Correct semantics for TimeStampedWeakValueHashMap + add tests

This largely accounts for the cases when WeakReference becomes no longer strongly
reachable, in which case the map should return None for all get() operations, and
should skip the entry for all listing operations.

Author: andrewor14

core/src/main/scala/org/apache/spark/broadcast/Broadcast.scala

@@ -78,7 +78,7 @@ abstract class Broadcast[T](val id: Long) extends Serializable {
    */
   protected def assertValid() {
     if (!_isValid) {
-      throw new SparkException("Attempted to use %s when is no longer valid!".format(toString))
+      throw new SparkException("Attempted to use %s after it has been destroyed!".format(toString))
     }
   }
 

core/src/main/scala/org/apache/spark/util/TimeStampedHashMap.scala

@@ -21,7 +21,7 @@ import java.util.Set
 import java.util.Map.Entry
 import java.util.concurrent.ConcurrentHashMap
 
-import scala.collection.{immutable, JavaConversions, mutable}
+import scala.collection.{JavaConversions, mutable}
 
 import org.apache.spark.Logging
 
@@ -50,11 +50,11 @@ private[spark] class TimeStampedHashMap[A, B](updateTimeStampOnGet: Boolean = fa
   }
 
   def iterator: Iterator[(A, B)] = {
-    val jIterator = getEntrySet.iterator()
+    val jIterator = getEntrySet.iterator
     JavaConversions.asScalaIterator(jIterator).map(kv => (kv.getKey, kv.getValue.value))
   }
 
-  def getEntrySet: Set[Entry[A, TimeStampedValue[B]]] = internalMap.entrySet()
+  def getEntrySet: Set[Entry[A, TimeStampedValue[B]]] = internalMap.entrySet
 
   override def + [B1 >: B](kv: (A, B1)): mutable.Map[A, B1] = {
     val newMap = new TimeStampedHashMap[A, B1]
@@ -86,8 +86,7 @@ private[spark] class TimeStampedHashMap[A, B](updateTimeStampOnGet: Boolean = fa
   }
 
   override def apply(key: A): B = {
-    val value = internalMap.get(key)
-    Option(value).map(_.value).getOrElse { throw new NoSuchElementException() }
+    get(key).getOrElse { throw new NoSuchElementException() }
   }
 
   override def filter(p: ((A, B)) => Boolean): mutable.Map[A, B] = {
@@ -101,9 +100,9 @@ private[spark] class TimeStampedHashMap[A, B](updateTimeStampOnGet: Boolean = fa
   override def size: Int = internalMap.size
 
   override def foreach[U](f: ((A, B)) => U) {
-    val iterator = getEntrySet.iterator()
-    while(iterator.hasNext) {
-      val entry = iterator.next()
+    val it = getEntrySet.iterator
+    while(it.hasNext) {
+      val entry = it.next()
       val kv = (entry.getKey, entry.getValue.value)
       f(kv)
     }
@@ -115,27 +114,39 @@ private[spark] class TimeStampedHashMap[A, B](updateTimeStampOnGet: Boolean = fa
     Option(prev).map(_.value)
   }
 
-  def toMap: immutable.Map[A, B] = iterator.toMap
+  def putAll(map: Map[A, B]) {
+    map.foreach { case (k, v) => update(k, v) }
+  }
+
+  def toMap: Map[A, B] = iterator.toMap
 
   def clearOldValues(threshTime: Long, f: (A, B) => Unit) {
-    val iterator = getEntrySet.iterator()
-    while (iterator.hasNext) {
-      val entry = iterator.next()
+    val it = getEntrySet.iterator
+    while (it.hasNext) {
+      val entry = it.next()
       if (entry.getValue.timestamp < threshTime) {
         f(entry.getKey, entry.getValue.value)
         logDebug("Removing key " + entry.getKey)
-        iterator.remove()
+        it.remove()
       }
     }
   }
 
-  /**
-   * Removes old key-value pairs that have timestamp earlier than `threshTime`.
-   */
+  /** Removes old key-value pairs that have timestamp earlier than `threshTime`. */
   def clearOldValues(threshTime: Long) {
     clearOldValues(threshTime, (_, _) => ())
   }
 
   private def currentTime: Long = System.currentTimeMillis
 
+  // For testing
+
+  def getTimeStampedValue(key: A): Option[TimeStampedValue[B]] = {
+    Option(internalMap.get(key))
+  }
+
+  def getTimestamp(key: A): Option[Long] = {
+    getTimeStampedValue(key).map(_.timestamp)
+  }
+
 }

core/src/main/scala/org/apache/spark/util/TimeStampedWeakValueHashMap.scala

@@ -18,47 +18,61 @@
 package org.apache.spark.util
 
 import java.lang.ref.WeakReference
+import java.util.concurrent.atomic.AtomicInteger
 
-import scala.collection.{immutable, mutable}
+import scala.collection.mutable
+
+import org.apache.spark.Logging
 
