Comment on explicit closing of URLClassLoader instances

Specifically, why we need it for benchmarks but not for plugins.

renaissance-core/src/main/java/org/renaissance/core/BenchmarkSuite.java

@@ -283,6 +283,10 @@ private static ClassLoader createClassLoaderFromPaths(
       throw new ExtensionException("malformed URL(s) in classpath specification");
     }
 
+    //
+    // No need to explicitly close this URLClassLoader on exit, because it does not
+    // operate on files created by the harness that need to be deleted on exit.
+    //
     ClassLoader parent = ModuleLoader.class.getClassLoader();
     return new URLClassLoader(classPathUrls, parent);
   }

renaissance-core/src/main/java/org/renaissance/core/ModuleLoader.java

@@ -158,6 +158,14 @@ ClassLoader createClassLoaderForModule(String name) throws ModuleLoadingExceptio
           name, filePaths.size(), makeClassPath(filePaths)
         ));
 
+        //
+        // Make sure to explicitly close this URLClassLoader on exit, because it operates
+        // on files created by the harness in the scratch directory hierarchy that need to
+        // be deleted on exit. Leaving the class loader open keeps the library JAR files
+        // open, preventing removal of the scratch directories. This is because on NFS,
+        // deleting an open file produces a NFS temporary file in the same directory, and
+        // on Windows, an open file cannot be deleted at all.
+        //
         return Cleaner.closeOnExit(new URLClassLoader(urls, thisClass.getClassLoader()));
 
       } catch (IOException e) {