Skip contents of symlinked directories in clean

The clean task was previously deleting the contents of directories that
were symlinked into the target directory. This was an oversight because
it never occurred to me that users might symlink a directory whose
contents they did not want deleted into the target directory.

main/src/main/scala/sbt/internal/Clean.scala

@@ -41,7 +41,7 @@ private[sbt] object Clean {
         .list(Glob(path, AnyPath))
         .filterNot { case (p, _) => exclude(p) }
         .foreach {
-          case (dir, attrs) if attrs.isDirectory =>
+          case (dir, attrs) if attrs.isDirectory && !attrs.isSymbolicLink =>
             deleteRecursive(dir)
             delete(dir)
           case (file, _) => delete(file)
@@ -89,15 +89,11 @@ private[sbt] object Clean {
         val excludeFilter = cleanFilter(scope).value
         val delete = cleanDelete(scope).value
         val targetDir = (target in scope).?.value.map(_.toPath)
-        def recursiveFiles(dir: Path): Seq[Path] =
-          view.list(dir.toGlob / **).collect { case (p, _) if !excludeFilter(p) => p }
-        val targetFiles = (if (full) targetDir else None).fold(Nil: Seq[Path])(recursiveFiles)
-        val cleanPaths = (cleanFiles in scope).?.value.getOrElse(Nil).flatMap { f =>
-          val path = f.toPath
-          if (Files.isDirectory(path)) path +: recursiveFiles(path) else path :: Nil
+
+        targetDir.filter(_ => full).foreach(deleteContents(_, excludeFilter, view, delete))
+        (cleanFiles in scope).?.value.getOrElse(Nil).foreach { f =>
+          deleteContents(f.toPath, excludeFilter, view, delete)
         }
-        val allFiles = cleanPaths.view ++ targetFiles
-        allFiles.sorted.reverseIterator.foreach(delete)
 
         // This is the special portion of the task where we clear out the relevant streams
         // and file outputs of a task.

sbt/src/sbt-test/nio/clean-symlinks/build.sbt

@@ -0,0 +1,5 @@
+import java.nio.file.Files
+
+TaskKey[Unit]("createSymlinkedDirectory") := {
+  Files.createSymbolicLink(target.value.toPath / "foo", baseDirectory.value.toPath / "foo")
+}

sbt/src/sbt-test/nio/clean-symlinks/foo/bar

@@ -0,0 +1 @@
+bar

sbt/src/sbt-test/nio/clean-symlinks/test

@@ -0,0 +1,9 @@
+> createSymlinkedDirectory
+
+$ exists target/foo/bar
+$ exists foo/bar
+
+> clean
+
+$ absent target/foo
+$ exists foo/bar