diff -r b2cdd24e83b6 -r 74dce5658d4c src/Pure/Thy/thy_resources.scala
--- a/src/Pure/Thy/thy_resources.scala	Thu Mar 22 17:00:48 2018 +0100
+++ b/src/Pure/Thy/thy_resources.scala	Thu Mar 22 17:05:06 2018 +0100
@@ -7,6 +7,9 @@
 package isabelle
 
 
+import java.io.{File => JFile}
+
+
 object Thy_Resources
 {
   /* PIDE session */
@@ -75,6 +78,14 @@
   {
     session =>
 
+    val tmp_dir: JFile = Isabelle_System.tmp_dir("server_session")
+
+    override def stop(): Process_Result =
+    {
+      try { super.stop() }
+      finally { Isabelle_System.rm_tree(tmp_dir) }
+    }
+
     def use_theories(
       theories: List[(String, Position.T)],
       qualifier: String = Sessions.DRAFT,