Context escape detection.

During creation of each of DottyTests context is stolen from test and a
WeakReference for it is created.
After running all tests references are examined to detect if any of them
has leaked.

Author: DarkDimius

project/Build.scala

@@ -15,7 +15,7 @@ object DottyBuild extends Build {
     resourceDirectory in Compile := baseDirectory.value / "resources",
     unmanagedSourceDirectories in Compile := Seq((scalaSource in Compile).value),
     unmanagedSourceDirectories in Test := Seq((scalaSource in Test).value),
-    
+
     // include sources in eclipse (downloads source code for all dependencies)
     //http://stackoverflow.com/questions/10472840/how-to-attach-sources-to-sbt-managed-dependencies-in-scala-ide#answer-11683728
     com.typesafe.sbteclipse.plugin.EclipsePlugin.EclipseKeys.withSource := true,
@@ -28,17 +28,19 @@ object DottyBuild extends Build {
                                 "org.scala-lang.modules" %% "scala-xml" % "1.0.1"),
 
     // get junit onboard
-    libraryDependencies += "com.novocode" % "junit-interface" % "0.9" % "test",
+    libraryDependencies += "com.novocode" % "junit-interface" % "0.11-RC1" % "test",
 
     // scalac options
     scalacOptions in Global ++= Seq("-feature", "-deprecation", "-language:_"),
 
     // enable verbose exception messages for JUnit
-    testOptions += Tests.Argument(TestFrameworks.JUnit, "-a", "-v"),
+    testOptions += Tests.Argument(TestFrameworks.JUnit, "-a", "-v", "--run-listener=test.ContextEscapeDetector"),
     // Adjust classpath for running dotty
     mainClass in (Compile, run) := Some("dotty.tools.dotc.Main"),
     fork in run := true,
     fork in Test := true,
+    parallelExecution in Test := false,
+
     // http://grokbase.com/t/gg/simple-build-tool/135ke5y90p/sbt-setting-jvm-boot-paramaters-for-scala
     javaOptions <++= (managedClasspath in Runtime, packageBin in Compile) map { (attList, bin) =>
        // put the Scala {library, reflect, compiler} in the classpath

test/test/ContextEscapeDetection.java

@@ -0,0 +1,40 @@
+package test;
+
+import dotty.tools.dotc.core.Contexts;
+import org.junit.*;
+
+import java.lang.ref.WeakReference;
+import java.util.LinkedList;
+import java.util.List;
+
+
+public abstract class ContextEscapeDetection {
+    public static class TestContext{
+        public TestContext(WeakReference<Contexts.Context> context, String testName) {
+            this.context = context;
+            this.testName = testName;
+        }
+
+        public final WeakReference<Contexts.Context> context;
+        public final String testName;
+
+    }
+    public static final List<TestContext> contexts = new LinkedList<>();
+
+    public abstract Contexts.Context getCtx();
+
+    public abstract void clearCtx();
+
+    @Before
+    public synchronized void stealContext() {
+        contexts.add(new TestContext(new WeakReference<>(this.getCtx()), this.getClass().getName()));
+    }
+
+    @After
+    public synchronized void clearContext() {
+        this.clearCtx();
+    }
+
+
+}
+

