f: JFile => Boolean) = new FileFilter {
diff --git a/src/compiler/scala/tools/util/PathResolver.scala b/src/compiler/scala/tools/util/PathResolver.scala
index c351b6ace1..188cabbc8d 100644
--- a/src/compiler/scala/tools/util/PathResolver.scala
+++ b/src/compiler/scala/tools/util/PathResolver.scala
@@ -234,6 +234,7 @@ final class PathResolver(settings: Settings) {
 
     // Assemble the elements!
     def basis = List[Traversable[ClassPath]](
+      JImageDirectoryLookup.apply(),                // 0. The Java 9 classpath (backed by the jrt:/ virtual system)
       classesInPath(javaBootClassPath),             // 1. The Java bootstrap class path.
       contentsOfDirsInPath(javaExtDirs),            // 2. The Java extension class path.
       classesInExpandedPath(javaUserClassPath),     // 3. The Java application class path.