Fix parallel entity retriever error handling

silk-plugins/silk-plugins-rdf/src/main/scala/org/silkframework/plugins/dataset/rdf/sparql/ParallelEntityRetriever.scala

@@ -21,6 +21,7 @@ import org.silkframework.dataset.rdf.{RdfNode, Resource, SparqlEndpoint, SparqlR
 import org.silkframework.entity.paths.UntypedPath
 import org.silkframework.entity.rdf.{SparqlEntitySchema, SparqlPathBuilder, SparqlRestriction}
 import org.silkframework.entity.{Entity, EntitySchema}
+import org.silkframework.plugins.dataset.rdf.sparql.ParallelEntityRetriever.{ExceptionPathValues, ExistingPathValues, PathValues, QueueEndMarker}
 import org.silkframework.runtime.activity.UserContext
 import org.silkframework.util.Uri
 
@@ -124,7 +125,6 @@ class ParallelEntityRetriever(endpoint: SparqlEndpoint,
                              (implicit userContext: UserContext) extends Thread {
     private val queue = new LinkedBlockingQueue[PathValues](maxQueueSize)
 
-    @volatile private var exception: Throwable = _
     private var nextElement: Option[PathValues] = None
 
     def hasNext: Boolean = {
@@ -133,21 +133,18 @@ class ParallelEntityRetriever(endpoint: SparqlEndpoint,
       } else {
         nextElement = Option(queue.take())
         //Throw exceptions which occurred during querying
-        if (exception != null) throw exception
         moreEntriesAvailable
       }
     }
 
     private def moreEntriesAvailable: Boolean = nextElement match {
-      case Some(e) => e != EndPathValues
+      case Some(e) => e != QueueEndMarker
       case _ => false
     }
 
     def next(): PathValues = {
-      //Throw exceptions which occurred during querying
-      if (exception != null) throw exception
       nextElement match {
-        case Some(e) if e != EndPathValues=>
+        case Some(e) if e != QueueEndMarker =>
           nextElement = None
           e
         case _ =>
@@ -162,7 +159,8 @@ class ParallelEntityRetriever(endpoint: SparqlEndpoint,
         parseResults(sparqlResults.bindings)
       }
       catch {
-        case ex: Throwable => exception = ex
+        case ex: Throwable =>
+          queue.put(ExceptionPathValues(ex))
       }
     }
 
@@ -194,7 +192,7 @@ class ParallelEntityRetriever(endpoint: SparqlEndpoint,
         if (currentSubject.isEmpty) {
           currentSubject = subject
         } else if (subject.isDefined && subject != currentSubject) {
-          queueElement(PathValues(currentSubject.get, currentValues))
+          queueElement(ExistingPathValues(currentSubject.get, currentValues))
 
           currentSubject = subject
           currentValues = Seq.empty
@@ -208,15 +206,11 @@ class ParallelEntityRetriever(endpoint: SparqlEndpoint,
       }
 
       for (s <- currentSubject if sparqlResults.nonEmpty) {
-        queueElement(PathValues(s, currentValues))
+        queueElement(ExistingPathValues(s, currentValues))
       }
-      queueElement(EndPathValues)
+      queueElement(QueueEndMarker)
     }
   }
-
-  private case class PathValues(uri: String, values: Seq[String])
-  private val EndPathValues = PathValues("", Seq.empty) // Signals end
-
 }
 
 object ParallelEntityRetriever {
@@ -308,4 +302,27 @@ object ParallelEntityRetriever {
     }
     sparql.toString
   }
+
+  sealed trait PathValues {
+    def uri: String
+    def values: Seq[String]
+    def failure: Option[Throwable]
+  }
+  /** Actual values. */
+  case class ExistingPathValues(uri: String, values: Seq[String]) extends PathValues {
+    override def failure: Option[Throwable] = None
+  }
+  /** Object that marks the end of the queue. */
+  object QueueEndMarker extends PathValues {
+    val uri: String = ""
+    val values: Seq[String] = Seq.empty
+    val failure: Option[Throwable] = None
+  }
+  /** Failure case */
+  case class ExceptionPathValues(exception: Throwable) extends PathValues {
+    override def uri: String = throw exception
+    override def values: Seq[String] = throw exception
+
+    override def failure: Option[Throwable] = Some(exception)
+  }
 }