From 445a738b9ebfd93f65fecd68a5fbbe61b7951bf8 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Kjetil=20=C3=98rbekk?= <kjetil.orbekk@gmail.com>
Date: Wed, 2 May 2012 12:54:04 +0200
Subject: Fix VariableUpdaterTask.

Use CyclicCountDownLatch to improve VariableUpdaterTask. This
implementation is much better, but unfortunately really hard to test.
I'm not sure how to test this.
---
 .../java/com/orbekk/same/VariableUpdaterTask.java  | 84 +++++++++++-----------
 .../java/com/orbekk/util/DelayedOperation.java     | 12 ++++
 2 files changed, 53 insertions(+), 43 deletions(-)

(limited to 'same/src/main')

diff --git a/same/src/main/java/com/orbekk/same/VariableUpdaterTask.java b/same/src/main/java/com/orbekk/same/VariableUpdaterTask.java
index f0bcdd0..25be4f8 100644
--- a/same/src/main/java/com/orbekk/same/VariableUpdaterTask.java
+++ b/same/src/main/java/com/orbekk/same/VariableUpdaterTask.java
@@ -15,72 +15,70 @@
  */
 package com.orbekk.same;
 
-import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.concurrent.atomic.AtomicReference;
+import java.util.logging.Logger;
+
+import com.orbekk.util.CyclicCountDownLatch;
+import com.orbekk.util.DelayedOperation;
+
 
 /** Updates a variable on-demand.
  */
 public class VariableUpdaterTask<T> extends Thread
         implements Variable.OnChangeListener<T> {
+    private static Logger logger = Logger.getLogger(VariableUpdaterTask.class.getName());
     private Variable<T> variable;
-    private volatile T newValue;
-    private AtomicBoolean hasNewValue = new AtomicBoolean(false);
-    private AtomicBoolean isReady = new AtomicBoolean(true);
+    private AtomicReference<T> newValue = new AtomicReference<T>();
+    private CyclicCountDownLatch hasNewValue = new CyclicCountDownLatch(1);
+    private CyclicCountDownLatch isReady = new CyclicCountDownLatch(1);
     
     public VariableUpdaterTask(Variable<T> variable) {
         super("VariableUpdater");
         this.variable = variable;
     }
     
-    public synchronized void set(T newValue) {
-        this.newValue = newValue;
-        hasNewValue.set(true);
-        notifyAll();
-    }
-    
-    /** Update the variable once. */
-    public void performWork() {
-        boolean shouldDoWork = false;
-        synchronized(this) {
-            shouldDoWork = hasNewValue.get() && isReady.get();
-            hasNewValue.set(false);
-            isReady.set(false);
-        }
-        if (shouldDoWork) {
-            variable.set(newValue);
-        }
+    public void set(T newValue) {
+        this.newValue.set(newValue);
+        hasNewValue.countDown();
     }
     
-    private synchronized void waitFor(AtomicBoolean v) {
-        if (Thread.currentThread().isInterrupted()) {
-            return;
-        }
-        while(!v.get()) {
-            try {
-                wait();
-            } catch (InterruptedException e) {
-                Thread.currentThread().interrupt();
+    void setValue(T newValue) {
+        while (!Thread.currentThread().isInterrupted()) {
+            DelayedOperation op = variable.set(newValue);
+            if (op.getStatus().isOk()) {
                 return;
+            } else if (op.getStatus().isConflict()) {
+                // We have a new value, but we need to wait for an update
+                // in order to overwrite it.
+                hasNewValue.countDown();
+                return;
+            } else if (op.getStatus().isError()) {
+                // Error during update. Just retry.
+                logger.info("Error updating value. Status: " + op.getStatus());
+            } else {
+                throw new AssertionError("Unknown state.");
             }
         }
     }
+
+    @Override
+    public void valueChanged(Variable<T> variable) {
+        isReady.countDown();
+    }
     
     @Override
     public void run() {
+        isReady.countDown();  // Initially ready.
         variable.addOnChangeListener(this);
-        while (true) {
-            waitFor(isReady);
-            waitFor(hasNewValue);
-            if (Thread.currentThread().isInterrupted()) {
-                break;
+        while (!Thread.interrupted()) {
+            try {
+                hasNewValue.await();
+                isReady.await();
+                setValue(newValue.get());
+            } catch (InterruptedException e) {
+                Thread.currentThread().interrupt();
             }
-            performWork();
         }
-        variable.removeOnChangeListener(this);
-    }
-
-    @Override
-    public synchronized void valueChanged(Variable<T> unused) {
-        isReady.set(true);
-        notifyAll();
+        logger.info("VariableUpdaterTask finished.");
     }
 }
\ No newline at end of file
diff --git a/same/src/main/java/com/orbekk/util/DelayedOperation.java b/same/src/main/java/com/orbekk/util/DelayedOperation.java
index 48e7c05..caffa4f 100644
--- a/same/src/main/java/com/orbekk/util/DelayedOperation.java
+++ b/same/src/main/java/com/orbekk/util/DelayedOperation.java
@@ -47,6 +47,18 @@ public class DelayedOperation {
             return status == OK;
         }
         
+        public boolean isError() {
+            return status == ERROR;
+        }
+        
+        public boolean isConflict() {
+            return status == CONFLICT;
+        }
+        
+        public boolean canRetry() {
+            return isError();
+        }
+        
         public int getStatusCode() {
             return status;
         }
-- 
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