Update ApplianceValueIterator.java

Adding additional synchronization to resolve the issue of the msgIterator being created with missing data.
This lock can and should be removed once the dataretrieval is made thread safe.

applications/archive/archive-plugins/org.csstudio.archive.reader.appliance/src/org/csstudio/archive/reader/appliance/ApplianceValueIterator.java

@@ -53,6 +53,8 @@ public abstract class ApplianceValueIterator implements ValueIterator {
     private final IteratorListener listener;
 
     protected boolean closed = false;
+
+    private static Object lock = new Object();
 
     /**
      * Constructs a new ApplianceValueIterator.
@@ -92,7 +94,9 @@ protected void fetchDataInternal(String pvName) throws ArchiverApplianceExceptio
         java.sql.Timestamp sqlEndTimestamp = TimestampHelper.toSQLTimestamp(end);
 
         DataRetrieval dataRetrieval = reader.createDataRetriveal(reader.getDataRetrievalURL());
-        mainStream = dataRetrieval.getDataForPV(pvName, sqlStartTimestamp, sqlEndTimestamp);
+        synchronized(lock){
+            mainStream = dataRetrieval.getDataForPV(pvName, sqlStartTimestamp, sqlEndTimestamp);
+        }
         if (mainStream != null) {
             mainIterator = mainStream.iterator();
         } else {
@@ -316,4 +320,4 @@ protected static AlarmSeverity getSeverity(int severity) {
             return AlarmSeverity.UNDEFINED;
         }
     }
-}
+}