test/test/ContextEscapeDetector.java

@@ -0,0 +1,109 @@
+package test;
+
+import org.junit.runner.Description;
+import org.junit.runner.Result;
+import org.junit.runner.notification.RunListener;
+import org.junit.Assert;
+
+import java.lang.ref.WeakReference;
+
+public class ContextEscapeDetector extends RunListener {
+
+    //context can be captured by objects, eg NoDenotation
+    public static final int CONTEXTS_ALLOWED = 1;
+
+    @Override
+    public void testRunFinished(Result result) throws Exception {
+        if (contextsAlive() > CONTEXTS_ALLOWED) {
+            forceGCHeuristic0();
+            if (contextsAlive() > CONTEXTS_ALLOWED) {
+                forceGCHeuristic1();
+                if (contextsAlive() > CONTEXTS_ALLOWED) {
+                    forceGCHeuristic2();
+                    forceGCHeuristic1();
+                    int contextAlive = contextsAlive();
+                    if (contextAlive > CONTEXTS_ALLOWED) {
+                        StringBuilder names = new StringBuilder();
+                        for (ContextEscapeDetection.TestContext ref : ContextEscapeDetection.contexts) {
+                            if (ref.context.get() != null) names.append(ref.testName).append(' ');
+                        }
+                        Assert.fail("Multiple contexts survived test suite: " + names.toString());
+                    }
+                }
+            }
+        }
+        super.testRunFinished(result);
+    }
+
+    private static synchronized int contextsAlive() {
+        int count = 0;
+        for (ContextEscapeDetection.TestContext ref : ContextEscapeDetection.contexts) {
+            if (ref.context.get() != null) count++;
+        }
+        return count;
+    }
+
+    private static volatile Object o = null;
+
+    private static synchronized void forceGCHeuristic0() {
+        System.gc();
+        Runtime.getRuntime().gc();
+        System.gc();
+        Runtime.getRuntime().gc();
+        System.gc();
+        Runtime.getRuntime().gc();
+        System.gc();
+        Runtime.getRuntime().gc();
+        System.gc();
+    }
+
+    private static synchronized void forceGCHeuristic1() {
+        Object obj = new Object();
+        WeakReference ref = new WeakReference<Object>(obj);
+        obj = null;
+        while (ref.get() != null) {
+            System.gc();
+        }
+    }
+
+    private static synchronized void forceGCHeuristic2() {
+        try {
+            Object[] arr = new Object[1024]; // upto 8 GB
+            WeakReference ref = new WeakReference<Object>(arr);
+            o = arr; // make sure array isn't optimized away
+
+            Runtime runtime = Runtime.getRuntime();
+            // allocate memory until no more that 64MB is left
+            for (int i = 0; i < 1024 &&
+                    runtime.totalMemory() != runtime.maxMemory() ||
+                    runtime.freeMemory() < 1024 * 1024 * 64; i++) {
+                int[] data = new int[1024 * 1024]; // 8MB
+                for (int j = 0; j < 1024 * 1024; j++) {
+                    data[j] = j; // force actual pages allocation
+                }
+                arr[i] = data;
+            }
+            o = null;
+            arr = new Object[128];
+            o = arr;
+            // allocate 1 more GB
+            for (int i = 0; i < 128; i++) {
+                int[] data = new int[1024 * 1024]; // 8MB
+                for (int j = 0; j < 1024 * 1024; j++) {
+                    data[j] = j; // force actual pages allocation
+                }
+                arr[i] = data;
+            }
+            o = null;
+            arr = null;
+
+            forceGCHeuristic0();
+            while (ref.get() != null) {
+                System.gc();
+            }
+        } catch (OutOfMemoryError e) {
+            o = null;
+            // just swallow
+        }
+    }
+}

test/test/DottyTest.scala

@@ -14,11 +14,11 @@ import dotty.tools.dotc.Compiler
 import dotty.tools.dotc
 import dotty.tools.dotc.core.Phases.Phase
 
-class DottyTest {
+class DottyTest extends ContextEscapeDetection{
 
   dotty.tools.dotc.parsing.Scanners // initialize keywords
 
-  implicit val ctx: Context = {
+  implicit var ctx: Contexts.Context = {
     val base = new ContextBase
     import base.settings._
     val ctx = base.initialCtx.fresh
@@ -37,6 +37,10 @@ class DottyTest {
     ctx
   }
 
+  override def getCtx: Context = ctx
+  override def clearCtx() = {
+    ctx = null
+  }
   private def compilerWithChecker(phase: String)(assertion:(tpd.Tree, Context) => Unit) = new Compiler {
     override def phases = {
       val allPhases = super.phases