Use a store fence to try to avoid race condition with sys.props access

Author: retronym

src/main/scala/scala/tools/partest/nest/StreamCapture.scala

@@ -43,6 +43,9 @@ object StreamCapture {
     val modified = new java.util.Properties()
     modified.putAll(saved)
     extra.foreach { case (k, v) => modified.setProperty(k, v) }
+    // Trying to avoid other threads seeing the new properties object prior to the new entries
+    // https://github.com/scala/scala/pull/6391#issuecomment-371346171
+    UnsafeAccess.U.storeFence()
     System.setProperties(modified)
     try {
       action

src/main/scala/scala/tools/partest/nest/UnsafeAccess.java

@@ -0,0 +1,30 @@
+package scala.tools.partest.nest;
+
+import java.lang.reflect.Field;
+
+@SuppressWarnings("unsafe")
+public class UnsafeAccess {
+    public final static sun.misc.Unsafe U;
+
+    static {
+        U = lookupUnsafe();
+    }
+
+    private static sun.misc.Unsafe lookupUnsafe() {
+        try {
+            sun.misc.Unsafe found = null;
+            for (Field field : sun.misc.Unsafe.class.getDeclaredFields()) {
+                if (field.getType() == sun.misc.Unsafe.class) {
+                    field.setAccessible(true);
+                    found = (sun.misc.Unsafe) field.get(null);
+                    break;
+                }
+            }
+            if (found == null) throw new IllegalStateException("Can't find instance of sun.misc.Unsafe");
+            else return found;
+        } catch (Throwable t) {
+            throw new ExceptionInInitializerError(t);
+        }
+    }
+}
+