[GitHub] [jena] rvesse commented on a change in pull request #580: JENA-1719: Work around for iterators being held across transactions

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rvesse commented on a change in pull request #580: JENA-1719: Work around for iterators being held across transactions
URL: https://github.com/apache/jena/pull/580#discussion_r298946940
 
 

 ##########
 File path: jena-core/src/main/java/org/apache/jena/reasoner/rulesys/impl/SafeTripleIterator.java
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 @@ -0,0 +1,56 @@
+package org.apache.jena.reasoner.rulesys.impl;
+
+import org.apache.jena.graph.Triple;
+import org.apache.jena.reasoner.TriplePattern;
+import org.apache.jena.reasoner.rulesys.BackwardRuleInfGraphI;
+import org.apache.jena.util.iterator.ClosableIterator;
+import org.apache.jena.util.iterator.ExtendedIterator;
+
+/**
+ * Wrapper around triple searches using in the back chainer to attempt a restart
+ * in the case of a failure such as cross-transaction query.
+ */
+public class SafeTripleIterator implements ClosableIterator<Triple> {
+
+    protected ExtendedIterator<Triple> matchIterator;
+    protected long offset = 0;
+    protected TriplePattern goal;
+    protected BackwardRuleInfGraphI infGraph;
+
+    public SafeTripleIterator(LPInterpreter interpreter, TriplePattern goal) {
+        this.infGraph = interpreter.getEngine().getInfGraph();
+        this.goal = goal;
+        restart();
+    }
+
+    protected void restart() {
+        matchIterator = infGraph.findDataMatches(goal);
+        for (int i = 0; i < offset; i++) matchIterator.next();
 
 Review comment:
   Doesn't this needs to defend against the case where the data changes rendering the offset invalid i.e. it might no longer be possible to call `next()` the expected number of times resulting in a `NoSuchElementException`

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