Add try-finally to handle MapOutputTracker timeouts.

core/src/main/scala/spark/MapOutputTracker.scala

@@ -148,18 +148,23 @@ private[spark] class MapOutputTracker(actorSystem: ActorSystem, isMaster: Boolea
       // We won the race to fetch the output locs; do so
       logInfo("Doing the fetch; tracker actor = " + trackerActor)
       val host = System.getProperty("spark.hostname", Utils.localHostName)
-      val fetchedBytes = askTracker(GetMapOutputStatuses(shuffleId, host)).asInstanceOf[Array[Byte]]
-      val fetchedStatuses = deserializeStatuses(fetchedBytes)
-
-      logInfo("Got the output locations")
-      mapStatuses.put(shuffleId, fetchedStatuses)
-      fetching.synchronized {
-        fetching -= shuffleId
-        fetching.notifyAll()
-      }
-      if (fetchedStatuses.contains(null)) {
-        throw new FetchFailedException(null, shuffleId, -1, reduceId,
-          new Exception("Missing an output location for shuffle " + shuffleId))
+      // This try-finally prevents hangs due to timeouts:
+      var fetchedStatuses: Array[MapStatus] = null
+      try {
+        val fetchedBytes =
+          askTracker(GetMapOutputStatuses(shuffleId, host)).asInstanceOf[Array[Byte]]
+        fetchedStatuses = deserializeStatuses(fetchedBytes)
+        logInfo("Got the output locations")
+        mapStatuses.put(shuffleId, fetchedStatuses)
+        if (fetchedStatuses.contains(null)) {
+          throw new FetchFailedException(null, shuffleId, -1, reduceId,
+            new Exception("Missing an output location for shuffle " + shuffleId))
+        }
+      } finally {
+        fetching.synchronized {
+          fetching -= shuffleId
+          fetching.notifyAll()
+        }
       }
       return fetchedStatuses.map(s =>
         (s.address, MapOutputTracker.decompressSize(s.compressedSizes(reduceId))))