 /**
  * A wrapper of TimeStampedHashMap that ensures the values are weakly referenced and timestamped.
  *
- * If the value is garbage collected and the weak reference is null, get() operation returns
- * a non-existent value. However, the corresponding key is actually not removed in the current
- * implementation. Key-value pairs whose timestamps are older than a particular threshold time
- * can then be removed using the clearOldValues method. It exposes a scala.collection.mutable.Map
- * interface to allow it to be a drop-in replacement for Scala HashMaps.
+ * If the value is garbage collected and the weak reference is null, get() will return a
+ * non-existent value. These entries are removed from the map periodically (every N inserts), as
+ * their values are no longer strongly reachable. Further, key-value pairs whose timestamps are
+ * older than a particular threshold can be removed using the clearOldValues method.
  *
- * Internally, it uses a Java ConcurrentHashMap, so all operations on this HashMap are thread-safe.
+ * TimeStampedWeakValueHashMap exposes a scala.collection.mutable.Map interface, which allows it
+ * to be a drop-in replacement for Scala HashMaps. Internally, it uses a Java ConcurrentHashMap,
+ * so all operations on this HashMap are thread-safe.
  *
  * @param updateTimeStampOnGet Whether timestamp of a pair will be updated when it is accessed.
  */
 private[spark] class TimeStampedWeakValueHashMap[A, B](updateTimeStampOnGet: Boolean = false)
-  extends mutable.Map[A, B]() {
+  extends mutable.Map[A, B]() with Logging {
 
   import TimeStampedWeakValueHashMap._
 
   private val internalMap = new TimeStampedHashMap[A, WeakReference[B]](updateTimeStampOnGet)
+  private val insertCount = new AtomicInteger(0)
+
+  /** Return a map consisting only of entries whose values are still strongly reachable. */
+  private def nonNullReferenceMap = internalMap.filter { case (_, ref) => ref.get != null }
 
   def get(key: A): Option[B] = internalMap.get(key)
 
-  def iterator: Iterator[(A, B)] = internalMap.iterator
+  def iterator: Iterator[(A, B)] = nonNullReferenceMap.iterator
 
   override def + [B1 >: B](kv: (A, B1)): mutable.Map[A, B1] = {
     val newMap = new TimeStampedWeakValueHashMap[A, B1]
+    val oldMap = nonNullReferenceMap.asInstanceOf[mutable.Map[A, WeakReference[B1]]]
+    newMap.internalMap.putAll(oldMap.toMap)
     newMap.internalMap += kv
     newMap
   }
 
   override def - (key: A): mutable.Map[A, B] = {
     val newMap = new TimeStampedWeakValueHashMap[A, B]
+    newMap.internalMap.putAll(nonNullReferenceMap.toMap)
     newMap.internalMap -= key
     newMap
   }
 
   override def += (kv: (A, B)): this.type = {
     internalMap += kv
+    if (insertCount.incrementAndGet() % CLEAR_NULL_VALUES_INTERVAL == 0) {
+      clearNullValues()
+    }
     this
   }
 
@@ -71,31 +85,53 @@ private[spark] class TimeStampedWeakValueHashMap[A, B](updateTimeStampOnGet: Boo
 
   override def apply(key: A): B = internalMap.apply(key)
 
-  override def filter(p: ((A, B)) => Boolean): mutable.Map[A, B] = internalMap.filter(p)
+  override def filter(p: ((A, B)) => Boolean): mutable.Map[A, B] = nonNullReferenceMap.filter(p)
 
   override def empty: mutable.Map[A, B] = new TimeStampedWeakValueHashMap[A, B]()
 
   override def size: Int = internalMap.size
 
-  override def foreach[U](f: ((A, B)) => U) = internalMap.foreach(f)
+  override def foreach[U](f: ((A, B)) => U) = nonNullReferenceMap.foreach(f)
 
   def putIfAbsent(key: A, value: B): Option[B] = internalMap.putIfAbsent(key, value)
 
-  def toMap: immutable.Map[A, B] = iterator.toMap
+  def toMap: Map[A, B] = iterator.toMap
 
-  /**
-   * Remove old key-value pairs that have timestamp earlier than `threshTime`.
-   */
+  /** Remove old key-value pairs with timestamps earlier than `threshTime`. */
   def clearOldValues(threshTime: Long) = internalMap.clearOldValues(threshTime)
 
+  /** Remove entries with values that are no longer strongly reachable. */
+  def clearNullValues() {
+    val it = internalMap.getEntrySet.iterator
+    while (it.hasNext) {
+      val entry = it.next()
+      if (entry.getValue.value.get == null) {
+        logDebug("Removing key " + entry.getKey + " because it is no longer strongly reachable.")
+        it.remove()
+      }
+    }
+  }
+
+  // For testing
+
+  def getTimestamp(key: A): Option[Long] = {
+    internalMap.getTimeStampedValue(key).map(_.timestamp)
+  }
+
+  def getReference(key: A): Option[WeakReference[B]] = {
+    internalMap.getTimeStampedValue(key).map(_.value)
+  }
 }
 
 /**
  * Helper methods for converting to and from WeakReferences.
  */
-private[spark] object TimeStampedWeakValueHashMap {
+private object TimeStampedWeakValueHashMap {
 
-  /* Implicit conversion methods to WeakReferences */
+  // Number of inserts after which entries with null references are removed
+  val CLEAR_NULL_VALUES_INTERVAL = 100
+
+  /* Implicit conversion methods to WeakReferences. */
 
   implicit def toWeakReference[V](v: V): WeakReference[V] = new WeakReference[V](v)
 
@@ -107,12 +143,15 @@ private[spark] object TimeStampedWeakValueHashMap {
     (kv: (K, WeakReference[V])) => p(kv)
   }
 
-  /* Implicit conversion methods from WeakReferences */
+  /* Implicit conversion methods from WeakReferences. */
 
   implicit def fromWeakReference[V](ref: WeakReference[V]): V = ref.get
 
   implicit def fromWeakReferenceOption[V](v: Option[WeakReference[V]]): Option[V] = {
-    v.map(fromWeakReference)
+    v match {
+      case Some(ref) => Option(fromWeakReference(ref))
+      case None => None
+    }
   }
 
   implicit def fromWeakReferenceTuple[K, V](kv: (K, WeakReference[V])): (K, V) = {
@@ -128,5 +167,4 @@ private[spark] object TimeStampedWeakValueHashMap {
       map: mutable.Map[K, WeakReference[V]]) : mutable.Map[K, V] = {
     mutable.Map(map.mapValues(fromWeakReference).toSeq: _*)
   }
-
 }